The Sixteenth Marcel Grossmann Meeting

The following sections are included:

• Publications in this Series
• Sponsors and Acknowledgements
• Organizing Committees
• Marcel Grossmann Awards
• Preface
• Contents

We experience a golden era in testing and exploring relativistic gravity. Whether it is results from gravitational wave detectors, satellite or lab experiments, radio astronomy plays an important complementary role. Here one can mention the cosmic microwave background, black hole imaging and, obviously, binary pulsars. This talk will concentrate on the latter and new results from studies of strongly self-gravitating bodies with unrivalled precision. This presentation compares the results to other methods, discusses implications for other areas of relativistic astrophysics and will give an outlook of what we can expect from new instruments in the near future.

We review and analyze four specific general-relativistic problems in which gravitomagnetism plays the important role: the dragging of magnetic fields around rotating black holes, dragging inside a collapsing slowly rotating spherical shell of dust, compared with the dragging by rotating gravitational waves. We demonstrate how the quantum detection of inertial frame dragging can be accomplished by using the Unruh-DeWitt detectors. Finally, we shall briefly show how “instantaneous Machian gauges” can be useful in the cosmological perturbation theory.

SN are one of the most powerful explosions in the universe and astronomers have invoked the collapse of a stellar core down to a neutron star as a potential power source behind these cosmic blasts. The current paradigm behind core-collapse SN relies on convection in the region just above the newly formed neutron star. This engine was driven and confirmed by observations. We review this observational evidence, and the potential for further observational constraints in this paper.

The IceCube neutrino telescope discovered PeV-energy neutrinos originating beyond our Galaxy with an energy flux that is comparable to that of GeV-energy gamma rays and EeV-energy cosmic rays. These neutrinos provide the only unobstructed view of the cosmic accelerators that power the highest energy radiation reaching us from the universe. We will review the results from IceCube’s first decade of operations, emphasizing the measurement of the diffuse multiflavored neutrino flux from the universe and the identification of the supermassive black hole TXS 0506+056 as a source of cosmic neutrinos and, therefore, cosmic rays. We will speculate on the lessons learned for multimessenger astronomy, among them that extragalactic neutrino sources may be a relatively small subset of the cosmic accelerators observed in high-energy gamma rays and that these may be gamma-ray-obscured at the times that they emit neutrinos.

Dark matter, an invisible substance which constitutes 85% of the matter in the observable universe, is one of the greatest puzzles in physics and astronomy today. Dark matter can be made of a new type of fundamental particle, not yet observed due to its feeble interactions with visible matter. In this talk, we present the first results of PandaX-4T, a 4-ton-scale liquid xenon dark matter observatory, searching for these dark matter particles from deep underground. We will briefly summarize the performance of PandaX-4T, introduces details in the data analysis, and present the latest search results on dark matter-nucleon interactions.

The X-Ray Imaging and Spectroscopy Mission (XRISM) is a JAXA/NASA X-ray observatory with collaboration from ESA and several institutes and academic institutions worldwide. It is proposed to fulfill the promise of high-resolution X-ray spectroscopy with imaging once realized but unexpectedly terminated by a mishap of ASTRO-H/Hitomi. XRISM carries two sets of X-ray Mirror Assemblies and is equipped on the focal plane with a 6 × 6 pixelized X-ray micro-calorimeter array and an aligned X-ray CCD camera. With the combination of high-resolution spectroscopy imaging and the broader field of view, XIRSM is expected to pioneer a new horizon of the Universe in X-ray astrophysics. Aiming to launch the satellite in the Japanese Fiscal Year 2022, we fabricate the instruments and test for the satellite integration starting at the beginning of 2022. The paper reports the development status, reviewing the science objectives and the operation plan.

The Sino-French space mission SVOM is mainly designed to detect, localize and follow-up Gamma-Ray Bursts and other high-energy transients. The satellite, to be launched mid 2023, embarks two wide-field gamma-ray instruments and two narrow-field telescopes operating at X-ray and optical wavelengths. It is complemented by a dedicated ground segment encompassing a set of wide-field optical cameras and two 1-meter class follow-up telescopes. In this contribution, we describe the main characteristics of the mission and discuss its scientific rationale and some original GRB studies that it will enable.

A procedure to derive a unitary evolution law for a quantised black hole has been proposed by the author. The proposal implies several assumptions, which seem almost unavoidable to this author. We start off with the question how to describe the energy eigenstates of a black hole. The background metric required for this cannot be the Vaidya metric let alone the metric proposed by Hawking, who included the effect of the final evaporation of the black hole. This however leads to the formation if firewalls at both the future and the past event horizon, unless one anticipates the effects that the firewalls have. These effects can be handled as new boundary conditions at the horizons, describing the flow of the participating particles. It is subsequently explained how these boundary conditions must involve the antipodes of the outside world. Imposing unitarity and continuity then automatically leads to a unique, unitary evolution operator. We exhibit the resulting, quite coherent picture.

There is much talk of a “mysterious form of energy”, called “dark energy” that forms the bulk of the energy content of the Universe. Perhaps the most mysterious aspect of it is why it should be regarded as mysterious in the first place. This question is discussed in the context of the development of relativity and relativistic cosmology. It will be argued that there is no good reason to treat it as other than Einstein’s cosmological constant.

We reveal three new discoveries in black hole physics previously unexplored in the Hawking era. These results are based on the remarkable 1971 discovery of the irreducible mass of the black hole by Christodoulou and Ruffini, and subsequently confirmed by Hawking.

1. The Horizon Mass Theorem shows that the mass at the event horizon of any black hole: neutral, charged, or rotating, depends only on twice its irreducible mass observed at infinity.

2. The External Energy Conjecture proposes that the electrostatic and rotational energy of a general black hole exist completely outside the horizon due to the nature of the irreducible mass.

3. The Moment of Inertia Property shows that every Kerr black hole has a moment of inertia. When the rotation stops, there is an irreducible moment of inertia as a result of the irreducible mass.

Thus after 50 years, the irreducible mass has gained a new and profound significance. No longer is it just a limiting value in energy extraction, it can also determine black hole dynamics and structure. What is believed to be a black hole is a physical body with an extended structure. Astrophysical black holes are likely to be massive compact objects from which light cannot escape.

We study the non-linear structure formation in cosmology accounting for the quantum nature of the dark matter (DM) particles in the initial conditions at decoupling, as well as in the relaxation and stability of the DM halos. Differently from cosmological N-body simulations, we use a thermodynamic approach for collisionless systems of self-gravitating fermions in General Relativity, in which the halos reach the steady state by maximizing a coarse-grained entropy. We show the ability of this approach to provide answers to crucial open problems in cosmology, among others: the mass and nature of the DM particle, the formation and nature of supermassive black holes in the early Universe, the nature of the intermediate mass black holes in small halos, and the core-cusp problem.

The Large High Altitude Air Shower Observatory (LHAASO) has recently published the first results, including the discovery of 12 ultrahigh-energy gamma-ray sources (with emission above 100 TeV) above 7σ confidence level and a detailed analysis of Crab Nebula. This contribution gives a brief introduction to the LHAASO experiment and its recent discoveries.

On Monday July 5, 2021 took place the official ceremony for the attribution of the 4 MG16 Individual Awards to Demetrios Christodoulou, Gerard ’t Hooft, Tsvi Piran and Steven Weinberg, as well as for the attribution of the 3 MG16 Institutional Awards to the Spektrum-Roentgen-Gamma (SRG) mission of the Max Planck Institute for Extraterrestrial Physics (MPE), of the Space Research Institute (IKI) of the Russian Academy of Sciences and of the S.A. Lavochkin Association. The MG Awards consist in a silver casting of the TEST sculpture by the artist Attilio Pierelli. The MG16 Awards were presented to the Awardees by Prof. Roy P. Kerr.

Following the GRB 170817A prompt emission lasting a fraction of a second, 10⁸ s of data in the X-rays, optical, and radio wavelengths have been acquired. We here present a model that fits the spectra, flux, and time variability of all these emissions, based on the thermal and synchrotron cooling of the expanding matter ejected in a binary white dwarf merger. The 10^-3M_⊙ of ejecta, expanding at velocities of 10⁹ cm s-¹, are powered by the newborn massive, fast rotating, magnetized white dwarf with a mass of 1.3M_⊙, a rotation period of ≳12 s, and a dipole magnetic field ∽ 10¹⁰ G, born in the merger of a 1.0 + 0.8M_⊙ white dwarf binary. Therefore, the long-lasting mystery of the GRB 170817A nature is solved by the merger of a white dwarf binary that also explains the prompt emission energetics.

Astrophysical problems such as modelling of core-collapse supernovae, collapse of protostellar clouds as well as other processes, involving collapsing matter, deal with regions (e.g. protostars, protoneutron stars), where a speed of sound has much larger values, than in remaining parts of a computational domain. A time-step in explicit numerical schemes, thus, has to be bounded by acoustic Courant-Friedrichs-Lewy condition, due to high speed of sound in these compact regions. In some cases, this condition can be very restrictive, and (semi-) implicit numerical schemes may outperform the explicit ones. We propose a semi-implicit solver on a collocated mesh for self-gravitating gas dynamical flows, in which only acoustic waves are treated implicitly. We use an operator-difference approach to construct difference analogues of vector differential operators on unstructured meshes in two and three dimensions, which allows us to save the conjugacy properties of the operators. A Rusanov-type dissipation was used to get monotonic flow profiles and usual linear flux reconstruction to improve an order of spatial approximation. Results of test calculations are presented.

Many neutron stars propagate through the interstellar medium with supersonic velocities, and their magnetospheres interact with the interstellar medium (ISM), forming bow shocks and magnetotails. Using numerical MHD simulations, we investigated the propagation of a magnetized neutron stars through a non-uniform ISM, the interaction of the magnetospheres with the ISM and the influence of ISM density on the shape of the magnetosphere tail. We consider the interaction of magnetized neutron stars with small-scale and large-scale inhomogeneities in the ISM. We conclude that the inhomogeneities in the ISM can change the shapes of the bow shocks and magnetotails at different values of the magnetization.

A solution of the Boltzmann equation is obtained for a magnetized plasma with arbitrary degenerate electrons and nondegenerate nuclei. For the arbitrary and non-degenerate electrons kinetic coefficients are obtained by solving Boltzmann equation by Chapman-Enskog method of successive approximations. The expressions have a considerably more complicated dependence on magnetic field than analogous dependences derived in previous publications on this subject.

The evolution of the accretion discs surrounding different compact objects (such as white dwarfs, neutron stars and black holes) is closely connected with the magnetic fields and their features. It is highly likely that the magnetic field generation is connected with the dynamo action. As for the dynamo, we have a lot of specific models which have been constructed for different astrophysical objects. For accretion discs we can take the approaches that were used for galactic discs which have nearly the same shape. There are two main models. The first one is connected with no-z approximation which is based on the fact that the discs is quite thin. RZ-model describes the discs with large half-thickness and can be used for the objects where we should study the vertical structure of the magnetic field. Here we show the results obtained for the accretion discs which used both of these approaches and compare the results.

We consider agglomerates of misaligned tori orbiting a supermassive black hole. The aggregate of tilted tori is modeled as a single orbiting configuration by introducing a leading function governing the distribution of toroids (and maximum pressure points inside the disks) around the black hole attractor. The orbiting clusters are composed by geometrically thick, pressure supported, perfect fluid tori. This analysis places constraints on the existence and properties of tilted tori and more general aggregates of orbiting disks. We study the constraints on the tori collision emergence and the instability of the agglomerates of tori with general relative inclination angles, the possible effects of the tori geometrical thickness and on the oscillatory phenomena. Some notes are discussed on the orbiting ringed structure in dependence of the dimensionless parameter ξ representing the (total) BH rotational energy extracted versus the mass of the BH, associating ξ to the characteristics of the accretion processes.

The origin of hydrodynamical instability and turbulence in the Keplerian accretion disk is a long-standing puzzle. The flow therein is linearly stable. Here we explore the evolution of perturbation in this flow in the presence of an additional force. Such a force, which is expected to be stochastic in nature hence behaving as noise, could result from thermal fluctuations (however small be), grain–fluid interactions, feedback from outflows in astrophysical disks, etc. We essentially establish the evolution of nonlinear perturbation in the presence of Coriolis and external forces, which is the modified Landau equation. We obtain that even in the linear regime, under suitable forcing and Reynolds number, the otherwise least stable perturbation evolves to a very large saturated amplitude, leading to nonlinearity and plausible turbulence. Hence, forcing essentially leads a linear stable mode to unstable. We further show that nonlinear perturbation diverges at a shorter time-scale in the presence of force, leading to a fast transition to turbulence. Interestingly, the emergence of nonlinearity depends only on the force but not on the initial amplitude of perturbation, unlike the original Landau equation-based solution.

We consider a static and axially symmetric metric containing two quadrupole parameters. In the present contribution, we study the quadrupole moments constraints on the properties of the relativistic accretion disc models, also explore the relation of oscillatory frequencies of charged particles to the frequencies of the twin high-frequency quasi-periodic oscillations observed in some microquasars. We also compare the results with Schwarzschild and Kerr metrics.

We generalize the relativistic accretion thick disc model to the background of a spinning charged accelerating black hole described by the C-metric to study the effects of this background on the disc model. We show the properties of this accretion disc model and its dependence on the initial parameters. This background can be distinguishable from the Kerr space-time by analyzing the observing features of accretion discs.

In this work, without any claim to completeness, I will review the zoo of binary systems emitting X radiation, the multi-frequency behaviors of high-mass X-ray systems (HMXBs), with particular emphasis on the X-ray/Be system A 0535 + 26/HDE 245770, which for a favorable series of concomitant causes it is the most studied system. I will also discuss the time lag between events occurring in the high-energy and low-energy bands in galactic accreting systems.

The first image of the black hole (BH) M87* obtained by the Event Horizon Telescope (EHT) has the shape of a crescent extending from the E to WSW position angles, with a tentative ‘ESE hotspot’. Assuming that the BH spin vector is aligned with both the inner accretion axis and the projected direction of the kpc-scale relativistic jet, the position of the ESE hotspot is inconsistent with the axisymmetric accretion flow. Recent polarimetric EHT images of M87* show that the ESE hotspot is essentially unpolarized, which strongly supports its distinct origin. If the hotspot emission is due to the synchrotron radiation, its depolarization requires either isotropically tangled magnetic fields or an additional Faraday dispersion measure. The 6-day EHT observing campaign in April 2017 allowed in principle to detect orbital motions advancing by up to ∽ 60°. The apparent rotation rate of the major axis of the EHT crescent image is consistent with the rotation rate of the Faraday-corrected polarization angle measured by the ALMA. However, the counterclockwise (CCW) sense of these rotations is opposite to the clockwise (CW) rotation of the plasma flows implied by the N-S brightness asymmetry, which might indicate that accretion in M87 is retrograde.

Black hole surroundings and relativistic jets host magnetically dominated regions and fast magnetic reconnection are likely to play an important role concerning astrophysical phenomena associated with such regions. In this contribution, we highlight the works related to turbulence-driven reconnection processes. These processes have been studied by us using analytical as well as numerical methods which showed that fast reconnection processes are powerful ways to giving rise to relativistic particles and associated non-thermal emissions around stellar-mass and supermassive black holes. The power released from the reconnection can even compete with those of extraction from black hole spin.

We discuss the possibility that the topological structure of the Universe may possess fractal properties. Relic wormholes and their fractal distribution are predicted in a natural way by lattice quantum gravity models. This gives a new approach to some long-standing problems. Those are the nature of dark matter phenomena, the origin of Faber-Jackson and Tully-Fisher relations, and the observed deficit of baryons. We consider open Friedman model and construct an exact fractal model by means of a factorization of the space over a discrete subgroup of the group of motion. We derive some basic features of the resulting fractal space and discuss applications of machine learning methods for the verification of the fractal properties.

Measuring distances of cosmological sources such as galaxies, stars and quasars plays an increasingly critical role in modern cosmology. Obtaining the optical spectrum and consequently calculating the redshift as a distance indicator could instantly classify these objects. As long as spectroscopic observations are not available for many galaxies and the process of measuring the redshift is time-consuming and infeasible for large samples, machine learning (ML) approaches could be applied to determine the redshifts of galaxies from different features including their photometric colors. In this paper, by using the flux magnitudes from the Sloan Digital Sky Survey (SDSS) catalog, we develop two ML regression algorithms (decision tree and random forest) for estimating the redshifts taking color indices as input features. We find that the random forest algorithm produces the optimum result for the redshift prediction, and it will be further improved when the dataset is limited to a subset with z ≤ 2 giving the normalised standard deviation ${\overline{\text{Δ}Z}}_{norm }=0.005$ and the standard deviation ${\sigma }_{\Delta z} = 0.12.$ This work shows great potential of using the ML approach to determine the photometric redshifts of distant sources.

In view of increasing data volume of existing and upcoming telescopes/detectors we here apply the 1–dimensional convolutional neural network (CNN) to estimate the redshift of (high-)redshifts quasars in Sloan Digital Sky Survey IV (SDSS-IV) quasar catalog from DR16 of eBOSS. Our CNN takes the flux of the quasars as an array and their redshift as labels. We here evidence that new structure of the network, and augmenting the training set, provide a high precision result in estimating the redshift of quasars.

Two major challenges in modern cosmology involve understanding the origin and growth of Cosmic structure and the progenitors of Gravitational Waves. Both scenarios currently require heavy computational resources to perform simulations and inference. In this work, we adopt simple Machine Learning methods to alleviate these requirements, to enable significantly faster sampling and inference. We show that using Dimensionality Reduction and simple Supervised Learning methods, it is possible to generate high-precision emulations of density fields given a set of parameters (such as the Dark Matter density parameter and redshift). Our method provides orders of magnitude improvement of CPU run times and much less computational resources when compared with N-Body simulations or more complex supervised learning approaches. We also show that it is possible to generate fast inference of gravitational wave parameters (such as the Chirp Mass) from Binary Black Hole systems using the same method. This method provides a promising approach to fast emulation and parameter inference to further explore in the context of upcoming large surveys like Euclid, LSST/Rubin, and LISA.

Galaxy Clusters are essential to study galaxy evolution and are sensitive probes of cosmology and the dynamics of the Dark sector. Large galaxy surveys, such as Euclid, DES, LSST/Rubin will detect many new clusters. For example, the Euclid mission survey may reveal more than 6 × 10⁴ clusters with S/N>3 up to z ∼ 2, representing a whole new era for cluster cosmology. A large fraction of these clusters will be unknown high-redshift cluster candidates, lacking spectroscopic information. Thus, a major challenge for cluster detection is the identification of possible member galaxies from photometry alone, and ideally without strong assumptions of what a cluster is. Here we present the first results for the detection of galaxy clusters of a modified version of the UPMASK (Unsupervised Photometric Membership Assignment) method. The method, created to study star clusters, uses heuristics and statistical analysis to separate cluster candidate galaxies from other field galaxies without assuming cluster profiles or any strong theoretical cluster priors. We show that the method operates in a fully unsupervised way and it can even work with minimal amounts of astrometry and photometry information, using Euclid-like galaxy survey simulations. We then use Pan-STARRS data to assess the performance of the method to identify Planck clusters and present possible detections of optical counterparts for cluster candidates in the second PlanckSZ data release catalog. Finally, we compare our findings with other Planck cluster candidate follow-up efforts.

The recent associations of neutrino events with blazars (e.g. TXS 0506+056, 3HSP J095507.9+355101) provided a unique opportunity to study the possible physical connection between the multiwavelength electromagnetic and neutrino emissions. We present SOPRANO, a new conservative implicit kinetic code which follows the time evolution of the isotropic distribution functions of protons, neutrons and the secondaries produced in photo-pion and photo-pair interactions, alongside with the evolution of photon and electron/positron distribution functions. In the current work, we apply SOPRANO to model the broadband spectrum of TXS 0506+056, 3HSP J095507.9+355101 and 3C 279 blazars. It was possible to constrain main physical parameters within both a pure hadronic and lepto-hadronic scenarios.

High-redshift blazars are among the most powerful objects in the Universe. The spectral and temporal properties of 33 distant blazars (z > 2.5) detected in the high-energy γ-ray band are discussed using Fermi-LAT and Swift Ultraviolet and Optical Telescope/X-ray Telescope (UVOT/XRT) data accumulated during 2008-2018. The properties of the jets of these blazar obtained by modeling the multiwavelength spectral energy distributions within a one-zone leptonic scenario are presented and discussed.

In this review, we discuss a derivation of effective low energy quantum gravitational dynamics from thermodynamics. The derivation is based on the formalism developed in semiclassical thermodynamics of spacetime that allows to obtain Einstein equations from the proportionality of entropy to the area. We first introduce the relevant ingredients of semiclassical thermodynamics of spacetime, paying special attention to the various concepts of entropy involved and their relations. We then extend the semiclassical formalism by considering low energy quantum gravity effects which imply a modified entropy formula with an additional term logarithmic in the area. Upon discussing the derivation of effective gravitational dynamics from this modified entropy, we comment on the most important features of our proposal. Moreover, we show its physical implications on a simple cosmological model and show that it suggests the replacement of the Big Bang singularity by a regular bounce.

A covariant canonical gauge theory of gravity free from torsion is studied. Using a metric conjugate momentum and a connection conjugate momentum, which takes the form of the Riemann tensor, a gauge theory of gravity is formulated, with form-invariant Hamiltonian. By the metric conjugate momenta, a correspondence between the Affine-Palatini formalism and the metric formalism is established. For, when the dynamical gravitational Hamiltonian ${\tilde{H}}_{Dyn}$ does not depend on the metric conjugate momenta, a metric compatibility is obtained from the equation of motions, and the equations of motion correspond to the solution is the metric formalism.

In this work we made use of a general static cillindrically symmetric metric to find U (1) local cosmic string solutions in the context of the hybrid metric-Palatini theory of gravity in it’s scalar-tensor representation. After finding the dynamical equations for this particular case, we imposed boost invariance along t and z directions, which simplified the equations of motions, leaving only one single metric tensor component, W ²(r). For an arbitrary potential V (ϕ), the solutions obtained can be put in a closed parametric form, with ϕ taken as a parameter. Several particular cases of the potential were studied, some yielding simple mathematical forms, others with only numerical solutions. In this way, we obtain a large class of novel stable stringlike solutions in the context of hybrid metric-Palatini gravity, in which the basic parameters, such as the scalar fleld, metric tensor components, and string tension, depend essentially on the initial values of the scalar fleld, and of its derivative, on the r(0) circular axis.

We consider inflationary scenarios of the supersymmetric quantum cosmology of FRLW models with a scalar field. We use the superfield formalism with a superpotential for the scalar superfield. The probability amplitude solution of the supersymmetric Wheeler-DeWitt equation, gives a probability density from which we can compute mean trajectories that can be parametrized by the scalar. By suitable choices of the superpotential, the resulting evolutions of the scale factor correspond to consistent inflationary scenarios. We show the acceleration, the resulting e-folds and the horizon for several superpotentials.

Although General Relativity (GR) is an extremely successful theory, at least for weak gravitational fields, it breaks down at very high energies. For example, extrapolating the expansion of the Universe backwards in time yields an infinite energy density, which is referred to as the initial singularity problem. Quantum Gravity is expected to provide a solution to this open question. In fact, one alternative scenario to the Big Bang, that avoids the singularity, is offered by Loop Quantum Cosmology (LQC), which predicts that the Universe undergoes a collapse to an expansion through a bounce. In this work we use metric f(R) gravity to reproduce the modified Friedmann equations, which have been obtained in the context of modified loop quantum cosmologies (mLQC). Using a order reduction method, we obtain covariant effective actions that lead to a bounce, for specific models of mLQC, considering a massless scalar field.

From the theory of the multiverse cosmology, it is possible that our universe collides with other universes locally in its history, which may result in local changes of the curvature of the spacetime. In this paper, we propose a method to probe the multiverse using gravitational wave observations for the first time. Our method firstly makes triangles using two detected gravitational wave sources and the Sun, and then measures the curvature of the triangles. We use 11 gravitational wave sources detected by LIGO and Virgo during O1 and O2, and make 55 triangles by combining them to measure their curvature. The curvature is measured by comparing the distance between two gravitational wave sources estimated by the gravitational wave observations with the one obtained with assumption of a simple model of the cosmological evolution.

As a result, we found that, for 43 of 55 triangles, the distances estimated by the model are greater than the ones obtained by the gravitational wave observations. This indicates a negative curvature, which may be due to the simplification of the cosmological evolution. For the rest 12, the distances are not determined because of uncertainty of the parameters of the gravitational wave observations. Further gravitational wave observations and more sophisticated model of the cosmological evolution is essential to test the multiverse cosmology observationally.

We discuss the status of observables and operator ordering ambiguity in the quantum cosmology model with Brown-Kuchař dust as the matter field. In order to study the dynamics of the FLRW universe, Hubble parameter and Ricci scalar are expressed as a function of phase space variables. As these functions exhibit operator ordering ambiguity, we write several Hermitian extensions corresponding to these observables. For the unitarily evolving semiclassical wave packet constructed in, we have computed the expectation value of these observables, which shows that very early in the collapsing branch and very late in the expanding branch, the expectation values of the Hubble parameter and the Ricci scalar matches the classically obtained results irrespective of the operator ordering chosen. The expectation value of the Hubble parameter vanishes, and Ricci scalar attains an extremum at the point of classical singularity for all orderings, showing a robust singularity resolution. The signature of the operator ordering ambiguity is most pronounced at the classical singularity. For Weyl ordering, the expectation value of the Ricci scalar becomes negative for certain parameter values. We have computed the expectation value of other curvature invariants as well, which follows the trend.

In this paper, we generate an anisotropic solution for a static sphere filled with quark matter in the framework of self-interacting Brans-Dicke theory. For this purpose, we add an anisotropic source in the seed distribution and decouple the field equations through deformation in the radial metric function. As a result of this transformation, the field equations are disintegrated into two systems which separately include the effects of isotropic and anisotropic sources. The system related to the additional source is solved via the MIT bag model equation of state. We consider Tolman V spacetime to formulate a solution for the isotropic sector which is extended to the anisotropic domain via decoupling technique. The junction conditions at the boundary determine the unknown parameters in terms of mass and radius of the spherical object. We investigate the viability and stability of the constructed strange star model in the presence of massive scalar field corresponding to the strange star candidate PSR J1614-2230. It is concluded that the anisotropic extension is well-behaved as it fulfills the necessary requirements of a physically acceptable model.

We show that the Nieh-Yan topological invariant breaks projective symmetry and loses its topological character in presence of non vanishing nonmetricity. The notion of the Nieh-Yan topological invariant is then extended to the generic metric-affine case, defining a generalized Nieh-Yan term, which allows to recover topologicity and projective invariance, independently. As a concrete example a class of modified theories of gravity is considered and its dynamical properties are investigated in a cosmological setting. In particular, bouncing cosmological solutions in Bianchi I models are derived. Finite time singularities affecting these solutions are analysed, showing that the geodesic completeness and the regular behavior of scalar perturbations in these space-times are not spoiled.

A class of modified gravity theories with higher order derivative terms of a function of the matter Lagrangian f (L_m) is considered. We will consider the Newtonian limit of the theory and show that the model predicts the standard Poisson equation for a massive test particle due to the higher order nature of the derivative matter coupling. Generally the energy momentum tensor is not conserved, leading to the fifth force similar to f (R, T ) theories. We will however show that in the FRW background the energy-momentum tensor is conserved. Cosmological implications of this model with different functions of the matter Lagrangian f will be investigated in details and we will show that current observational data can be satisfied. Evolution of the matter density perturbation in the longitudinal gauge is also considered for dust matter sources and we will show that the observational data can be satisfied in this model.

We analyze the Bianchi I cosmology in the presence of a massless scalar field and describe its dynamics via a semiclassical and quantum polymer approach. We investigate the morphology of the emerging Big Bounce by adopting three different sets of configurational variables: the natural Ashtekar connections, the Universe volume plus two anisotropy coordinates and a set of anisotropic volume-like coordinates (the latter two sets of variables would coincide in the case of an isotropic Universe). In the semiclassical analysis we demonstrate that the Big Bounce emerges in the dynamics for all the three sets of variables. Moreover, when the Universe volume itself is considered as a configurational variable, we have derived the polymer-modified Friedmann equation and demonstrated that the Big Bounce has a universal nature, i.e. the total critical energy density has a maximum value fixed by fundamental constants and the Immirzi parameter only. From a pure quantum point of view, we investigate the Bianchi I dynamics only in terms of the Ashtekar connections. In particular, we apply the Arnowitt–Deser–Misner (ADM) reduction of the variational principle and then we quantize the system. We study the resulting Schrödinger dynamics, stressing that the wave packet peak behavior over time singles out common features with the semiclassical trajectories.

A quantum state in a Bianchi II model is studied as it approaches the cosmological singularity, by means of the evolution of its moments. Classically this system presents a transition between two Bianchi I models. This phenomenon is described by a very specific and well-known transition law, which is derived based on the conservation of certain physical quantities. In the quantum theory fluctuations, as well as higher-order quantum moments, of the different variables arise. Consequently, these constants of motion are modified and hence also the transition rule. We focus on the so-called locally rotationally symmetric and vacuum case, as a first step towards a more complete study. Indeed, the future goal of this research line is to generalize this analysis to the Bianchi IX spacetime, which can be seen as a succession of Bianchi II models. Ultimately, these results will shed light on the role played by quantum effects in the BKL conjecture.

Using the quaternion framework (Q-math), we show that the specific mathematical equations born in “quaternion medium” in physical units become known physical laws. In particular, it is shown how one can discover immanently hidden “geometric physical laws”: Cartesian frames, equations of electrodynamics, Q-vector formulation of the relativity theory. One can also find the linked logical chain between laws of quantum, classical, and relativistic mechanics.

We discuss the existence of a static, spherically symmetric spacetime that is the solution of the Einstein field equations coupled with an electric field obeying the equations of electromagnetism of Maxwell-Bopp-Landé-Thomas-Podolsky for a static point charge. Contrary to what happens with the Reissner-Weyl-Nordström spacetime, the electric field energy is finite, just as for this same theory on a background flat spacetime.

I give an overview about recent results on the well-posedness and breakdown of solutions for relativistic fluid equations.

We use Weinberg’s trick for adiabatic modes, in a Manton approximation for general relativity on manifolds with spatial boundary. This results in a description of the slow-time dependent solutions as null geodesics on the space of boundary diffeomorphisms, with respect to a metric we prove to be composed solely of the boundary data. We show how the solutions in the bulk space is determined with the constraints of general relativity.

To give our description a larger perspective, we furthermore identify our resulting Lagrangian as a generalized version of the covariantized Lagrangian for continuum mechanics. We study the cases of 3+1 and 2+1 dimensions and show for the solutions we propose, the Hamiltonian constraint becomes the real homogeneous Monge-Ampere equation in the special case of two spatial dimensions.

In this short paper, we review the Dirac equation on the zero-gravity Kerr-Newman spacetime. Our main objective is to provide a correspondence between the classification of the bound states for the zGKN spectrum and the usual hydrogenic states $1s_{1/2},2s_{1/2}$, etc. of the Hydrogen atom.

In this short review, we explain how and in which sense the causal action principle for causal fermion systems gives rise to classical gravity and the Einstein equations. Moreover, methods are presented for going beyond classical gravity, with applications to a positive mass theorem for static causal fermion systems, a connection between area change and matter flux and the construction of a quantum state.

Using Newman-Penrose formalism in tetrad and spinor forms, we perform separation of variables in the wave equations for massless fields of various spins s=1/2, 1, 3/2, 2 on the background of exact plane-fronted gravitational wave metrics. Then, applying Wald’s method of conjugate operators, we derive equations for Debye potentials and we find the back-projection operators expressing multicomponent fields in terms of these potentials. For shock wave backgrounds, as a special case of the non-vacuum pp-waves, the exact solutions for Debye potentials are constructed explicitly. The possibilities of generalization to the case of massive fields are discussed, in particular, construction of exact solutions of the Dirac and Proca equations. These results can be used in various supergravity problems on the pp-wave backgrounds, including holographic applications.

We show that spinors propagating in curved gravitational background acquire an interaction with spacetime curvature, which leads to a quantum mechanical geometric effect. This is similar to what happens in the case of magnetic fields, known as Pancharatnam-Berry phase. As the magnetic and gravitational fields have certain similar properties, e.g. both contribute to curvature, this result is not difficult to understand. Interestingly, while spacetime around a rotating black hole offers Aharonov-Bohm and Pancharatnam-Berry both kinds of geometric effect, a static spacetime offers only the latter. In the bath of primordial black holes, such gravity induced effects could easily be measured due to their smaller radius.

Free massless fields of any spin in flat D-dimensional spacetime propagate at the speed of light. But the retarded fields produced by the corresponding point-like moving sources share this property only for even D. Since the Green’s functions of the d’Alembert equation are localized on the light cone in even-dimensional spacetime, but not in odd dimensions, extraction of the emitted part of the retarded field in odd D requires some care. We consider the wave equations for spins 0, 1, and 2 in five-dimensional spacetime and analyze the fall-off conditions for the retarded fields at large distances. It is shown that the farthest part of the field contains a component propagating at the speed of light, while the non-derivative terms propagate with all velocities up to that of light. The generated radiation will contain a radiation tail corresponding to the complete prehistory of the source’s motion preceding the retarded moment of time. We also demonstrate that dividing the Green’s function into a part localized on the light cone and another part that is not zero inside the light cone gives separately the divergent terms in the Coulomb field of a point source. Their sum, however, is finite and corresponds to the usual power-law behaviour.

In this work, we use the fact that kinematics of light propagation in a non-dispersive medium associated with a bi-metric spacetime is expressed by means of a 1-parameter family of contact transformations. We present a general technique to find such transformations and explore some explicit examples for Minkowski and anti-deSitter spacetimes geometries.

Élie Cartan’s invariant integral formalism is extended to gauge field theory, including general relativity. This constitutes an alternative procedure that is equivalent to the Rosenfeld, Bergmann, Dirac algorithm. Also a new derivation of the generator of diffeomorphism-induced canonical transformations is given that proceed’s from the Poincaré-Cartan form. In addition, a Hamilton-Jacobi formalism is developed for constructing explicit phase space functions in general relativity that are invariant under the full four-dimensional diffeomorphism group. These identify equivalence classes of classical solutions of Einstein’s equations. Each member is dependent on intrinsic spatial coordinates and also undergoes non-trivial evolution in intrinsic time. The intrinsic coordinates are determined by the spacetime geometry in terms of Weyl scalars. The implications of this analysis for an eventual quantum theory of gravity are profound.

Whether the 3—space where we live is a globally orientable manifold M₃, and whether the local laws of physics require that M₃ be equipped with a canonical orientation, are among the important unsettled questions in cosmology and quantum field theory. It is often assumed that a test for spatial orientability requires a global journey across the whole 3—space to check for orientation-reversing closed paths. Since such a global expedition is not feasible, physically motivated theoretical arguments are usually offered to support the choice of canonical time orientation for the 4—dimensional spacetime manifold, and space orientation for 3—space. One can certainly take advantage of such theoretical arguments to support these assumptions on orientability, but the ultimate answer should rely on cosmological observations or local experiments, or can come from a topological fundamental theory of physics. In a recent paper we have argued that it is potentially possible to locally access the the 3—space orientability of Minkowski empty spacetime through physical effects involving point-like ‘charged’ objects under vacuum quantum electromagnetic fluctuations. More specifically, we have studied the stochastic motions of a charged particle and an electric dipole subjected to these fluctuations in Minkowski spacetime, with either an orientable or a non-orientable 3—space topology, and derived analytical expressions for a statistical orientability indicator in these two flat topologically inequivalent manifolds. For the charged particle, we have shown that it is possible to distinguish the two topologies by contrasting the evolution of their respective indicators. For the point electric dipole we have found that a characteristic inversion pattern exhibited by the curves of the orientability indicator is a signature of non-orientability, making it possible to locally probe the orientability of Minkowski 3—space in itself. Here to shed some additional light on the spatial orientability, we briefly review these results, and also discuss some of its features and consequences. The reviewed results might be seen as opening the way to a conceivable experiment involving quantum vacuum electromagnetic fluctuations to look into the spatial orientability of Minkowski empty spacetime.

Physical reasoning gives expressions for the hamiltonian of a system of quantummechanical particles. These hamiltonians are often differential operators that are symmetric in a densely-defined domain. However, to study the dynamics of the unitary group corresponding to a hamiltonian, it is required that the hamiltonian be self-adjoint or essentially self-adjoint. This study analyzes the effect of the static non-linear electromagnetic-vacuum spacetime of a point nucleus on the self-adjointness and the spectrum of the general–relativistic Dirac hamiltonian for a test electron.

We present the hypothesis that some of ring galaxies were formed by relic magnetic torus - shaped wormholes. In the primordial plasma before the recombination magnetic fields of wormholes trap baryons whose energy is smaller than a threshold energy. They work as the Maxwell’s demons collecting baryons from the nearest (horizon size) region and thus forming clumps of baryonic matter which have the same torus-like shapes as wormhole throats. Such clumps may serve as seeds for the formation of ring galaxies and smaller objects having the ring form. Upon the recombination torus-like clumps may decay and merge. Unlike galaxies, such objects may contain less or even no dark matter in halos. However, the most stringent feature of such objects is the presence of a large - scale toroidal magnetic field. We show that there are threshold values of magnetic fields which give the upper and lower boundary values for the baryon clumps in such protogalaxies.

In this work, we explore wormhole geometries in a recently proposed modified gravity theory arising from a non-conservative gravitational theory, tentatively denoted action-dependent Lagrangian theories. The generalized gravitational field equation essentially depends on a background four-vector λ^μ, that plays the role of a coupling parameter associated with the dependence of the gravitational Lagrangian upon the action, and may generically depend on the spacetime coordinates. Considering wormhole configurations, by using “Buchdahl coordinates”, we find that the four-vector is given by λ_μ = (0, 0, λ_θ, 0), and that the spacetime geometry is severely restricted by the condition gttguu = −1, where u is the radial coordinate. We find a plethora of specific asymptotically flat, symmetric and asymmetric, solutions with power law choices for the function λ, by generalizing the Ellis-Bronnikov solutions and the recently proposed black bounce geometries, amongst others. We show that these compact objects possess a far richer geometrical structure than their general relativistic counterparts.

We investigate binary lenses with 1/rⁿ potentials in the asymmetric case with two lenses with different indexes n and m. These kinds of potentials have been widely used in several contexts, ranging from galaxies with halos described by different power laws to lensing by wormholes or exotic matter.

Solitons in space–time capable of transporting time-like observers at superluminal speeds have long been tied to violations of the weak, strong, and dominant energy conditions of general relativity. This trend was recently broken by a new approach that identified soliton solutions capable of superluminal travel while being sourced by purely positive energy densities. This is the first example of hyper-fast solitons satisfying the weak energy condition, reopening the discussion of superluminal mechanisms rooted in conventional physics. This article summarizes the recent finding and its context in the literature. Remaining challenges to autonomous superluminal travel, such as the dominant energy condition, horizons, and the identification of a creation mechanism are also discussed.

Key results from the literature pertaining to a class of nonsingular black hole mimickers are explored. The family of candidate spacetimes is for now labelled the ‘black-bounce’ family, stemming from the original so-called ‘Simpson–Visser’ spacetime in static spherical symmetry. All model geometries are analysed through the lens of standard general relativity, are globally free from curvature singularities, pass all weak-field observational tests, and smoothly interpolate between regular black holes and traversable wormholes. The discourse is segregated along geometrical lines, with candidate spacetimes each belonging to one of: static spherical symmetry, spherical symmetry with dynamics, and stationary axisymmetry.

Both traversable wormholes and warp drives, concepts originally developed within the context of science fiction, have now (for some 30 odd years) been studied, debated, and carefully analyzed within the framework of general relativity. An overarching theme of the general relativistic analysis is unavoidable violations of the classical point-wise energy conditions. Another science fiction trope, now over 80 years old, is the tractor beam and/or pressor beam. We shall discuss how to formulate both tractor beams and/or pressor beams, and a variant to be called a stressor beam, within the context of reverse engineering the spacetime metric. (While such reverse engineering is certainly well beyond our civilization’s current capabilities, we shall be more interested in asking what an arbitrarily advanced civilization might be able to accomplish.) We shall see that tractor beams and/or pressor beams can be formulated by suitably modifying the notion of warp drives, and that, as for wormholes and warp drives, violations of the classical point-wise energy conditions are utterly unavoidable.

The classical singularity theorems of General Relativity rely on energy conditions that are easily violated by quantum fields. Here, we provide motivation for an energy condition obeyed in semiclassical gravity: the smeared null energy condition (SNEC), a proposed bound on the weighted average of the null energy along a finite portion of a null geodesic. Using SNEC as an assumption we proceed to prove a singularity theorem. This theorem extends the Penrose singularity theorem to semiclassical gravity and has interesting applications to evaporating black holes.

Within the framework of F(R) theories of gravity with (2+1)-dimensions and constant scalar curvature R, we construct a family of thin-shell wormholes with circular symmetry and we analyze the stability of the static configurations under radial perturbations. We show an example of asymptotically anti-de Sitter thin-shell wormholes with charge, finding that stable configurations with normal matter are possible for a suitable range of the parameters.

Alcubierre proposed in 1994 that the well known special relativistic limitation that particles cannot travel with velocities bigger than the light speed can be bypassed when such trips are considered globally within specific general relativistic frameworks. Although initial results indicated this scenario as being unphysical, since it would seem to require negative mass-energy density, recent theoretical analyses suggest that such an unphysical situation may not always be necessarily true. In this paper, we present solutions of the Einstein equations using the original Alcubierre warp drive metric endowed with various matter-energy sources, namely dust, perfect fluid, anisotropic fluid, and perfect fluid within a cosmological constant spacetime. A connection of some of these solutions featuring shock waves described by the Burgers equation is also shown.

In this work the geometrical methods and symmetry principles in gravitation are explored motivating a new perspective into the spacetime paradigm. The effects of post-Riemann spacetime geometries with torsion are briefly studied in applications to fundamental fermionic and bosonic fields, cosmology and gravitational waves. The physical implications and related phenomenological considerations are addressed, and the fundamental ideas related to spacetime physics, motivated by geometrical methods and symmetry principles, are also briefly discussed in the context of the possible routes towards a new spacetime paradigm in gravitation and unified field theories

We propose a new approach to the thermodynamics of scalar-tensor gravity and its possible “diffusion” toward general relativity, previously regarded as an equilibrium state in spacetime thermodynamics. The main idea is describing scalar-tensor gravity as an effective dissipative fluid and applying Eckart’s first order thermodynamics to it. This gives explicit effective quantities: heat current density, “temperature of gravity”, viscosity coefficients, entropy density, plus an equation describing the “diffusion” to Einstein gravity. These quantities, otherwise missing in spacetime thermodynamics, pop out with minimal assumptions.

It is shown in this study that deviations from the Einstein-Hilbert action at the quadratic level using a proper analyses and suitable dynamical variables lead to a tiny modification to the post Newtonian equations of motion, and non-GR like behavior at very short length scales.

We present a comparative analysis of current observational constraints on three recently discussed alternative models for explaining the low-redshift acceleration of the universe: the so-called steady-state torsion model, the generalized coupling model, and the scale invariant model by Maeder (an example of a broader class which we also briefly study). These are compared to the traditional parameterization of Chevallier, Polarski and Linder. Each of the candidate models is studied under two different assumptions: as genuine alternatives to ΛCDM (where a new degree of freedom would be expected to explain the recent acceleration of the universe without any cosmological constant) and as parametric extensions of ΛCDM (where both a cosmological constant and the new mechanism can coexist, and the relative contributions of both are determined by the data). Our comparative analysis suggests that, from a phenomenological point of view, all such models neatly divide into two classes, with different observational consequences.

Einstein’s general relativity predicts that a gravitational wave is allowed to have two polarizations called tensor-modes: plus and cross modes. On the other hand, the general metric theory of gravity predicts that a gravitational wave is allowed to have up to six polarizations: two scalar and two vector modes in addition to the tensor-modes. In case the number of laser-interferometric gravitational wave telescopes is larger than the one of the polarizations the gravitational waves have, all the polarizations can be reconstructed separately. Since it depends on theories of gravity which polarizations the gravitational waves have, the investigation of polarizations is important for the test of theories of gravity. In this paper, in order to test the scalar-tensor gravity theory, one of important alternative theories of gravity, we search for the scalar-mode of GW170817 observed by LIGO Livingstone, Hanford and Virgo without prior information about any tensor-scalar gravity theories. As a result, we found the maximum SNR of the scalar-mode of GW170817 was 2.77, the p-value was 0.01, and the band-limited root sum square of h was $1.55\times {10}^{-21}[1/\sqrt{Hz}]$ with the time window of 2[s] and frequency window of 60∼120[Hz].

We apply cosmological reconstruction methods to the f(R, T) modified gravity, in its recently developed scalar-tensor representation. We do this analysis assuming a perfect fluid in a Friedmann-Lemaître-Robsertson-Walker (FLRW) universe. In this contribution we show the equations of motion obtained and we present the solutions found for one of the particular cases we analysed: an exponential evolution of the cosmological scale factor.

We consider first order perturbations on locally rotationally symmetric (LRS) class II spacetimes. In particular, we investigate the interactions between electromagnetic, gravitational, and plasma related perturbations when allowing for a non-zero magnetic field on the background spacetime. In doing so we focus on mechanisms for generating and magnifying the magnetic fields, as these kinds of mechanisms may have some relevance when trying to explain the origin of the observed large scale cosmic magnetic fields. The equations governing the behavior of the perturbations are gathered from the Ricci identities for certain preferred vector fields, the Bianchi identities, Maxwell’s equations, and from relations describing energy-momentum conservation and particle conservation for the cosmological plasma. After employing a simplified cold magnetohydrodynamic (MHD) description, and harmonically decomposing the spatial dependencies, we arrive at a closed set of ordinary differential equations in time. On analyzing this system, which decouples into an even and an odd subsector, we indeed observe possible mechanisms for generating magnetic field perturbations to first order.

Quintessence fields, or scalar fields minimally coupled to gravity, are considered to be viable candidates for dark energy. It is well-known that some classes of modified theories of gravity can be recast as Einstein’s general relativity with a minimally coupled scalar field, through conformal transformation. The ‘universe’ described by the initial and the final actions are referred to as the Jordan and Einstein frames, respectively. Although these conformally connected frames are mathematically equivalent, the equations of motion in these two frames may describe drastically different physical scenarios. Depending upon the choice of the Jordan frame action (or equivalently the scalar field potential in Einstein frame) it is possible that while the Einstein frame expands, the Jordan frame collapses. We classify quintessence models in the Einstein frame that are dual to f(R) gravity theories in the Jordan frame, based on whether they possess such expansion-collapse duality. We derive a general condition for expansion-collapse duality, applicable to quintessence models with arbitrary time-dependent equations of state. The condition also takes into account the presence of other components in the Einstein frame universe. Such expansion-collapse duality between these conformally connected frames can lead to an effective description of a collapsing universe in terms of an expanding one, which is a topic of further exploration.

The Dark Energy became now a commonly-accepted paradigm of cosmology, but its physical essence remains absolutely unknown, and its numerical values are drastically different in the early and modern Universe. The Dark Energy is usually introduced in the contemporary literature either by postulating some additional terms in the Lagrangians or by employing the empirical equations of state. In the present work, we try to look at this problem from a more general point of view, namely, employing the quantum-mechanical uncertainty relation between the time and energy in the Mandelstam–Tamm form, which is appropriate for the long-term evolution of quantum systems. This leads us to the time-dependent effective Lambda-term, decaying as 1/t. The corresponding cosmological model possesses a number of quite appealing features: (1) While in the standard cosmology there are a few very different expansion stages (governed by the Lambda-term, radiation, dust-like matter, and Lambda-term again), our model provides a universal description of the entire evolution of the Universe by the same “quasi-exponential” function. (2) As follows from the analysis of causal structure, the present-day cosmological horizon comprises a single domain developing from the Big Bang. Therefore, the problems of homogeneity and isotropy of the matter, the absence of topological defects, etc. should be naturally resolved. (3) At last, our model naturally explains the observed approximately flat 3D space, i.e., solution with zero curvature is formed “dynamically”, starting from the arbitrary initial conditions.

An exact time-dependent solution of a black hole is found in a conformally invariant gravity model on a warped Randall-Sundrum spacetime, by writing the metric $g_{\mu \nu }={\omega }^{\frac{4}{n-2}}{\tilde{g}}_{\mu \nu }$. Here ${\tilde{g}}_{\mu \nu }$ represents the “un-physical” spacetime and ω the dilaton field, which will be treated on equal footing as any renormalizable scalar field. It is remarkable that the 5D and 4D effective field equations for the metric components and dilaton fields can be written in general dimension n = 4, 5. The location of the horizon(s) are determined by a quintic polynomial. This polynomial is related to the symmetry group of the icosahedron, isomorphic with the Galois group A₅. We applied the antipodal mapping on the axially symmetric black hole spacetime and make some connection with the information and firewall paradoxes. The dilaton field can be used to describe the different notion the in-going and outside observers have of the Hawking radiation by using different conformal gauge freedom. The disagreement about the interior of the black hole is explained by the antipodal map of points on the horizon. The free parameters of the solution can be chosen in such a way that ${\tilde{g}}_{\mu \nu }$ is singular-free and topologically regular, even for ω → 0.

The universe according to the tetron model consists of invisible tiny constituents, elastically bound with bond length about the Planck length and binding energy the Planck energy. A tetron transforms as the fundamental fermion(=octonion) representation 8 of SO(6,1). With respect to the decomposition SO(6, 1) → SO(3, 1) × SO(3) it possesses spin 1/2 and isospin 1/2, i.e. a tetron represents an isospin doublet of Dirac spinors. The 24 known quarks and leptons arise as eigenmode excitations of a tetrahedral fiber structure, which is made up from 4 tetrons (plus 4 antitetrons) and extends into 3 additional ‘internal’ dimensions. While the laws of gravity are due to the elastic properties of the tetron bonds, particle physics interactions take place within the internal fibers. I will concentrate on two of the most intriguing features of the model:

- understanding small neutrino masses from the conservation of isospin, and, more in general, calculating the spectrum of quark and lepton masses. This is obtained from the tetron model’s interpretation of the Higgs mechanism.

- the possibility to determine the full size of the universe from future dark energy measurements. This is obtained from the tetron model’s interpretation of the dark energy phenomenon.

Finally, the dark energy equation of state, i.e. the equation of state of the tetron background will be derived.

Boundary conditions have physical consequences. On Lifshitz spacetimes, the Klein-Gordon equation gives rise to an initial-boundary value problem that admits a plethora of physically-sensible boundary conditions. Considering a free, scalar, massive quantum field theory on a four-dimensional Lifshitz spacetime with critical exponent z = 2, I layout how to construct two-point functions for ground and thermal states, of local Hadamard form, satisfying the canonical commutation relations, and compatible with Robin and mode-dependent boundary conditions. Each one relates to an inequivalent dynamics, but they are all equivalently physically-sensible—only an experiment could single one out. The results I present here are part of a joint work with C. Dappiaggi and D. Sina.

Based on our original work published in Ref., we investigate an autonomous system analysis in terms of new expansion-normalized variables for homogeneous and anisotropic Bianchi-I spacetimes in f(R) gravity in the presence of anisotropic matter. It is demonstrated that with a suitable choice of the evolution parameter, the Einstein’s equations are reduced to an autonomous 5-dimensional system of ordinary differential equations for the new variables. Furthermore, for a large class of functions f(R), which includes several cases commonly considered in the literature, all the fixed points are polynomial roots, and thus they can be determined with good accuracy and classified for stability. In addition, typically for these cases, any fixed point corresponding to isotropic solutions in the presence of anisotropic matter will be unstable. The assumption of a perfect fluid as source and or the vacuum cases imply some dimensional reductions and even more simplifications. In particular, it is found that the vacuum solutions of f(R) = R^δ+1 with δ a constant are governed by an effective bi-dimensional phase space which can be constructed analytically, leading to an exactly soluble dynamics. It is also shown that several results already reported in the literature can be re-obtained in a more direct and easy way by exploring our dynamical formulation.

We imvestigate the cosmological implications for the Sharma-Mittal holographic dark energy such as some cosmological parameters and thermodynamic analysis. Taking into account the apparent horizon with interacting scenario of dark energy and dark matter, the framework of deformed Hořava-Lifshitz gravity is considered. The cosmological parameters include the Hubble parameter, equation of state parameter for the accelerating/decelerating phases, deceleration parameter to explore the expansion rate and squared speed of sound for stability analysis.

In this work, we elaborate on the finite action for wormholes in higher derivative theories as well as for wormholes. Both non-traversable and traversable wormholes in theories with higher curvature invariants posses finite action.

Non-rotating strange quark stars made of isotropic matter in Lorentz-violating theories of gravity are studied. In particular, Hořava gravity and Einstein-æther theory are considered. For quark matter we adopt both linear and non-linear equations-of-state, corresponding to the MIT bag model and color flavor locked phase, respectively. The new, modified structure equations generalize the usual Tolman-Oppenheimer-Volkoff equations valid in Einstein’s General Relativity. A dimensionless parameter ν measures the deviation from the standard TOV equations, which are recovered in the appropriate limit. We compute some properties, such as masses, radii as well as the factor of compactness of the stars, and we show pictorially the impact of the parameter ν on the mass-to-radius relationships for several different equations-of-state. Other physical considerations, such as stability criteria, causality and energy conditions, are also considered, and they are all found to be fulfilled.

The M87* black hole shadow observation by the Event Horizon Telescope (EHT) has enabled us to test the modified gravity theories in the extreme-field regime and estimating the black hole parameters. Having this assertion, we investigate the Kerr-like rotating black holes in 4D Einstein-Gauss-Bonnet (EGB) gravity and deduce their shadows. Considering the inclination angle θ₀ = 17^o, we show that the EGB black hole shadows are smaller and more distorted than for the Kerr black holes. Modelling the M87* black hole as the EGB black hole, we predict the shadow angular size 35.7888μas ≤ θ_d ≤ 39.6192μas. The M87* black hole shadow angular size θ_d = 42±3μas, within the 1σ region, constrains the GB coupling parameter and the black hole spin parameter. Interestingly, the circularity deviation of the EGB black hole shadows is smaller than the bounded deduced for the M87* black hole.

A lattice regularization for the 2d projectable Horăva-Lifshitz (HL) quantum gravity is known to be the 2d causal dynamical triangulations (CDT), and the 2d CDT can be generalized so as to include all possible genus contributions non-perturbatively. We show that in the context of HL gravity, effects coming from such a non-perturbative sum over topologies can be successfully taken into account, if we quantize the 2d projectable HL gravity with a simple bi-local wormhole interaction. This conference paper is based on the article, Phys. Lett. B 816 (2021), 136205.

The Vector-Tensor (VT) theories of gravity are a class of alternative theories to General Relativity (GR) that are characterized by the presence of a dynamical vector field besides the metric. They are studied in attempts to understand spontaneous Lorentz violation, to generate massive gravitons, and as models of dark matter and dark energy. In this article, I outline how the nature of singularities and horizons in VT theories differ greatly from GR even under the same ordinary conditions. This is illustrated with Einstein-aether theory where vacuum black hole solutions have naked singularities and vacuum cosmological solutions have new singularities that are otherwise absent in GR. It would be interesting to explore these deviations using gravitational waves.

We investigate Hořava-Lifshitz and Einstein-Æther gravity in light of the Event Horizon Telescope (EHT) observations of the M87*. We calculate the corresponding photon effective potential, the unstable photon sphere radius, and finally the induced angular size, which combined with the mass and the distance can lead to a single prediction that quantifies the black hole shadow, namely the diameter per unit mass d. Since d_M87* is observationally known from the EHT Probe, we extract the corresponding parameter regions in order to obtain consistency. We find that Einstein-Æther black hole solutions agree with the shadow size of EHT M87*, if the involved Æther parameters are restricted within specific ranges, along with an upper bound on the dimensionless spin parameter a, which is verified by a full scan of the parameter space within 1σ-error.

This paper is a summary of a talk given in the proceedings of Sixteenth Marcel Grossmann Meeting (MG16). We study a (3+1)-dimensional Hořava-Lifshitz (HL) gravity which coupling with an anisotropic electromagnetic (EM) field which is obtained from a (4+1)-dimensional HL gravity. The model has a noticeable feature that gravitational and electromagnetic waves have the same velocity in the Minkowski background and the Friedman-Robertson-Walker (FRW) background. Based on this characteristic we put forward a new way to restrict the parameter of the HL gravity by considering the possible Lorentz violation effect of the GRB 170817A. It turns out that in this way we can place a stringent constraint on the parameter of (4+1)-dimensional HL gravity.

In this brief report, as the session convener of Hořava-Lifshitz (HL) Gravity (AT6), I summarize the main results of the 15 talks presented in the 16th Marcel Grossmann Meeting, July 5–10, 2021. This session mainly focused on classical and quantum aspects of HL gravity and some related theories, such as Einstein-aether theory and khronometric gravity, as well as their applications to cosmology and astrophysics.

The emergence of R² (Starobinsky) inflation from the semi-classical modification of gravity due to matter quantum fields (trace anomaly) clearly points out the importance of fundamental physics and the first principles in the construction of successful cosmological models. Along with the observational success, R² gravity is also an important step beyond general relativity (GR) towards quantum gravity. Furthermore, several approaches of quantum gravity to date are strongly indicating the presence of non-locality at small time and length scales. In this regard, ultraviolet (UV) completion of R² inflation has been recently studied in a string theory-inspired ghost-free analytic non-local gravity. We discuss the promising theoretical predictions of non-local R²-like inflation with respect to the key observables such as tensor-to-scalar ratio, tensor tilt which tell us about the spectrum of primordial gravitational waves, and scalar Non-Gaussianities which tell us about the three-point correlations in the CMB fluctuations. Any signature of non-local physics in the early Universe will significantly improve our understanding of fundamental physics at UV energy scales and quantum gravity.

We investigate static spherically symmetric solutions in the Palatini kinetically coupled scalar-tensor theory, which reduces to gravity minimally coupled to a scalar field in Einstein frame. Using the fact that the Jordan and Einstein frame are related by a reversible disformal transformation, which can be solved in closed form, we derive the general solution in the Jordan frame and show that it does not contain black holes. There is a wormhole branch and a naked singularity branch between which lies a non-singular asymptotically flat solution with kernel M_1,1 × S². This theory strongly violates the null energy condition.

The problem of calculating the redshift of electromagnetic spectrum of the star, moving in the vicinity of Schwarzschild black hole is solved within the framework of the General Theory of Relativity. The inverse problem — determination the parameters of the motion of a star from observational data of redshift is considered. The approach that gives possibilities to solve the inverse problem is proposed. The approach is tested on the numerical model that gives possibilities to calculate redshift as function of time of observation for a star moving in the vicinity of Schwarzschild black hole. The parameters of the star in numerical model are close to parameters of the S-stars, moving in the vicinity of the Sgr A*.

In the wake of the Event Horizon Telescope (EHT) observations of the supermassive black hole M87*, efforts are underway to distinguish the black holes in general relativity (GR) and modified theories of gravity (MoG). We study the rotating hairy Kerr black holes with a deviation α and primary hair l₀, apart from rotation parameter a and mass M. Interestingly, the hairy Kerr black holes possess smaller sizes but more distorted shadows than the Kerr black holes. We find that, within 1s uncertainty of the EHT observations, the inferred circularity deviation ΔC ≤ 0.1 for the M87* black hole is satisfied, whereas the shadow angular diameter θ_d = 42 ± 3μas, for a given choice of α, places bounds on the parameters a and l₀. Thusfore, the hairy Kerr black holes are inferred to be suitable candidates for astrophysical black holes.

This article reviews one of the most intriguing properties of black hole spacetimes known in the literature- gravitational memory effect, and its connection with asymptotic symmetries, also termed as Bondi-van der Burg-Metzner-Sachs (BMS) symmetries, emerging near the horizon of black holes. Gravitational memory is a non-oscillatory part of the gravitational wave amplitude which generates a permanent displacement for freely falling test particles or test detectors. We highlight a model scenario where asymptotic symmetries appear as a soldering freedom in the context of stitching of two black hole spacetimes, and examine the impact of the interaction between test detectors and horizon shells. Further, we provide a more realistic approach of computing displacement memory for near-horizon asymptotic symmetries which is analogous to the conventional memory originally obtained at asymptotic null infinity.

We derive and critically examine the consequences that follow from the formation of a regular black or white hole horizon in finite time of a distant observer. In spherical symmetry, only two distinct classes of solutions to the semiclassical Einstein equations are self-consistent. Both are required to describe the formation of physical black holes and violate the null energy condition in the vicinity of the outer apparent horizon. The near-horizon geometry differs considerably from that of classical solutions. If semiclassical physics is valid, accretion into a black hole is no longer possible after the horizon has formed. In addition, the two principal generalizations of surface gravity to dynamical spacetimes are irreconcilable, and neither can describe the emission of nearly-thermal radiation. Comparison of the required energy and timescales with established semiclassical results suggests that if the observed astrophysical black holes indeed have horizons, their formation is associated with new physics.

In classical gravity, nothing can escape from a black hole, not even light. In particular, this happens for stationary black holes because their horizons are null. We show, on the other hand, that the apparent horizon and the region near r = 0 of an evaporating charged, rotating black hole are both timelike. This implies that there exists a channel, via which classical or quantum information can escape to the outside, as the black hole evaporates. Since astrophysical black holes have at least some rotation, our results apply to all black holes in nature. We discuss implications of our result.

Since its inception, the Bekenstein-Hawking area relation for black-hole entropy has been the primary testing ground for various theories of quantum gravity. However, a key challenge to such theories is identifying the microscopic structures and explaining the exponential growth of microstates, providing a fundamental understanding of thermodynamic quantities. Since entropy is a single number, we explore other quantities to provide complete information about the black-hole microstates. We establish a one-to-one correspondence between entanglement energy, entropy, and temperature (quantum entanglement mechanics) and the Komar energy, Bekenstein-Hawking entropy, and Hawking temperature of the horizon (black-hole thermodynamics), respectively. We also show that this correspondence leads to the Komar relation and Smarr formula for generic 4-D spherically symmetric space-times. While offering an independent derivation of black-hole thermodynamics from field observables, the universality of results suggests that quantum entanglement is a fundamental building block of space-time.

This paper investigates thermodynamics as well as thermal stability of the Reissner-Nordström black hole with the effects of non-linear electrodynamics. We first calculate the expressions for Hawking temperature, Helmholtz free energy, internal energy, enthalpy and Gibbs free energy of this black hole and then study their graphical behavior in the presence of non-linear electrodynamic effects. We also investigate thermal stability of the considered system with two different methods, i.e., through heat capacity and through Hessian matrix. It is found from both the methods that the considered system is thermodynamically stable in the presence of non-linear electrodynamic parameter (α). Finally, we analyze the phase transitions of Hawking temperature as well as heat capacity in terms of entropy for different values of charge (q), horizon radius (r₊) and coupling parameter. We obtain that Hawking temperature changes its phase from positive to negative for increasing values of q and r₊ while it shows opposite trend for higher values of α. The heat capacity changes its phase from negative to positive for large values of charge, horizon radius and coupling parameter.

We investigate radial linear uniformly accelerated trajectories and their corresponding Rindler horizons in the black hole geometry. In a curved spacetime, a covariant definition for Rindler trajectories is provided in the context of the generalised Letaw-Frenet equations for trajectories with constant curvature scalar and vanishing torsion and hypertorsion. Interestingly, we arrive at a bound on magnitude of acceleration for Rindler trajectories such that, for acceleration greater than the bound value, the Rindler trajectory always falls into the black hole and the distance of closest approach for the trajectory to turn away is always greater than the Schwarzschild radius for all finite boundary data. We further investigate the past and future Rindler horizons using the analytical solution for the trajectories and discuss their features.

The symmetric two-point function for a massless, minimally coupled scalar field in the Unruh state is examined for Schwarzschild-de Sitter spacetime in two dimensions. This function grows linearly in terms of a time coordinate that is well-defined on the future black hole and cosmological horizons, when the points are split in the space direction. This type of behavior also occurs in two dimensions for other static black hole spacetimes when the field is in the Unruh state, and at late times it occurs in spacetimes where a black hole forms from the collapse of a null shell. The generalization to the case of the symmetric two-point function in two dimensions for a massive scalar field in Schwarzschild-de Sitter spacetime is discussed.

A method is presented which allows for the numerical computation of the stress-energy tensor for a quantized massless minimally coupled scalar field in the region outside the event horizon of a 4D Schwarzschild black hole that forms from the collapse of a null shell. This method involves taking the difference between the stress-energy tensor for the in state in the collapsing null shell spacetime and that for the Unruh state in Schwarzschild spacetime. The construction of the modes for the in vacuum state and the Unruh state is discussed. Applying the method, the renormalized stress-energy tensor in the 2D case has been computed numerically and shown to be in agreement with the known analytic solution. In 4D, the presence of an effective potential in the mode equation causes scattering effects that make the the construction of the in modes more complicated. The numerical computation of the in modes in this case is given.

Einstein equations projected on black-hole horizons give rise to the equations of motion of a viscous fluid. This suggests a way to understand the microscopic degrees of freedom on the black-hole horizon by focusing on the physics of this fluid. In this talk, we shall approach this problem by building a crude microscopic model for the Horizon-fluid(HF) corresponding to asymptotically flat black-holes in 3+1 dimensions. The symmetry requirement for our model is that it should incorporate the S1 diffeo-symmetry on the black-hole horizon. The second constraint comes from the demand that the correct value of the Coefficient of the Bulk Viscosity of the HF can be deduced from the model. Both these requirements can be satisfied by an adoption of the eight vertex Baxter model on a S2 surface. We show that the adiabatic entropy quantisation proposed by Bekenstein also follows from this model. Finally, we argue the results obtained so far suggest that a perturbed black-hole can be described by a CFT perturbed by relevant operators and discuss the physical implications.

Mass formulas are obtained for stationary axisymmetric solutions of the Einstein-Maxwell dilaton-axion theory, which have a regular rod structure on the axis of symmetry. Asymptotic mass, angular momentum and charge are expressed as the sums of masses, angular momenta and charges of rods dressed with field contributions. The calculation is based on a three-dimensional sigma model representation of the stationary EMDA system and the Tomimatsu approach proposed for the Einstein-Maxwell system. Our results provide an alternative interpretation of mass formulas and thermodynamics for black holes with Dirac and Misner strings. It is also applicable to aligned multiple black holes with struts.

We extend the black hole holography to the case of an asymptotically anti–de Sitter(AdS) rotating charged black holes in f(T) = T + αT ² gravity, where α is a constant. We find that the scalar wave radial equation at the near-horizon region implies the existence of the 2D conformal symmetries. We show that choosing proper central charges for the dual CFT, we produce exactly the macroscopic Bekenstein-Hawking entropy from the microscopic Cardy entropy for the dual CFT. These observations suggest that the rotating charged AdS black hole in f(T) gravity is dual to a 2D CFT at finite temperatures.

The evidence is mounting that the universe is currently undergoing a phase of accelerated expansion. One possible alternative is the modification in gravity in the largest possible scales. This leads to the many questions related to black-holes: violation of Birkhoff theorem and no-hair theorem. To confirm/infirm, we need to obtain exact black-hole solutions in these modified gravity theories. In this talk, we focus on the exact spherically symmetric solutions in f (R) theories of gravity. We explicitly show that some f (R) models contain an infinite number of exact static, Ricci-flat spherically symmetric vacuum solutions and, hence, violate Birkhoff’s theorem in f (R) theories. We analytically derive two exact vacuum black-hole solutions for the same class of f (R) theories. The two black-hole solutions have the event-horizon at the same point; however, their asymptotic features are different. Our results suggest that the no-hair theorem may not hold for generic modified gravity theories. We discuss the implications to distinguish modified gravity theories from general relativity.

We consider gravitational collapse of a spherically symmetric sphere of a fluid with spin and torsion into a black hole. We use the Tolman metric and the Einstein–Cartan field equations with a relativistic spin fluid as a source. We show that gravitational repulsion of torsion prevents a singularity and replaces it with a nonsingular bounce. Quantum particle production during contraction strengthens torsion in opposing shear. Particle production during expansion can produce enormous amounts of matter and generate a finite period of inflation. The resulting closed universe on the other side of the event horizon may have several bounces. Such a universe is oscillatory, with each cycle larger in size then the previous cycle, until it reaches the cosmological size and expands indefinitely. Our universe might have therefore originated from a black hole existing in another universe.

From time to time, different observations suggest that Einstein’s theory of general relativity (GR) may not be the ultimate theory of gravity. Various researchers have suggested that the f (R) theory of gravity is the best alternative to replace GR. Using f (R) gravity, one can elucidate the various unexplained physics of compact objects, such as black holes, neutron stars, and white dwarfs. Researchers have already put effort into finding the vacuum solution around a black hole in f (R) gravity. However, for a long time, they could not find an asymptotically flat vacuum solution. In this article, we show that the asymptotically flat vacuum solution of f (R) gravity is possible and thereby use it to explain the spherical accretion flow around the black hole.

A certain type of matter with anisotropic pressures can add to the Reissner-Nordström metric a term proportional to a power of the radial coordinate. Using the standard method of separating variables for the Hamilton-Jacobi equation, we study the shadow of the corresponding rotating solution, obtained through the Newman-Janis algorithm. We define and calculate three observables in order to characterize the position, size and shape of the shadow.

We review the constraints modified theories of gravity must satisfy to be compatible with the spherically symmetric black hole solutions of semiclassical gravity that describe the formation of an apparent horizon in finite time of a distant observer. The constraints are satisfied in generic modified gravity theories with up to fourth-order derivatives in the metric, indicating that the semiclassical solutions correspond to zeroth-order terms in perturbative solutions of these theories. As a result, it may not be possible to distinguish between the semiclassical theory and modifications including up to fourth-order derivatives based on the observation of an apparent horizon alone.

What we are doing is three-fold. First, we examine the gist of the Penrose suggestion as to signals from a prior universe showing up in the CMBR. That is, this shows up as data in the CMBR. Second, we give a suggestion as to how super massive black holes could be broken up s of a prior universe cycle by pre-big-bang conditions, with say millions of pre-Planck black holes coming up out of a breakup of prior universe black holes. Three, we utilize a discussion as to Bose–Einstein condensates set as gravitons as to composing the early universe black holes. The BEC formulation gives a number N of gravitons, linked to entropy, per black hole, which could lead to contributions to the alleged CMBR perturbations, which were identified by Penrose et al.

We summarize the talks presented at the BH3 session (Black Holes in Alternative Theories of Gravity) of the 16th Marcel Grossmann Meeting held online on July 5-10 2021.

The detection of the merger of a neutron star binary in both gravitational waves and a broad spectrum of electromagnetic waves (GW170817) provided the most compelling evidence to date that such mergers produce heavy r-process elements. The inferred rate of these mergers coupled to the estimated r-process production suggests that these mergers could produce nearly all of the r-process elements in the universe. However, uncertainties in the merger rate and the amount of r-process production per merger means that scientists can not constrain the fraction of the merger r-process contribution to better than 1–100% of the total amount in the universe. The total r-process mass synthesized is best constrained by the observations themselves and uncertainties in the inferred production quantity follows from the uncertainties in modeling the emission from the NSM ejecta. In this paper, we review these modeling uncertainties.

We review some recent results obtained at the sixth Post-Newtonian level of approximation for the Hamiltonian description of a two-body system, by using several methods whose combination has led to the so-called “Tutti-Frutti” approach.

Multi-channel astronomy is one of the most important and rapidly developing field of modern physics. The well known result of multi-channel observations is simultaneous detection of gravitational wave and gamma ray bursts associated with neutron star merger event. This observation open a new window to study the Universe, and actually triggered the broad scale study of astrophysical phenomena in hard X-Rays and gamma rays by orbital experiments together with ground based observations of gravitational waves, neutrino and ultra-high energy cosmic rays. From this perspective it appears that small satellites of CubeSat type are quite appropriate for multi-channel observations of astrophysical transients because it is the cheapest way to realize all-sky monitoring observations by orbital instruments. Presently at D.V. Skobeltsyn Institute of Nuclear Physics of the M.V. Lomonosov Moscow State University (SINP MSU) a new project named Universat–SOCRAT is under development which is intended for operational monitoring of near-Earth’s radiation environment and monitoring of electromagnetic transients in the optical, UV, X-ray and gamma ranges. Here we discuss the first results of charged particles, gamma quanta fluxes and UV-emission measurements from the upper atmosphere in several CubeSat missions, which were successfully launched in 2019, 2020.

The detailed continuous fast optical photometry analysis obtained by MASTER Global Network for the GRB160625B optical counterpart MASTER OT J203423.51+065508.0 is presented. There are also hard X-ray and gamma-ray emission obtained by the Lomonosov and Konus-Wind spacecrafts detectors. We detected quasiperiodic emission components in the intrinsic optical emission of GRB160625B and propose a three-stage collapse scenario for this long and bright GRB. We associate quasiperiodic fluctuations with forced precession of a Spinar, i.e. self-gravitating rapidly rotating super dense body, whose evolution is determined by a powerful magnetic field. The spinar’s mass lead it to collapse into a black hole at the end of an evolution.

Gamma-ray bursts (GRBs) are bright extragalactic flashes of gamma-ray radiation and briefly the most energetic explosions in the Universe. Their catastrophic origin —the merger of compact objects or the collapse of massive stars— drives the formation of a newborn compact remnant (black hole or magnetar) that powers two highly relativistic jets. As these jets continue to travel outwards, they collide with the external material surrounding the dying star, producing a long-lasting afterglow that can be seen across the entire electromagnetic spectrum, from the most energetic gamma-ray emission to radio wavelengths. But how can such material be accelerated and focused into narrow beams? The internal shock model proposes that repeated collisions between material blasted out during the explosion can produce the gamma-ray flash. The competing magnetic model credits primordial large-scale ordered magnetic fields that collimate and accelerate the relativistic outflows. To distinguish between these models and ultimately determine the power source for these energetic explosions, our team studies the polarization of the light during the first minutes after the explosion (using novel instruments on fully autonomous telescopes around the globe) to directly probe the magnetic field properties in these extragalactic jets. This technology allowed the detection of highly polarized optical light in GRB 120308A and confirmed the presence of mildly magnetized jets with large-scale primordial magnetic fields in a reduced sample of GRBs (e.g. GRB 090102, GRB 110205A, GRB 101112A, GRB 160625B). Here we discuss the observations of the most energetic and first GRB detected at very high TeV energies, GRB 190114C, which opens a new frontier in GRB magnetic field studies suggesting that some jets can be launched highly magnetized and that the collapse and destruction of these magnetic fields at very early times may have powered the explosion itself. Additionally, our most recent polarimetric observations of the jet of GRB 141220A indicate that, when the jetted ejected material is decelerated by the surrounding environment, the magnetic field amplification mechanisms at the front shock —needed to generate the observed synchrotron emission— produce small magnetic domains. These measurements validate theoretical expectations and contrast with previous observations that suggest large magnetic domains in collisionless shocks (i.e. GRB 091208B).

We present MASTER Global Robotic Net (Lipunov et al. 2010) earliest optical alert observations of IceCube-170922A error box. We discovered fast variability of blazar TXS 0506+056 27 sec after notice time (73s after the trigger time) at 2017-09-22 20:55:43 UT by MASTER-Tavrida robotic telescope. MASTER found the blazar TXS 0506+056 to be in the off-state after one minute and then switched to the on-state no later than two hours after the event. The effect is observed at a 50-sigma significance level. We also analysed own unique 16-years light curve of blazar TXS 0506+056 (518 data set).

In this proceedings contribution we summarize a series of results on nonlinear perturbation theory for modified gravity models, in which a scalar field changes the gravitational dynamics at cosmic scales. We discuss its effects on large scale structure, focusing on screenings and the power spectrum of matter fields and tracers. We also discuss a redshift space distortions model for modified gravity and the departures from the LCDM model.

In the last two decades, Modified Gravity (MG) models have been proposed to explain the accelerated expansion of the Universe. However, one of the main difficulties these theories face is that they must reduce to General Relativity (GR) at sufficiently high energy densities, such as those found in the solar system. To achieve this, MG theories typically employ so-called screening mechanisms: nonlinear effects that bring them to GR at the appropriate limits. For this reason, low-energy regions where the screenings do not operate efficiently, such as cosmic voids, are identified as ideal laboratories for testing GR. Hence, the use of marked statistics that up-weight low energy densities have been proposed for being implemented with data from future galaxy surveys. In this proceeding note, we show how to construct theoretical templates for such statistics and test their accuracy with the use of N-body simulations.

We analyze the properties of a generic bosonic cloud that we interpreted as a condensed phase of generic bosons or a Bose–Einstein condensate (BEC) type halo surrounding a Schwarzschild–type black hole. We model the halo as a condensed phase of generic bosons in terms of a massive scalar field that satisfies a self–interacting Klein–Gordon equation. To model the density of particles of the corresponding cloud, we apply the so–called Thomas–Fermi approximation that allows us to extract relevant properties of the system. By using galaxy data from a subsample of SPARC data base, we find the best fits of the BEC model by using the Thomas–Fermi approximation. We show that in the centre of galaxies we must have a supermassive compact central object, i.e., supermassive black hole, additionally the cloud or the halo behaves as a weakly interacting BEC composed of ultralight bosons.

We constructed a new all-sky Compton parameter map (y-map) of the thermal Sunyaev-Zel’dovich (tSZ) effect by applying the MILCA component separation algorithm to the 100 to 857 GHz frequency channel maps from the Planck data release 4. The Planck team performed several improvements for the new channel maps in terms of noises and systematics, and it allowed us to produce a new y-map with reduced noises by ∼7% and minimal survey strips compared to the previous version released in 2015. We computed the tSZ angular power spectrum of the new y-map and performed a cosmological analysis. The results showed $S_8={0.764}_{-0.018}^{+0.015}{(stat)}_{-0.016}^{+0.031}(sys)$, including systematic uncertainties from a hydrostatic mass bias and pressure profile model. The value is fully consistent with recent KiDS and DES weak-lensing observations. It is also consistent with the Planck CMB’s result within 2σ, while our result is slightly lower.

The largest temperature anisotropy in the cosmic microwave background (CMB) is the dipole. The simplest interpretation of the dipole is that it is due to our motion with respect to the rest frame of the CMB. As well as creating the ℓ=1 mode of the CMB sky, this motion affects all astrophysical observations by modulating and aberrating sources across the sky. It can be seen in galaxy clustering, and in principle its time derivative through a dipole-shaped acceleration pattern in quasar positions. Additionally, the dipole modulates the CMB temperature anisotropies with the same frequency dependence as the thermal Sunyaev-Zeldovich (tSZ) effect and so these modulated CMB anisotropies can be extracted from the tSZ maps produced by Planck. Unfortunately this measurement cannot determine if the dipole is due to our motion, but it does provide an independent measure of the dipole and a validation of the y maps. This measurement, and a description of the first-order terms of the CMB dipole, are outlined here.

The MIllimeter Sardinia radio Telescope Receiver based on Array of Lumped elements kids, MISTRAL, is a millimetric (≃ 90GHz) multipixel camera being built for the Sardinia Radio Telescope. It is going to be a facility instrument and will sample the sky with 12 arcsec angular resolution, 4 arcmin field of view, through 408 Kinetic Inductance Detectors (KIDs). The construction and the beginning of commissioning is planned to be in 2022. MISTRAL will allow the scientific community to propose a wide variety of scientific cases including protoplanetary discs study, star forming regions, galaxies radial profiles, and high angular resolution measurements of the Sunyaev Zel’dovich (SZ) effect with the investigation of the morphology of galaxy cluster and the search for the Cosmic Web.

The high-z submillimeter galaxies (SMGs) can be used as background sample for gravitational lensing studies thanks to their magnification bias, which can manifest itself through a non-negligible measurement of the cross-correlation function between a background and a foreground source sample with non-overlapping redshift distributions. In particular, the choice of SMGs as background sample enhances the cross-correlation signal so as to provide an alternative and independent observable for cosmological studies regarding the probing of mass distribution.

In particular the magnification bias can be exploited in order to constrain the free astrophysical parameters of a Halo Occupation Distribution model and some of the main cosmological parameters. Urged by the improvements obtained when adopting a pseudo-tomographic analysis, It has been adopted a tomographic set-up to explore not only a ΛCDM scenario, but also the possible time evolution of the dark energy density in the ω₀CDM and ω₀ω_aCDM frameworks.

In the standard cosmological scenario, no circular polarization is predicted for Cosmic Microwave Background (CMB) radiation. However, in the frame of moving particle, Lorentz symmetry violation leads to circular polarization for CMB radiation. We estimate the circular polarization power spectrum $C_{Vl}^{(S)}$ in CMB radiation due to Compton scattering in the presence of the Lorentz symmetry violation. We show that the V-mode power spectrum can be obtained in terms of linear polarization power spectrum at the last scattering surface.

Cosmological and astrophysical surveys in various wavebands, in particular from the radio to the far-infrared, offer a unique view of the universe’s properties and the formation and evolution of its structures. After a preamble on the so-called tension problem, which occurs when different types of data are used to determine cosmological parameters, we discuss the role of fast radio bursts in cosmology, in particular for the missing baryon Bologna, Italy problem, and the perspectives from the analysis of the 21 cm redshifted line from neutral hydrogen. We then describe the Planck Legacy Archive, its wealth of scientific information and next developments, and the promising perspectives expected from higher resolution observations, in particular for the analysis of the thermal Sunyaev-Zel’dovich effect. Three cosmological results of the Planck mission are presented next: the implications of the map of Comptonization fluctuations, the dipole analysis from cross-correlating cosmic microwave background anisotropy and Comptonization fluctuation maps, and the constraints on the primordial tensor-to-scalar perturbation ratio. Finally, we discuss some future perspectives and alternative scenarios in cosmology, such as the study of the Lorentz invariance violation with the cosmic microwave background polarization, the introduction of new gravitational degrees of freedom to solve the dark matter problem, and the exploitation of the magnification bias with high-redshift sub-millimeter galaxies to constrain cosmological parameters.

We initiate a new method for probing inflationary models that can produce primordial black hole populations, using only CMB physics at relatively large scales. In these scenarios, profile of the primordial scalar power spectrum exhibit a universal dip feature that is followed by a rapid growth towards small scales, leading to a peak responsible for PBH formation. Focusing on scales around the dip that are well separated from the peak, we first analytically compute expressions for the curvature bispectrum. We then show that the amplitude of the bispectrum is enhanced for the squeezed configuration around the position of the dip, and it acquires a characteristic scale dependence that can be probed by cross correlations between CMB μ-distortions and temperature anisotropies. We quantitatively study the properties of such cross-correlations and how they depend on the underlying model, discussing how they can be tested by the next generation of CMB μ-distortion experiments. This method allows one to experimentally probe inflationary PBH scenarios using well-understood CMB physics, without considering non-linearities associated with PBH formation and evolution.

Compton scattering of photons with thermal electrons is one of the most ubiquitous phenomenon in nature. Numerical approaches to solve the evolution of photons in such situations typically assume the energy exchange to be much smaller compared to the temperature of electrons. In this work, we solve the photon evolution in its full generality, without the diffusion approximation, and show the differences between the exact solution and approximate solutions. We point out the importance of solving exact kinematics of Compton scattering for the computation of Cosmic Microwave Background (CMB) spectral distortions at redshifts (z) ≲ 3 × 10⁴.

The BISOU (Balloon Interferometer for Spectral Observations of the Universe) project aims to study the viability and prospects of a balloon-borne spectrometer, pathfinder of a future space mission dedicated to the measurements of the CMB spectral distortions. We present here a preliminary concept based on previous space mission proposals, together with some sensitivity calculation results for the observation goals, showing that a 5-σ measurement of the y-distortions is achievable.

We present new cosmic microwave background (CMB) spectral distortions constraints on low mass (10⁻¹⁰-10⁴ eV) dark matter particles that decay into photons. The constraints are first presented in a model-independent manner and then applied to axions and decaying excited states. For the first time, we place constraints using the full distortion spectra compared to the COBE/FIRAS and EDGES measurements. This proceeding summarizes results obtained with Jens Chluba and Richard Battye, and published in MNRAS 507 (2021).

The COSmic Monopole Observer (COSMO) is an experiment to measure low-level spectral distortions in the isotropic component of the Cosmic Microwave Background (CMB). Deviations from a pure blackbody spectrum are expected at low level (< 1 ppm) due to several astrophysical and cosmological phenomena, and promise to provide important independent information on the early and late phases of the universe. They have not been detected yet, due to the extreme accuracy required, the best upper limits being still those from the COBE-FIRAS mission. COSMO is based on a cryogenic differential Fourier Transform Spectrometer, measuring the spectral brightness difference between the sky and an accurate cryogenic blackbody. The first implementation of COSMO, funded by the Italian PRIN and PNRA programs, will operate from the Concordia station at Dome-C, in Antarctica, and will take advantage of a fast sky-dip technique to get rid of atmospheric emission and its fluctuations, separating them from the monopole component of the sky brightness. Here we describe the instrument design, its capabilities, the current status. We also discuss its subsequent implementation in a balloon-flight, which has been studied within the COSMOS program of the Italian Space Agency.

First proposed in 1964 by Sjur Refsdal, gravitational lensing provides a straightforward and elegant geometrical way of estimating the Hubble constant from cosmologically distant variable sources. The method relies on observationally determined time delays between light arriving through different multiple images, and the mass models of the lens, which are constrained by observed image properties and other information. While the time delays are obtained with increasing precision, the mass models, which are subject to lensing degeneracies, remain the main source of systematic uncertainty. Various modeling groups have adopted different strategies for dealing with degeneracies. In this talk I will describe the basics of extracting H₀ from lensing, the observational successes, modeling challenges, current results, and future prospects.

The standard ΛCDM cosmological model now seems to face some puzzles. One of the most serious problems is the so-called Hubble tension; the values of the Hubble constant H₀ obtained by local measurements look inconsistent with that inferred from Cosmic Microwave Background (CMB). Although introducing extra energy components such as the extra radiation or Early Dark Energy appears to be promising, such extra components could alter the abundance of light elements synthesized by Big Bang Nucleosynthesis (BBN). We perform a Monte Carlo simulation to evaluate the effect of those extra component scenarios to solve the Hubble tension on the BBN prediction.

I review a string-inspired cosmological model with gravitational anomalies in its early epochs, which is based on fields from the (bosonic) massless gravitational multiplet of strings, in particular gravitons and Kalb Ramond (KR), string-model independent, axions (the dilaton is assumed constant). I show how condensation of primordial gravitational waves, which are generared at the very early eras immediately after the big bang, can lead to inflation of the so called running vacuum model (RVM) type, without external inflatons. The role of the slow-roll field is played by the KR axion, but it does not drive inflation. The non-linearities in the anomaly terms do. Chiral fermionic matter excitations appear at the end of this RVM inflation, as a result of the decay of the RVM vacuum, and are held responsible for the cancellation of the primordial gravitational anomalies. Chiral anomalies, however, survive in the post-inflationary epochs, and can lead to the generation of a non perturbative mass for the KR axion, which could thus play the role of dark matter in this Universe. As a result of the condensed gravitational anomaly, there is a Lorentz-invariance violating KR axion background, which remains undiluted during the RVM inflation, and can lead to baryogenesis through leptogenesis in the radiation era, in models with sterile right-handed neutrinos. I also discuss the phenomenology of the model in the modern era, paying particular attention to linking it with a version of RVM, called type II RVM, which arguably can alleviate observed tensions in the current-epoch cosmological data.

Cosmological data still allow for the presence of a non-negligible amount of dark energy at very high redshifts, namely during the matter- and radiation-dominated epochs. This is the so-called early dark energy (EDE), which could help to mitigate the tensions that affect the standard model of cosmology since (i) it reduces the sound horizon at the baryon-drag epoch, hence giving room to higher values of H₀ than those found in the ΛCDM; and (ii) it could potentially decrease the number of large-scale structures in the Universe due its negative pressure and its inability to cluster efficiently for large enough values of its sound speed. Here we put constraints on the fraction of EDE using two methods: first, we use a perfect fluid parameterization that produces plateaux in Ω_ede(z) during the relativistic and non-relativistic matter-dominated eras. Second, we apply a tomographic approach to constrain the EDE density in redshift bins, which allows us to reconstruct the evolution of the EDE fraction before and after the decoupling of the Cosmic Microwave Background (CMB) photons. We have employed Planck data 2018, the Pantheon compilation of supernovae of Type Ia (SNIa), data on galaxy clustering, the prior on the absolute magnitude of SNIa by SH0ES, and weak lensing data from KiDS+VIKING-450 and DES-Y1. Using our minimal parameterization we find that EDE is not able to loosen the cosmological tensions, and show that the constraints on the EDE fraction weaken considerably when its sound speed takes lower values. Thanks to our binned analysis we are able to put tight constraints on the EDE fraction around the CMB decoupling time, ≲ 0.4% at 2σ c.l. We confirm previous results that a significant EDE fraction in the radiation-dominated epoch loosens the H₀ tension, but tends to worsen the tension for σ₈. A subsequent presence of EDE in the matter-dominated era helps to alleviate this problem. When both the SH0ES prior and weak lensing data are considered in the fitting analysis in combination with data from CMB, SNIa and baryon acoustic oscillations, the EDE fractions are constrained to be ≲ 2.6% in the radiation-dominated epoch and ≲ 1.5% in the redshift range z ∈ (100, 1000) at 2σ c.l. The two tensions remain with a statistical significance of ∽ 2 − 3σ c.l. This contribution to the proceedings of the CM3 parallel session of the MG16 Marcel Grossmann virtual Conference: “Status of the H₀ and σ8 tensions: theoretical models and model-independent constraints” is based on the paper arXiv:2107.11065, which appeared in the arXiv shortly after my talk of July 6th 2021.

The cosmological constant (CC) term, Λ, in Einstein’s equations has been for about three decades a fundamental building block of the concordance or standard ΛCDM model of cosmology. Although the latter is not free of fundamental problems, it provides a good phenomenological description of the overall cosmological observations. However, an interesting improvement in such a phenomenological description and also a change in the theoretical status of the Λ-term occurs upon realizing that the vacuum energy is actually a “running quantity” in quantum field theory in curved spacetime. Several works have shown that this option can compete with the ΛCDM with a rigid Λ term. The so-called, “running vacuum models” (RVM) are characterized indeed by a vacuum energy density, ρ_vac, which is evolving with time as a series of even powers of the Hubble parameter and its time derivatives. This form has been motivated by semi-qualitative renormalization group arguments in previous works. Here we review a recent detailed computation by the authors of the renormalized energy-momentum tensor of a non-minimally coupled scalar field with the help of adiabatic regularization procedure. The final result is noteworthy: ρ_vac(H) takes the precise structure of the RVM, namely a constant term plus a dynamical component ∽ H² (which should be detectable in the present universe) including also higher order effects 𝒪(H⁴) which can be of interest during the early stages of the cosmological evolution. Besides, it is most remarkable that such renormalized form of the vacuum energy density does not carry dangerous terms proportional to m⁴, the quartic powers of the masses of the fields, which are a well-known source of exceedingly large contributions to the vacuum energy density and are directly responsible for extreme fine tuning in the context of the cosmological constant problem.

We present an analysis of the Brans-Dicke cosmological model with a cosmological constant and cold dark matter (BD-ΛCDM). We find that the BD-ΛCDM is favored by the overall cosmological data (SNIa+BAO+H(z)+LSS+CMB) when it is compared with the standard model of cosmology. The BD-ΛCDM model can be viewed from the GR perspective as a Running Vacuum Model (RVM) with a time evolving vacuum energy density. Due to this fact and also to its time evolving effective gravitational coupling, the model can alleviate the σ₈ and the H₀ tensions at a time. We also present the results for different types of RVM’s when they are tested in the light of the cosmological data and we show that a mild dynamics for the vacuum energy density can help to smooth out the aforementioned tensions, thus improving the performance of ΛCDM model.

The Cosmic Microwave Background (CMB) temperature and polarization anisotropy measurements have provided strong confirmation of the ΛCDM model of structure formation. Even if this model can explain incredibly well the observations in a vast range of scales and epochs, with the improvement of the experimental sensitivity, a few interesting tensions between the cosmological probes, and anomalies in the CMB data, have emerged with different statistical significance. While some portion of these discrepancies may be due to systematic errors, their persistence across probes strongly hints at cracks in the standard ΛCDM cosmological scenario. The most statistically significant is the Hubble constant puzzle and I will show a couple of interesting extended cosmological scenarios that can alleviate it.

We report how to alleviate both the H₀ and σ₈ tensions simultaneously within f (T) gravity. In particular, we consider the parametrization $f(T)=-T-2\text{Λ}/M_P^2+\alpha T^{\beta }$, where two out of the three parameters are independent. This model can effciently fit observations solving the two tensions. To our knowledge, this is the first time where a modified gravity theory can alleviate both H₀ and σ₈ tensions simultaneously, hence, offering an additional argument in favor of gravitational modification.

We present a fully relativistic framework to evaluate the impact of stochastic inhomogeneities on the prediction of the Hubble-Lemaître diagram. In this regard, we relate the fluctuations of the luminosity distance-redshift relation in the Cosmic Concordance model to the intrinsic uncertainty associated to the estimation of cosmological parameters from high-redshift surveys (up to z = 4). Within this framework and according to the specific of forthcoming surveys as Euclid Deep Survey and LSST, we show that the cosmic variance associated with the measurement of the Hubble constant will not exceed 0.1 %. Thanks to our results, we infer that deep surveys will provide an estimation of the the Hubble constant H₀ which will be more precise than the one obtained from local sources, at least in regard of the intrinsic uncertainty related to a stochastic distribution of inhomogeneities.

Primordial black holes (PBHs) can be produced when large density perturbations enter the horizon in the early universe. They can be dark matter (DM) and the black holes detected by the LIGO-Virgo collaborations. In this proceeding, we show that the large enhancement of the perturbations, required for the DM PBHs and LIGO/Virgo PBHs, can be realized in inflation models with a downward step. This enhancement mechanism is related to the particle production associated with the non-adiabatic evolution of the inflaton. This proceeding is based on our original paper.

The enhancement of the spectrum of primordial comoving curvature perturbation ℛ can induce the production of primordial black holes (PBH) which could account for part of present day dark matter. As an example of the effects of the modification of gravity on the production of PBHs, we investigate the effects on the spectrum of ℛ produced by the modification of gravity in the case of G-inflation, deriving the relation between the unitary gauge curvature perturbation ζ and the comoving curvature perturbation ℛ, and identifying a background dependent enhancement function ɛ which can induce large differences between the two gauge invariant variables.

When ζ is not constant in time it is different from ℛ, for example on sub-horizon scales, or in models exhibiting an anomalous super-horizon growth of ζ, but since this growth cannot last indefinitely, eventually they will coincide. We derive the general condition for super-horizon growth of ζ, showing that slow-roll violation is not necessary. Since the abundance of PBHs depends on the statistics of the peaks of the comoving density contrast, which is related to the spectrum of ℛ, it is important to take into account these effects on the PBHs abundance in modified gravity theories.

In this work we will explore U (1) local cosmic string solutions in the context of the generalized hybrid metric-Palatini theory of gravity in its scalar-tensor representation. Using a general static cillindrically symmetric metric to find the dynamical equations for this particular case, we will simplify the equations by imposing boost invariance along t and z directions. The physical and geometrical properties of the cosmic strings are determined by the two scalar fields, as well by an effective field potential, functionally dependent on both scalar fields. While for some forms of the potential, the dynamical equations can be solved exactly, for more general formas of the potential the solutions are found numerically. In this way, we obtain a large class of stable stringlike astrophysical configurations, whose basic parameters (string tension and radius) depend essentially on the effective field potential, and on the boundary conditions.

There has been observational evidence about spin axes of quasars in large quasar groups correlated over hundreds of Mpc. This is seen in the radio spectrum as well as in the optical range. There is not yet a satisfactory explanation of this “spooky” alignment. This alignment cannot be explained by mutual interaction at the time that quasars manifest themselves optically. A cosmological explanation could be possible in the formation of superconducting vortices (cosmic strings) in the early universe, just after the symmetry-breaking phase of the universe. We gathered from the NASA/IPAC and SIM-BAD extragalactic databases the right ascension, declination, inclination, position angle and eccentricity of the host galaxies of 3 large quasar groups to obtain the azimuthal and polar angle of the spin vectors. The alignment of the azimuthal angle of the spin vectors of quasars in their host galaxy is confirmed in the large quasar group U1.27 and compared with two other groups in the vicinity, i.e., U1.11 and U1.28, investigated by Clowes (2013). It is well possible that the azimuthal angle alignment fits the predicted azimuthal angle dependency in the theoretical model of the formation of general relativistic superconducting vortices, where the initial axially symmetry is broken just after the symmetry breaking of the scalar-gauge field.

Cosmic string networks form during cosmological phase transitions as a consequence of the Kibble mechanism. The evolution of the simplest networks is accurately described by the canonical Velocity Dependent One-Scale (VOS) model. However, numerical simulations have demonstrated the existence of significant quantities of short-wavelength propagation modes on the strings, known as wiggles, which motivated the recent development of a wiggly string extension of the VOS. Here we summarize recent progress in the physical interpretation of this model through a systematic study of the allowed asymptotic scaling solutions of the model. The modeling mainly relies on three mechanisms: the universe’s expansion rate, energy transfer mechanisms (e.g., the production of loops and wiggles), and the choice of the scale in which wiggles are coarse-grained. We consider the various limits in which each mechanism dominates and compare the scaling solutions for each case, in order to gain insight into the role of each mechanism in the overall behavior of the network. Our results show that there are three scaling regimes for the wiggliness, consisting of the well-known Nambu-Goto solution, and non-trivial regimes where the amount of wiggliness can grow as the network evolves or, for specific expansion rates, become a constant. We also demonstrate that full scaling of the network is more likely in the matter era than in the radiation epoch, in agreement with numerical simulations.

The canonical velocity-dependent one-scale (VOS) model for cosmic string evolution contains a number of free parameters which cannot be obtained ab initio. Therefore it must be calibrated using high resolution numerical simulations. We exploit our state of the art graphically accelerated implementation of the evolution of local Abelian-Higgs string networks to provide a statistically robust calibration of this model. In order to do so, we will make use of the largest set of high resolution simulations carried out to date, for a variety of cosmological expansion rates, and explore the impact of key numerical choices on model calibration, including the dynamic range, lattice spacing, and the choice of numerical estimators for the mean string velocity. This sensitivity exploration shows that certain numerical choices will indeed have consequences for observationally crucial parameters, such as the loop chopping parameter. To conclude, we will also briefly illustrate how our results impact observational constraints on cosmic strings.

We present results from adaptive mesh refinement (AMR) simulations of global cosmic strings. Using the public code, GRChombo, we perform a quantitative investigation of the dynamics of single sinusoidally displaced string configurations. We study a wide range of string energy densities μ ∝ ln λ, defined by the string width parameter λ over two orders of magnitude. We investigate the resulting massless (Goldstone boson or axion) and massive (Higgs) radiation signals, using quantitative diagnostic tools to determine the eigenmode decomposition. Given analytic radiation predictions for global Nambu-Goto strings, we compare the oscillating string decay with a backreaction model accounting for radiation energy losses, finding excellent agreement. We establish that backreaction decay is accurately characterised by the inverse square of the amplitude being proportional to the inverse tension μ for 3 ≲ λ ≲ 100. The investigation of massive radiation at small to intermediate amplitudes finds evidence that it is suppressed exponentially relative to the preferred massless channel with a $\sqrt{\lambda }$ dependence in the exponent. We conclude that analytic radiation modelling in the thin-string (Nambu-Goto) limit provides the appropriate cosmological limit for global strings.

In the QCD axion dark matter scenario with post-inflationary Peccei-Quinn symmetry breaking, the number density of axions, and hence the dark matter density, depends on the length of string per unit volume at cosmic time t, by convention written ζ/t². The expectation has been that the dimensionless parameter ζ tends to a constant ζ₀, a feature of a string network known as scaling. It has recently been claimed that in larger numerical simulations ζ shows a logarithmic increase with time. This case would result in a large enhancement of the string density at the QCD transition, and a substantial revision to the axion mass required for the axion to constitute all of the dark matter. With a set of new simulations of global strings we compare the standard scaling (constant-ζ) model to the logarithmic growth. We also study the approach to scaling, through measuring the root-mean-square velocity v as well as the scaled mean string separation x. We find good evidence for a fixed point in the phase-space analysis in the variables (x, v), providing a strong indication that standard scaling is taking place. We show that the approach to scaling can be well described by a two parameter velocity-one-scale (VOS) model, and show that the values of the parameters are insensitive to the initial state of the network. We conclude that the apparent corrections to ζ are artifacts of the initial conditions, rather than a property of the scaling network.

We study axion strings with the electroweak gauge flux in the DFSZ axion model and show that these strings, called electroweak axions, exhibit superconductivity without fermionic zero modes. We also show that the primordial magnetic field in the early universe can induce a large electric current along the string. A pair of the strings carrying such a large current feels a net attractive force between them and can form a Y-shaped junction in the early universe, whose formation probability is roughly estimated to be 1/2.

Baryon Acoustic Oscillations (BAO) datasets use very precise measurements of the spatial distribution of large-scale structures as a distance ladder to help constrain cosmological parameters. In a recent article,¹ we combined 17 uncorrelated BAO measurements in the effective redshift range 0.106 ≤ z ≤ 2.36 with the Cosmic Chronometers data, the Pantheon Type Ia supernova and the Hubble Diagram of Gamma Ray Bursts and Quasars to obtain that the ΛCDM model fit infers for the Hubble constant: 69.85 ± 1.27km/sec/Mpc and for the sound horizon distance: 146.1 ± 2.15Mpc. Beyond the ΛCDM model we test Ω_kCDM and wCDM and we get Ω_k = −0.076 ± 0.012, w = − 0.989 ± 0.049 accordingly. In this proceeding we present elaborate on our findings and we compare them to other recent results in the literature.

We present a phenomenological analysis of current observational constraints on classes of FLRW cosmological models in which the matter side of Einstein’s equations includes, in addition to the canonical term, a term proportional to some function of the energy-momentum tensor (T ² = T_αβ T ^αβ = ρ² + 3p²), or of its trace (T = ρ − 3p). Qualitatively, one may think of these models as extensions of general relativity with a nonlinear matter Lagrangian. As such they are somewhat different from the usual dynamical dark energy or modified gravity models: in the former class of models one adds further dynamical degrees of freedom to the Lagrangian (often in the form of scalar fields), while in the latter the gravitational part of the Lagrangian is changed. We study both of these models under two different scenarios: (1) as phenomenological two-parameter or three-parameter extensions of the standard ΛCDM, in which case the model still has a cosmological constant but the nonlinear matter Lagrangian leads to additional terms in Einstein’s equations, which cosmological observations tightly constrain, and (2) as alternatives to ΛCDM, where there is no cosmological constant, and the nonlinear matter term would have to provide the acceleration (which would be somewhat closer in spirit to the usual modified gravity models). A comparative analysis of the observational constraints obtained in the various cases provides some insight on the level of robustness of the Λ model and on the parameter space still available for phenomenological alternatives.

Various observations have shown that dark energy accounts for nearly two-thirds of the energy density of the Universe. The simplest model to explain the nature of dark energy is the cosmological constant (ΛCDM) model. Although Planck observations supports using ΛCDM model as the base cosmological model, there exist some inconsistencies in parameter estimates when compared with independent observations. The most important is the inconsistency in the H₀ estimates from the Planck collaboration which reports $H_0={67.5}_{-0.5}^{+0.5}{\text{kms}}^{-1}Mp{c}^{-1}$, a considerably lower value when compared with the direct local distance ladder measurements. This value shows a discrepancy at the level greater than 4σ with the constraints reported by SH0ES collaboration in 2019, $H_0={74.3}_{-1.42}^{+1.42}km{s}^{-1}Mp{c}^{-1}$. These disagreements, called the Hubble tension, point towards a new physics that deviates from the standard ΛCDM model and to resolve this various methods have been proposed. In this work, a quintessence scalar field with an inverse power potential (V(ϕ)∽ ϕ⁻ⁿ) is assumed as a description of dark energy and we focus on an interacting dark energy dark matter model where the interacting term is taken to be linear in the field (Φ). We study in detail the evolution of the model and provide constraints on the model parameters using low redshift cosmological observations of Type Ia Supernovae (SN), baryon acoustic oscillations (BAO), direct measurements of Hubble parameter (Hz) and high redshift HII galaxy measurements (HIIG). We find that the model agrees with the existing values of the nonrelativistic matter density parameter, Ω_m and dark energy equation of state parameter, w₀. The analysis shows that the observations prefer a negative value of coupling constant and gives the best fit value of $H_0={69.9}_{-1.02}^{+0.46}{\text{kms}}^{-1}{\text{Mpc}}^{-1}$ and thereby can be used to alleviates the H₀ tension between Planck measurements and the observations considered.

We focus on weak inhomogeneous models of the Universe at low redshifts, described by the Lemaître-Tolman-Bondi (LTB) metric. The principal aim of this work is to compare the evolution of inhomogeneous perturbations in the ΛCDM cosmological model and f (R) modified gravity theories, considering a flat Friedmann-Lemaître-Robertson-Walker (FLRW) metric for the background. More specifically, we adopt the equivalent scalar-tensor formalism in the Jordan frame, in which the extra degree of freedom of the f (R) function is converted into a non-minimally coupled scalar field. We investigate the evolution of local inhomogeneities in time and space separately, following a linear perturbation approach. Then, we obtain spherically symmetric solutions in both cosmological models. Our results allow us to distinguish between the presence of a cosmological constant and modified gravity scenarios, since a peculiar Yukawa-like solution for radial perturbations occurs in the Jordan frame. Furthermore, the radial profile of perturbations does not depend on a particular choice of the f (R) function, hence our results are valid for any f (R) model.

We present “soft cosmology”, namely we investigate small deviations from the usual framework due to the effective appearance of soft-matter properties in the Universe sectors. One effect of such a case would be the dark energy to exhibit a different equation-of-state parameter at large scales (which determine the universe expansion) and at intermediate scales (which determine the sub-horizon clustering and the large scale structure formation). Concerning soft dark matter, we show that it can effectively arise due to the dark-energy clustering, even if dark energy is not soft. We propose a novel parametrization introducing the “softness parameters” of the dark sectors. As we see, although the background evolution remains unaffected, due to the extreme sensitivity and significant effects on the global properties even a slightly non-trivial softness parameter can improve the clustering behavior and alleviate e.g. the fσ₈ tension.

We proposes a phenomenological generalisation of the standard model with only one extra degree of freedom that parametrises the evolution of a scalar field responsible for the cosmic acceleration. The model also foresees an additional parameter in the form of a coupling between dark energy and dark matter. This model captures a large diversity of dark energy evolutions at low redshift and could usefully complement common CPL parametrisations widely used. In this context, we have been constraining the parametrisation with data from Planck and KiDS, bringing different results between the early and late universe observations.

For a flat ΛCDM (standard) cosmology, a small sample of gravitationally lensed quasars with measured time delays has recently provided a value of the Hubble constant H₀ in tension with the Planck flat ΛCDM result. Trying to check if this tension is real or not, we used basic observational constraints for two double quasars of the GLENDAMA sample (SBS 0909+532 and SDSS J1339+1310) to discuss the underlying value of H₀ in a standard cosmology. For SBS 0909+532, we were not able to obtain a reliable measurement of H₀. However, the current data of SDSS J1339+1310 are consistent with H₀ around 67.8 km s⁻¹ Mpc⁻¹ and σ(H₀)/H₀ ∽ 10%. Although the formal uncertainty is still large and mainly due to the lack of details on the mass density profile of the main lens galaxy, the central value of H₀ coincides with that of the TDCOSMO+SLACS collaboration (using gravitational lens systems) and is within the 1σ interval from Planck cosmic microwave background data. After getting these preliminary encouraging results through only one double quasar, we are currently planning to use several GLENDAMA systems to accurately measure the Hubble constant and put constraints on other cosmological parameters.

A unification of dark matter and dark energy based on a dynamical space time theory is suggested. By introducing a dynamical space time vector field χ_µ as a Lagrange multiplier, a conservation of an energy momentum tensor $T_{(\chi )}^{\mu \nu }$is implemented. This Lagrangian generalizes the Unified dark energy and dark matter from a scalar field different from quintessence which did not consider a Lagrangian formulation. This generalization allows the solutions which were found previously, but in addition to that also non singular bouncing solutions that rapidly approach to the ΛCDM model. The BBN constraint is also studied.