The Fourteenth Marcel Grossmann Meeting

The following sections are included:

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

Note from Publisher: This chapter consists of photographs only.

The black hole information problem and the firewall problem can be addressed by assuming an extra local symmetry: conformal invariance. It must be an exact symmetry, spontaneously broken by the vacuum, in a way similar to the Brout–Englert–Higgs (BEH) mechanism. We note how this symmetry formally removes the horizon and the singularity inside black holes. For the Standard Model this symmetry is severely restrictive, demanding all coupling constants, masses and even the cosmological constant to be computable, in principle. Finally, this symmetry suggests that the Weyl action (the square of the Weyl curvature) should be added to the Einstein–Hilbert action. The ensuing indefinite metric states are briefly studied, and we conclude with some remarks concerning the interpretation of quantum mechanics.

Perturbative algebraic quantum field theory (pAQFT) is a mathematically rigorous framework that allows to construct models of quantum field theories (QFTs) on a general class of Lorentzian manifolds. Recently, this idea has been applied also to perturbative quantum gravity (QG), treated as an effective theory. The difficulty was to find the right notion of observables that would in an appropriate sense be diffeomorphism invariant. In this paper, I will outline a general framework that allows to quantize theories with local symmetries (this includes infinitesimal diffeomorphism transformations) with the use of the Batalin–Vilkovisky (BV) formalism. This approach has been successfully applied to effective QG in a recent paper by Brunetti, Fredenhagen and myself. In the same paper, we also proved perturbative background independence of the quantized theory, which is going to be discussed in the present work as well.

Theories with elementary scalar degrees of freedom seem nowadays required for simple descriptions of the Standard Model and of the Early Universe. It is then natural to embed theories of inflation in supergravity, also in view of their possible ultraviolet completion in String Theory. After some general remarks on inflation in supergravity, we describe examples of minimal inflaton dynamics which are compatible with recent observations, including higher-curvature ones inspired by the Starobinsky model. We also discuss different scenarios for supersymmetry breaking during and after inflation, which include a revived role for non-linear realizations. In this spirit, we conclude with a discussion of the link, in four dimensions, between “brane supersymmetry breaking” and the super-Higgs effect in supergravity.

The plethora of recent and forthcoming data on the cosmic microwave background (CMB) data are stimulating a new wave of inflationary model-building. Naturalness suggests that the appropriate framework for models of inflation is supersymmetry. This should be combined with gravity in a supergravity theory, whose specific no-scale version has much to commend it, e.g. its derivation from string theory and the flat directions in its effective potential. Simple no-scale supergravity models yield predictions similar to those of the Starobinsky R + R² model, though some string-motivated versions make alternative predictions. Data are beginning to provide interesting constraints on the rate of inflaton decay into Standard Model particles. In parallel, LHC and other data provide significant constraints on no-scale supergravity models, which suggest that some sparticles might have masses close to present experimental limits.

The information paradox results from the traditional picture of a black hole, where all the matter is pulled to the centeral position r = 0. The paradox is resolved in string theory by the fuzzball paradigm: microstates in string theory are fuzzballs with no horizon or singularity, and radiate from their surface just like normal bodies. One can then ask the question: what happens when an object falls onto the fuzzball surface? The conjecture of fuzzball complementarity shows how the dynamics of this surface can have a dual description where the object appears to approximately fall through empty space. The firewall argument attempts to rule out such a free infall description, but the argument starts with contradictory postulates: it assumes that information comes out of the hole, yet its postulates do not allow this unless we violate causality.

Radio-loud neutron stars known as pulsars allow a wide range of experimental tests for fundamental physics, ranging from the study of super-dense matter to tests of General Relativity (GR) and its alternatives. As a result, pulsars provide strong-field tests of gravity, they allow for the direct detection of gravitational waves in a “pulsar timing array” (PTA), and they promise the future study of black hole properties. This contribution gives an overview of the on-going experiments and recent results.

High energy transients make up a diverse and exotic class of objects, from terrestrial lightning to γ-ray bursts at cosmological distances. In this review, we provide a detailed look at some of the more exciting transients observed over the last few years by Swift and other high energy missions.

Until the discovery of gamma-ray bursts, supernovae were the most powerful explosions known in the universe. Supernovae have a long history in the astronomy community with both detailed observational and theoretical studies focused on pinning down the engines to these cosmic explosions. Gamma-ray bursts are more exotic (associated with a short burst of gamma-rays) and rare (1,000-10,000 times less frequent than supernovae) than supernovae. Discovered in the late 1970s, the exotic nature of these bursts led to a rapidly growing list of proposed models (in the hundreds) to explain the engine powering them. How do we determine which of these engines is correct? Here we take lessons learned from the search for the supernova engine and apply them to the gamma-ray bursts phenomenon.

After the big bang, production of heavy elements in the early universe takes place starting from the formation of the first (Pop III) stars, their evolution, and explosion. The Pop III supernova (SN) explosions have strong dynamical, thermal, and chemical feedback on the formation of subsequent stars and evolution of galaxies. However, the nature of Pop III stars/supernovae (SNe) have not been well-understood. The signature of nucleosynthesis yields of the first SN can be seen in the elemental abundance patterns observed in extremely metal-poor (EMP) stars. We show that the abundance patterns of EMP stars, e.g. the excess of C, Co, Zn relative to Fe, are in better agreement with the yields of hyper-energetic explosions (Hypernovae, (HNe)) rather than normal supernovae. We note the large variation of the abundance patterns of EMP stars propose that such a variation is related to the diversity of the GRB-SNe and posssibly superluminous supernovae (SLSNe). For example, the carbon-enhanced metal-poor (CEMP) stars may be related to the faint SNe (or dark HNe), which could be the explosions induced by relativistic jets. Finally, we examine the various mechanisms of SLSNe.

We start with a brief introduction to the historical background in the early pioneering days when the first neutron star thermal evolution calculations predicted the presence of neutron stars hot enough to be observable. We then report on the first detection of neutron star temperatures by ROSAT X-ray satellite, which vindicated the earlier prediction of hot neutron stars. We proceed to present subsequent developments, both in theory and observation, up to today. We then discuss the current status and the future prospect, which will offer useful insight to the understanding of basic properties of ultra-high density matter beyond the nuclear density, such as the possible presence of such exotic particles as pion condensates.

By transforming a cubic kilometer of natural Antarctic ice into a neutrino detector, the IceCube project created the opportunity to observe cosmic neutrinos. We describe the experiment and the complementary methods presently used to study the flux of the recently discovered cosmic neutrinos. In one method, events are selected in which neutrinos interacted inside the instrumented volume of the detector, yielding a sample of events dominated by neutrinos of electron and tau flavor. Alternatively, another method detects secondary muons produced by neutrinos selected for having traveled through the Earth to reach the detector, providing a pure sample of muon neutrinos. We will summarize the results obtained with the enlarged data set collected since the initial discovery and appraise the current status of high-energy neutrino astronomy. The large extragalactic neutrino flux observed points to a nonthermal universe with comparable energy in neutrinos, gamma rays and cosmic rays. Continued observations may be closing in on the source candidates. In this context, we highlight the potential of multimessenger analyses as well as the compelling case for constructing a next-generation detector larger in volume by one order of magnitude.

Nearly a century of experimental observations and theoretical arguments have pointed out that a large fraction of the Universe is composed by dark matter particles. Many possibilities are open on the nature and interaction types of such relic particles. Moreover, the poor knowledge of many fundamental astrophysical, nuclear and particle physics aspects as well as of some experimental and theoretical parameters, the different used approaches and target materials, etc. make it challenging to understand the implication of some different experimental efforts. Some general arguments are addressed here. Future perspectives are mentioned.

In this centenary year for general relativity, this paper presents a brief review of some key developments that have rendered the theory pivotal to our understanding of cosmic X-ray sources, galactic nuclei and cosmology — a complete transformation from the time when the 50th anniversary of the theory was celebrated The paper ends with a few speculative comments on possible future developments.

The Cherenkov telescope array (CTA) is a next-generation observatory for very high energy (VHE) gamma-ray astronomy. With one array of imaging atmospheric Cherenkov telescopes each in the Northern and Southern Hemispheres, CTA will provide full-sky coverage, enhance flux sensitivity by one order of magnitude compared to current instruments, cover gamma-ray energies from 20 GeV to 300 GeV, and provide a wide field of view with angular resolution of a few arc-minutes.

Science themes to be addressed by the CTA observatory include (i) understanding the origin of relativistic cosmic particles, and the role these play in the evolution of star forming systems and galaxies, (ii) probing extreme environments such as neutron stars and black holes, but also the cosmic voids, and (iii) exploring frontiers in physics such as the nature of dark matter. With its superior performance, the prospects for CTA combine guaranteed science — the in-depth understanding of known objects and mechanisms — with anticipated detection of new classes of gamma-ray emitters and new phenomena, and a very significant potential for fundamentally new discoveries.

After four years of uninterrupted cryogenic operation the ESA Planck satellite has produced a set of high-signal-to-noise maps of the microwave and submillimetre sky, both in temperature and polarisation, with a powerful combination of angular resolution (θ_FWHM ∼ 33’-5’), sensitivity (ΔT/T ∼ 2 × 10⁻⁶), frequency range (25 to 1000 GHz), sky coverage (100%), calibration accuracy (∼ 0.1%), and control of systematic effects. The use of two instruments based on different technologies ensures the required wide spectral range and provides a unique opportunity for testing internal consistency. The Planck multi-frequency observations allow for an accurate separation of the cosmic microwave background (CMB) from Galactic and extragalactic foregrounds components. The 2015 Planck full-mission data set leads to an exquisite measurement of the CMB temperature power spectrum up to the 8^th acoustic peak (2 < ℓ < 2500), and to unprecedented measurements of the EE and TE power spectra (2 < ℓ < 2000). The overall Planck results, combining temperature and polarisation with lensing likelihoods, are in excellent agreement with the standard six-parameter ΛCDM model. The Planck polarisation data at large scales and independent estimates from Planck lensing indicate a reionization redshift lower than in previous estimates. The Planck 2015 cosmological parameters using both temperature and polarization data are in very good agreement with those from the 2013 release, but with significantly improved precision.

Our concept of induced gravitational collapse (IGC paradigm) starting from a supernova occurring with a companion neutron star, has unlocked the understanding of seven different families of gamma ray bursts (GRBs), indicating a path for the formation of black holes in the universe. An authentic laboratory of relativistic astrophysics has been unveiled in which new paradigms have been introduced in order to advance knowledge of the most energetic, distant and complex systems in our universe. A novel cosmic matrix paradigm has been introduced at a relativistic cosmic level, which parallels the concept of an S-matrix introduced by Feynmann, Wheeler and Heisenberg in the quantum world of microphysics. Here the “in” states are represented by a neutron star and a supernova, while the “out” states, generated within less than a second, are a new neutron star and a black hole. This novel field of research needs very powerful technological observations in all wavelengths ranging from radio through optical, X-ray and gamma ray radiation all the way up to ultra-high-energy cosmic rays.

Short and long-duration gamma-ray bursts (GRBs) have been recently sub-classified into seven families according to the binary nature of their progenitors. For short GRBs, mergers of neutron star binaries (NS–NS) or neutron star-black hole binaries (NS-BH) are proposed. For long GRBs, the induced gravitational collapse (IGC) paradigm proposes a tight binary system composed of a carbon–oxygen core (CO_core) and a NS companion. The explosion of the CO_core as supernova (SN) triggers a hypercritical accretion process onto the NS companion which might reach the critical mass for the gravitational collapse to a BH. Thus, this process can lead either to a NS-BH or to NS–NS depending on whether or not the accretion is sufficient to induce the collapse of the NS into a BH. We shall discuss for the above compact object binaries: (1) the role of the NS structure and the equation-of-state on their final fate; (2) their occurrence rates as inferred from the X and gamma-ray observations; (3) the expected number of detections of their gravitational wave (GW) emission by the Advanced LIGO interferometer.

Over the past few decades, a consensus picture has emerged in which roughly a quarter of the universe consists of dark matter. I begin with a review of the observational evidence for the existence of dark matter: rotation curves of galaxies, gravitational lensing measurements, hot gas in clusters, galaxy formation, primordial nucleosynthesis and Cosmic Microwave Background (CMB) observations. Then, I discuss a number of anomalous signals in a variety of data sets that may point to discovery, though all of them are controversial. The annual modulation in the DAMA detector and/or the gamma-ray excess seen in the Fermi Gamma Ray Space Telescope from the Galactic Center could be due to WIMPs; a 3.5 keV X-ray line from multiple sources could be due to sterile neutrinos; or the 511 keV line in INTEGRAL data could be due to MeV dark matter. All of these would require further confirmation in other experiments or data sets to be proven correct. In addition, a new line of research on dark stars is presented, which suggests that the first stars to exist in the universe were powered by dark matter heating rather than by fusion: the observational possibility of discovering dark matter in this way is discussed.

We present a classical conformal field theory on an arbitrary two-dimensional spacetime background. The dynamical object is a space-filling string, and the evolution may be thought as occurring on the manifold of the conformal group. The theory is a “descendant” of the theory of gravitation in two-dimensional spacetime. The discussion is based on the relation of the deformations of the space-filling string with conformal transformations. The realization of the conformal algebra in terms of surface deformations possesses a classical central charge. The action principle, the conformal and Weyl invariances of the action, and the equations of motion are studied. The energy-momentum tensor, the coupling to Liouville matter, and the cancellation of anomalies are analyzed. The quantum theory is not discussed.

An accretion flow around a black hole has a saddle type sonic point just outside the event horizon to guarantee that the flow enters the black hole supersonically. This feature exclusively present in the strong gravity limit makes its marks in every observation of black hole candidates. Another physical sonic point is present (as in a Bondi flow) even in weak gravity. Every aspect of spectral or temporal properties of every black hole can be understood using this transonic or advective flow having more than one saddle type points. This most well known and generalized solution with viscosity and radiative transfer has been verified by numerical simulations also. Spectra, computed for various combinations of the standard Keplerian, and advective sub-Keplerian components match accurately with those from satellite observations. Standing, oscillating and propagatory oscillating shocks are produced due to centrifugal barrier of the advective component. The post-shock region acts as the Compton cloud producing the power-law spectra. Jets and outflows are also produced from this post-shock region, commonly known as the CENtrifugal barrier supported BOundary Layer or CENBOL. In soft states, the CENBOL is cooled down by soft photons from the Keplerian disk, and thus the outflow is absent. Type-C and Type-B QPOs are generated respectively due to strong and weak resonance oscillations of the CENBOL. Away from resonance, oscillation may be triggered when Rankine-Hugoniot conditions are not satisfied and Type-A QPOs could be seen.

This contribution is a review of some talks presented at the session “MHD processes near compact objects” of the Fourteenth Marcel Grossmann Meeting MG14. We discuss the contemporary developments of MHD processes that occur near black holes and neutron stars. The influence of magneto-plasma processes on the structure of the compact objects and accretion processes is considered.

We consider the possibility of multiply-connected spacetimes, ranging from the Flamm–Einstein–Rosen bridge, geons, and the modern renaissance of traversable wormholes. A fundamental property in wormhole physics is the flaring-out condition of the throat, which through the Einstein field equation entails the violation of the null energy condition (NEC). In the context of modified theories of gravity, it has also been shown that the normal matter can be imposed to satisfy the energy conditions, and it is the higher order curvature terms, interpreted as a gravitational fluid, that sustain these nonstandard wormhole geometries, fundamentally different from their counterparts in general relativity (GR). We explore interesting features of these geometries, in particular, the physical properties and characteristics of these ‘exotic spacetimes’.

An overview of the AT4 session on localized self-gravitating field systems in the Einstein and alternatives theories of gravity of the 14th Marcel Grossmann meeting is given. The focus will be on recent progress on massive gravity.

We report on the Parallel Session BH4 “Gravitational fields with sources: From compact objects to black holes” of the 14th Marcel Grossmann Meeting held at Sapienza University of Rome in 2015.

In this summary, we present the main topics of the talks presented in the parallel session “Black holes - 5” of the 14th Marcel Grossmann Meeting held in Rome, Italy in July 2015. We first present a short review of the main approaches used to understand thermodynamics by using differential geometry. Then, we present a brief summary of each presentation, including some general remarks and comments.

This is a rapporteur article of the parallel session “Regular and analogue black holes” which took place within the 14th Marcel Grossmann Meeting.

The last three years have again seen new exciting developments in the area of higher dimensional black objects. For black objects with noncompact higher dimensions, the solution space was exlored further within the blackfold approach and with numerical schemes, yielding a large variety of new families of solutions, while limiting procedures created so-called super-entropic black holes. Concerning compact extra dimensions, the sequences of static nonuniform black strings in five and six dimensions were extended to impressively large values of the nonuniformity parameter with extreme numerical precision, showing that an oscillating pattern arises for the mass, the area or the temperature, while approaching the conjectured double-cone merger solution. Besides the presentation of interesting new types of higherdimensional solutions, also their physical properties were addressed in this session. While the main focus was on Einstein gravity, a significant number of talks also covered Lovelock theories.

Natures of progenitors of type Ia Supernovae (SNe Ia) have not yet been clarified. There has been long and intensive discussion on whether the so-called single degenerate (SD) scenario or the double degenerate (DD) scenario, or anything else, could explain a major population of SNe Ia, but the conclusion has not yet been reached. With rapidly increasing observational data and new theoretical ideas, the field of studying the SN Ia progenitors has been quickly developing, and various new insights have been obtained in recent years. This paper aims at providing a summary of the current situation regarding the SN Ia progenitors, both in theory and observations. It seems difficult to explain the emerging diversity seen in observations of SNe Ia by a single population, and we emphasize that it is important to clarify links between different progenitor scenarios and different sub-classes of SNe Ia.

In this parallel session recent work concerning the analysis of boson star properties as well as boson star representations in specific spacetimes is considered. This includes e.g. all issues related to the global structure and the physical interpretation of these objects. Furthermore, presentations of work within the framework of black hole mimickers (e.g. wormholes and gravastars) are of great interest within this framework.

In this review paper we revisit the main ideas concerning the role played by scalar fields in Cosmology, in particular as candidate for dark matter. Moreover, we include some information from the plenary talks and proceedings of the BS2 session of the MG14: The origin of scalar fields and their roles in Cosmology by Kei–ichi Maeda, Composite Dark Energy particle derived from Dark Gauge Group by Axel de la Macorra, Scalar Fields (Bose–Einstein Condensates) as the Dark Matter of the Universe by Tonatiuh Matos, and Quasistationary solutions of selfgravitating scalar fields around black holes by Nicolas Sanchis–Gual.

Recent cosmic microwave background (CMB) data in temperature and polarization have reached high precision in estimating all the parameters that describe the current socalled standard cosmological model. Recent results about the integrated Sachs–Wolfe (ISW) effect from CMB anisotropies, galaxy surveys, and their cross-correlations are presented. Looking at fine signatures in the CMB, such as the lack of power at low multipoles, the primordial power spectrum (PPS) and the bounds on non-Gaussianities, complemented by galaxy surveys, we discuss inflationary physics and the generation of primordial perturbations in the early universe. Three important topics in particle physics, the bounds on neutrinos masses and parameters, on thermal axion mass and on the neutron lifetime derived from cosmological data are reviewed, with attention to the comparison with laboratory experiment results. Recent results from cosmic polarization rotation (CPR) analyses aimed at testing the Einstein equivalence principle (EEP) are presented. Finally, we discuss the perspectives of next radio facilities for the improvement of the analysis of future CMB spectral distortion experiments.

In recent years, significant progress has been made in building new galaxy clusters samples, at low and high redshifts, from wide-area surveys, particularly exploiting the Sunyaev–Zel’dovich (SZ) effect. A large effort is underway to identify and characterize these new systems with optical/NIR and X-ray facilities, thus opening new avenues to constraint cosmological models using structure growth and geometrical tests. A census of galaxy clusters sets constraints on reionization mechanisms and epochs, which need to be reconciled with recent limits on the reionization optical depth from cosmic microwave background (CMB) experiments. Future advances in SZ effect measurements will include the possibility to (unambiguously) measure directly the kinematic SZ effect, to build an even larger catalogue of galaxy clusters able to study the high redshift universe, and to make (spatially-)resolved galaxy cluster maps with even spectral capability to (spectrally-)resolve the relativistic corrections of the SZ effect.

Astronomical observations reveal hierarchical structures in the universe, from galaxies, groups of galaxies, clusters and superclusters, to filaments and voids. On the largest scales, it seems that some kind of statistical homogeneity can be observed. As a result, modern cosmological models are based on spatially homogeneous and isotropic solutions of the Einstein equations, and the evolution of the universe is approximated by the Friedmann equations. In parallel to standard homogeneous cosmology, the field of inhomogeneous cosmology and backreaction is being developed. This field investigates whether small scale inhomogeneities via nonlinear effects can backreact and alter the properties of the universe on its largest scales, leading to a non-Friedmannian evolution. This paper presents the current status of inhomogeneous cosmology and backreaction. It also discusses future prospects of the field of inhomogeneous cosmology, which is based on a survey of 50 academics working in the field of inhomogeneous cosmology.

In this paper, we summarize some of the main observational challenges for the standard Friedmann–Lemaître–Robertson–Walker (FLRW) cosmological model and describe how results recently presented in the parallel session “Large-scale Structure and Statistics” (DE3) at the “Fourteenth Marcel Grossman Meeting on General Relativity” are related to these challenges.

Self-interacting dark matter (SIDM) is a hypothetical form of dark matter (DM), characterized by relatively strong (compared to the weak interaction strength) self-interactions (SIs), which has been proposed to resolve a number of issues concerning tensions between simulations and observations at the galactic or smaller scales. We review here some recent developments discussed at the 14th Marcel Grossmann Meeting (MG14), paying particular attention to restrictions on the SIDM (total) cross-section from using novel observables in merging galactic structures, as well as the rôle of SIDM on the Milky Way halo and its central region. We report on some interesting particle-physics inspired SIDM models that were discussed at MG14, namely the glueball DM, and a right-handed neutrino DM (with mass of a few tens of keV, that may exist in minimal extensions of the standard model (SM)), interacting among themselves via vector bosons mediators in the dark sector. A detailed phenomenology of the latter model on galactic scales, as well as the potential role of the right handed neutrinos in alleviating some of the small-scale cosmology problems, namely the discrepancies between observations and numerical simulations within standard ΛNCDM and ΛNWDM cosmologies are reported.

The last two decades have witnessed a steady advancement of techniques for particle and radiation detection. From very large detectors (such as neutrino telescopes (ANTARES, Ice-Cube) and LHC-detectors (Atlas, CMS) through medical imagers (MRI, CT) to everyday integrated sensors such as charge-coupled devices (CCDs) for imaging, progress has been impressive. However, research efforts have focused on detection of charged particles or electromagnetic radiation. Technological advances towards detection of neutral particles were much slower. We are developing a new class of detectors for neutral and weakly interacting particles such as neutrons, neutrinos, and dark-matter candidates. The detection is based on the interaction of the particles with the nuclei of detector by means of room-temperature bolometry. Small amounts of energy deposited into nano-scale grains trigger a release of chemical energy, leading to a “nano-explosion”. We used combined electromagnetic pulse, acoustic and optical read-out techniques for read-out of such RT-bolometers. A “nano-explosion” generates an overpressure, sonic boom, which can be detected acoustically and detectable burst of photons. We are interested in very rare events and the coincidence detection of at least two quantities has been used to reject artefacts. This paper describes the read-out schemes developed for such chemically amplified detectors aka RT-bolometers. The proposed RT-bolometers can find important applications in a broad range of fields, including the detection of neutrons for research studies in solid-state physics and biochemistry (e.g., neutron detectors for spallation sources, neutron microscopy); detection of neutrons for homeland security, detection of neutrinos at reactors and detection of Dark Matter candidates. Geo-neutrino detection will provide valuable geological information and will enable Neutrino Geology.

The dark matter (DM) parallel session DM2 of the 14th Marcel Grossmann Meeting was enriched by several contributions about the results and the strategies in the study and in the detection of DM particles in the Galactic halo. In the following, an overview of the latest results in this field will be summarized. A particular care will be given to the results obtained by exploiting the model independent DM annual modulation signature for the presence of DM particles in the galactic halo. Results from the other experiments using different procedures, different techniques and different target materials will be shortly addressed as well as implications and experimental perspectives.

We analyze structural parameters of the globular clusters belonging to the Milky Way system which were listed in the latest edition of the Harris Catalogue. We search for observational evidences of the effect of tidal forces induced by the Galaxy on the dynamical and thermodynamical evolution of a globular cluster. The behavior for the W₀ distribution exhibited by the globular cluster population seems to be in contrast with theoretical results in literature about gravothermal instability, and suggest a new limit value smaller than the previous one.

Boost modes Ψ_ϰ(x) are eigenfunctions of the Lorentz transformations generator in two-dimensional (2D) Minkowski space (MS). We demonstrate and discuss deep interrelation between the boost modes and the field correlators, also known as Wightman functions. In the case of a massive scalar field, the boost modes, as functions of the spectral parameter ϰ, contain the Dirac delta-function singularity δ(ϰ) at the light cone. The zero boost mode coincides up to a constant factor with the Wightman function. The light cone singularity of boost modes for a fermion field is stronger. For this case, they contain the Gelfand δ-function of complex argument δ(ϰ ± i/2), while the Wightman function components coincide with analytical continuation of the boost modes set towards the spectral values ϰ = ∓i/2. We argue that due to the discovered properties of the boost modes the so-called Unruh modes, which are at the core of the Unruh effect derivation, do not constitute a complete set in MS and thus cannot be used for quantization of neither scalar, nor fermion field. Finally, we discuss boost modes for the case of the constant electric background and rederive the well-known result for spontaneous pair creation rate. Solution of this problem in the boost modes representation reveals distinctions between the Unruh problem and the effect of pair creation by an electric field in vacuum.

This work summarises recent progress obtained by the mini-superpspace quantization of N = 1, d = 4 supergravity, formulated in the framework of the Bianchi IX cosmological model. The emphasis is put on three main results: the completeness of the solution space obtained, the elements suggesting a hidden Kac-Moody structure of the theory and those leading to conjecture an avoidance of the cosmological singularity by some branches of the wave function of the Universe.

We review recent developments in the field of string cosmology with particular emphasis on open problems having to do mainly with geometric asymptotics and singularities. We discuss outstanding issues in a variety of currently popular themes, such as tree-level string cosmology asymptotics, higher-order string correction effects, M-theory cosmology, braneworlds, and finally ambient cosmology.

We briefly introduce the disadvantages for Type Ia supernovae (SNe Ia) as standard candles to measure the universe, and suggest Gamma-ray bursts (GRBs) can serve as a powerful tool for probing the properties of high redshift universe. We use GRBs as distance indicators in constructing the Hubble diagram at redshifts beyond the current reach of SNe Ia observations. Since the progenitors of long GRBs (LGRBs) are confirmed to be massive stars, they are deemed as an effective approach to study the cosmic star formation rate (SFR). A detailed representation of how to measure high-z SFR using GRBs is presented. Moreover, first stars can form only in structures that are suitably dense, which can be parametrized by defining the minimum dark matter halo mass M_min. M_min must play a crucial role in star formation. The association of LGRBs with the collapses of massive stars also indicates that the GRB data can be applied to constrain the minimum halo mass M_min and to investigate star formation in dark matter halos.

It is recognized that very likely the correlation between peak energy E_p and bolometric intensity is intrinsic to GRBs. However its physical origin is still debated. In this paper we will discuss a possible interpretation of the correlation in the light of a GRB prompt emission spectral model, GRBCOMP, proposed in Ref. 1. GRBCOMP is essentially a photospheric model for the prompt emission of GRBs. Its main ingredients are a thermal bath of soft seed photons and a subrelativistically expanding outflow plasma, consequence of the star explosion. The emerging spectrum is the result of two phases: first, up to the photospheric radius, Comptonization of a subrelativistic electron outflow with thermal bath of soft photons, then, convolution of the Comptonized photons in the first phase with a Green function. The result of this convolution is consistent with different physical processes, in particular Inverse Compton. GRBCOMP has been successfully tested using a significant sample of GRB time resolved spectra in the broad energy band from 2 keV to 2 MeV.

A major breakthrough in our understanding of gamma-ray bursts (GRB) prompt emission physics occurred in the last few years, with the realization that a thermal component accompanies the over-all nonthermal prompt spectra. This thermal part is important by itself, as it provides direct probe of the physics in the innermost outflow regions. It further has an indirect importance, as a source of seed photons for inverse-Compton scattering, thereby it contributes to the nonthermal part as well. In this short review, we highlight some key recent developments. Observationally, although so far it was clearly identified only in a minority of bursts, there is indirect evidence that a thermal component exists in a very large fraction of GRBs, possibly close to 100%. Theoretically, the existence of a thermal component has a large number of implications as a probe of underlying GRB physics. Some surprising implications include its use as a probe of the jet dynamics, geometry and magnetization.

This article is a rapporteur review of selected talks presented at the session ‘Gravitational lensing: theory and numerical modeling’ of the Fourteenth Marcel Grossmann Meeting (MG14) as well as discussion of general ideas of related topics. We consider the influence of plasma on various gravitational lensing effects, including strong lensing systems and black hole shadows. We discuss recent developments in gravitational lensing beyond the small deflection regime, as well as some other aspects of this field. New results in modeling strong lensing systems and microlensing are presented.

Einstein’s General theory of relativity (GR) successfully describes gravity. Although GR has been accurately tested in weak gravitational fields, it remains largely untested in the general strong field cases. One of the most fundamental predictions of GR is the existence of black holes (BHs). After the recent direct detection of gravitational waves by LIGO, there is now near conclusive evidence for the existence of stellar-mass BHs. In spite of this exciting discovery, there is not yet direct evidence of the existence of BHs using astronomical observations in the electromagnetic spectrum. Are BHs observable astrophysical objects? Does GR hold in its most extreme limit or are alternatives needed? The prime target to address these fundamental questions is in the center of our own Milky Way, which hosts the closest and best-constrained supermassive BH candidate in the universe, Sagittarius A* (Sgr A*). Three different types of experiments hold the promise to test GR in a strong-field regime using observations of Sgr A* with newgeneration instruments. The first experiment consists of making a standard astronomical image of the synchrotron emission from the relativistic plasma accreting onto Sgr A*. This emission forms a “shadow” around the event horizon cast against the background, whose predicted size (∽50 μas) can now be resolved by upcoming very long baseline radio interferometry experiments at mm-waves such as the event horizon telescope (EHT). The second experiment aims to monitor stars orbiting Sgr A* with the next-generation near-infrared (NIR) interferometer GRAVITY at the very large telescope (VLT). The third experiment aims to detect and study a radio pulsar in tight orbit about Sgr A* using radio telescopes (including the Atacama large millimeter array or ALMA). The BlackHoleCam project exploits the synergy between these three different techniques and contributes directly to them at different levels. These efforts will eventually enable us to measure fundamental BH parameters (mass, spin, and quadrupole moment) with sufficiently high precision to provide fundamental tests of GR (e.g. testing the no-hair theorem) and probe the spacetime around a BH in any metric theory of gravity. Here, we review our current knowledge of the physical properties of Sgr A* as well as the current status of such experimental efforts towards imaging the event horizon, measuring stellar orbits, and timing pulsars around Sgr A*. We conclude that the Galactic center provides a unique fundamental-physics laboratory for experimental tests of BH accretion and theories of gravity in their most extreme limits.

Parallel session NS3 was devoted to review and discuss various aspects of the QCD phase diagram, ranging from matter at low temperatures and high densities as the one present in compact objects to matter at high temperatures and low densities, as the one obtained in heavy ion collisions. Different aspects of the phase transition, as the possible existence of a critical end point, the onset of deconfinement and the effects of strong magnetic fields on the phase diagram were discussed during this parallel session and outlined in this text. The complete discussions are presented separately in the texts prepared by the speakers and by the Rapporteur.

We investigate the quark deconfinement phase transition in cold (T = 0) and hot β-stable hadronic matter. Assuming a first-order phase transition, we calculate and compare the nucleation rate and the nucleation time due to quantum and thermal nucleation mechanisms. We show that above a threshold value of the central pressure a pure hadronic star (HS) (i.e. a compact star with no fraction of deconfined quark matter (QM)) is metastable to the conversion to a quark star (QS) (i.e. a hybrid star or a strange star). This process liberates a huge amount of energy, of the order of 10⁵³ erg, which produces a powerful neutrino burst, likely accompanied by intense gravitational waves emission, and possibly by a second delayed (with respect to the supernova explosion forming the HS) explosion which could be the energy source of a powerful gamma-ray burst (GRB). This stellar conversion process populates the QS branch of compact stars, thus one has in the universe two coexisting families of compact stars: HSs and QSs. We introduce the concept of critical mass M_cr for cold HSs and proto-hadronic stars (PHSs), and the concept of limiting conversion temperature for PHSs. We show that PHSs with a mass M < M_cr could survive the early stages of their evolution without decaying to QSs. Finally, we discuss the possible evolutionary paths of PHSs.

100 years after the invention of General Relativity (GR) and 110 years after the development of Special Relativity (SR) we have to state that until now no single experiment or observation allows any doubt about the validity of these theories within the accuracy of the available data. Tests of GR can be divided into three categories: (i) test of the foundations of GR, (ii) tests of the consequences of GR, and (iii) test of the interplay between GR and quantum mechanics. In the first category, we have tests of the Einstein Equivalence Principle and the structure of the Newton axioms, in the second category we have effects like the gravitational redshift, light defection, gravitational time delay, the perihelion shift, the gravitomagnetic effects as the Lense–Thirring and Schiff effect, and gravitational waves. Tests of the effects of gravity on quantum systems are a first step towards experiments searching for a quantum gravity theory. In this paper, we also highlight practical applications in positioning, geodesy, and the International Atomic Time. After 100 years, GR can now definitely be regarded also as practical and applied science.

The framework of locally covariant quantum field theory (QFT), an axiomatic approach to QFT in curved spacetime (CST), is reviewed. As a specific focus, the connection between spin and statistics is examined in this context. A new approach is given, which allows for a more operational description of theories with spin and for the derivation of a more general version of the spin–statistics connection in CSTs than previously available. This part of the text is based on [C. J. Fewster, arXiv:1503.05797.] and a forthcoming publication; the emphasis here is on the fundamental ideas and motivation.

In this talk I discuss recent developments in moduli stabilisation, SUSY breaking and chiral D-brane models together with several interesting features of cosmological models built in the framework of type IIB string compactifications. I show that a non-trivial pre-inflationary dynamics can give rise to a power loss at large angular scales for which there have been mounting observational hints from both WMAP and Planck. I then describe different stringy embeddings of inflationary models which yield large or small tensor modes. I finally argue that reheating is generically driven by the decay of the lightest modulus which can produce, together with Standard Model particles, also non-thermal dark matter and light hidden sector degrees of freedom that behave as dark radiation.

Isolated magnetic white dwarfs have field strengths ranging from 10³G to 10⁹ G, and constitute an interesting class of objects. The origin of the magnetic field is still the subject of a hot debate. Whether these fields are fossil, hence the remnants of original weak magnetic fields amplified during the course of the evolution of the progenitor of white dwarfs, or on the contrary, are the result of binary interactions or, finally, other physical mechanisms that could produce such large magnetic fields during the evolution of the white dwarf itself, remains to be elucidated. In this work, we review the current status and paradigms of magnetic fields in white dwarfs, from both the theoretical and observational points of view.

I discuss the progress in the numerical modeling of accretion flows made possible after solving the general relativistic radiation-hydrodynamics equations through the moment formalism and with Godunov-type methods. I first present the necessary numerical tools for coping with the stiffness of the source terms in the equations and for treating the intermediate regime between the optically thick and the optically thin. Then, I show some applications to spherical accretion and to supersonic Bondi-Hoyle accretion onto a black hole, discussing its astrophysical relevance.

To understand accretion flow dynamics around black hole candidates (BHCs) one needs to study spectral as well as temporal features in details. After the inclusion of Chakrabarti-Titarchuk two Component Advective Flow (TCAF) model into HEASARC’s spectral analysis package XSPEC as a local additive table model, we found that it is quite capable of fitting spectra from different phases of few transient black hole candidates (TBHCs) during their X-ray outbursts. From spectral fits with TCAF model, one can directly extract physical flow parameters, such as two types of accretion (Keplerian disk and sub-Keplerian halo) rates and shock parameters (location and strength of the shock). A much better understanding of spectral and timing properties is achieved by studying evolution of these physical flow parameters during the outbursts of TBHCs. One can also predict frequency of primary dominating QPOs from TCAF fitted shock parameters. Based on a comparison of halo to disk accretion rate ratio (ARR) with quasi-periodic oscillation (QPOs; if present) frequencies, a physical understanding of the classification of the entire outburst phase of the BHCs into different spectral states emerges. We conclude that TCAF in XSPEC, provides us with a better tool to understand accretion flow dynamics during the outbursts of TBHCs.

The origin of Ultraluminous X-ray sources (ULXs) in external galaxies whose X-ray luminosities exceed those of the brightest black holes in our Galaxy by hundreds and thousands of times is mysterious. The most popular models for the ULXs involve either intermediate mass black holes (IMBHs) or stellar-mass black holes accreting at super-Eddington rates. Here we review the ULX properties, their X-ray spectra indicate a presence of hot winds in their accretion disks supposing the supercritical accretion. However, the strongest evidences come from optical spectroscopy. The spectra of the ULX counterparts are very similar to that of SS 433, the only known supercritical accretor in our Galaxy.

We study time variability properties of black hole transients densely monitored by the RXTE instruments. We systematically study the time/phase lag at QPO frequency. We find hard lag, intregated over Quasi Periodic Oscillation (QPO) frequency for the low inclination source (such as GX 339-4). The hard lag monotonically decrease and become negative (i.e., soft lag happens) close to 3.0 Hz for the high inclination sources (e.g., XTE J1550-564). Thus we find two different behaviours for the high inclination and the low inclination systems. We also find that the evolution properties of low-frequency quasi-periodic oscillations (QPOs) do not depend on the orbital inclination though the amplitude of low-frequency quasi-periodic oscillations (QPOs) depends on the orbital inclination. We conclude these evolutions could be due to the systematic movement of the Comptonizing region itself confirming the propagatory shock oscillation model.

In the case of no-horizon regular spacetimes there exist a region close to origin where the spacetime is “flattened”. Existence of such a region leads to presence of so called “ghost” images of accretion discs orbiting in the spacetimes. We show how the presence of the ghost images influences distribution of photons in the profiles of spectral lines generated by Keplerian disks.

We study transient Galatic black hole candidate MAXI J1836-194 during its 2011 outburst using RXTE/PCA archival data. 2.5-25 keV spectra are fitted with Two Component Advective Flow (TCAF) model fits file as an additive table local model in XSPEC. From TCAF model spectral fits, physical parameters such as Keplerian disk rate, sub-Keplerian halo rate, shock location and compression ratio are extracted directly for better understanding of accretion processes around the BHC during this outburst. Low frequency quasi-periodic oscillation (QPO) are observed sporadically during the entire epoch of the outburst, with a general trend of increasing frequency during rising and decreasing frequency during declining phases of the outburst, as in other transient BHCs. The nature of the variation of the accretion rate ratio (ratio of halo and disk rates) and QPOs (if observed), allows us to properly classify entire epoch of the outburst into following two spectral state, such as hard (HS), hard-intermediate (HIMS). These states are observed in the sequence of HS (Ris.) → HIMS (Ris.) → HIMS (Dec.) → HS (Dec.). This outburst of MAXI J1836-194 could be termed as ‘failed’ outburst, since no observation of soft (SS) and soft-intermediate (SIMS) spectral state are found during the entire outburst.

Transient black hole candidate MAXI J1659-152 showed rapid spectral and temporal evolution during its very first outburst. Our understanding about accretion flow dynamics around black hole candidates has improved much more after the inclusion of Chakrabarti-Titarchuk Two Component Advective Flow (TCAF) model as an additive table model in XSPEC. In this paper we make a detail study of temporal and spectral properties of black hole candidate MAXI J1659-152 during its 2010 outburst with TCAF and POS model. From our fit, we extract accretion flow parameters (Keplerian disk and sub-Keplerian halo rates, shock location, shock strength). We find mass of the object to be 4.17-7.74 M_⊙.

Dynamics of charged matter in the oblique black hole magnetosphere is investigated. In particular, we adopt a model consisting of a rotating black hole embedded in the external large-scale magnetic field that is inclined arbitrarily with respect to the rotation axis. Breaking the axial symmetry appears to have profound consequences regarding the dynamics of particles and it also poses some methodological difficulties. In this contribution we discuss the applicability of the method of effective potential for the non-axisymmetric model and show that it may only be applied in the appropriate reference frame.

The inner region of a transient source is cooled down by the inverse Comptonization of soft photons from the Keplerian disk component. The reduction of pressure forces the outer boundary of the Compton cloud, namely, the centrifugal pressure supported shock to move inward slowly in order to satisfy Rankine-Hugoniot conditions. We consider the transient source H1743-322 to study this movement of the shock. The presence of cooling changes the geometry of the Compton cloud gradually. We also see how the flow parameters of this source change day by day during a complete outburst. As the shock-oscillation could also modulate harder X-rays, we want to resolve the question: are QPOs originated from shock oscillations and can the time variation of the QPO frequency be explained by this slow propagation of the shock? For that we solve the Rankine-Hugoniot conditions and derive the condition of shock formation in presence of Compton cooling. We also compute inward velocity of the shock Compton cloud to be a few meters per second, which agrees well with earlier observational results.

We use α prescription of viscosity and study steady state solutions of an accretion flow, in presence of dissipative shocks. The flow solutions depend on four initial parameters namely, ε_in, l_H, α and f. We study all such solutions possible and divide the parameter space into regions which allow the formation of such shock waves and which do not. Then, we follow a similar study in presence of outflows. From the analysis of the resultant parameter space, we find that the dissipative shocks can still form for the values of α as high as 0.27 in small regions of the flow parameter space in absence of outflows. This value reduces to 0.2 in presence of outflows. The typical values of αappear in the range 0.01 − 0.15. The values of α reported from numerical simulations are always lower than 0.1. This signifies that viscosity parameter in accretion flows is low enough to form dissipative shocks, both in presence and absence of outflows. This supports our claim that shocks could be omnipresent. It was also found that a dissipation factor of f ≻ 0.3 does not produce shocks in significant region of the flow parameter space and hence, these values are not physically relevant. Likewise, at the maximum, ∼ 13% of inflowing matter can be ejected as outflows at the CENBOL.

Nature of photon trajectories in a curved spacetime around black holes are studied without constraining their motion to any plane. Impacts of photon bending are separately scrutinized for Keplerian and CENBOL components of Two Component Advective Flow (TCAF) model. Parameters like Red shift, Bolometric Flux, temperature profile and time of arrival of photons are also computed.

Variability classes in the enigmatic black hole candidate GRS 1915+105 are known to be correlated with the variation of the Comptonizing Efficiency (CE) which is defined to be the ratio between the number of power-law (hard) photons and seed (soft) photons injected into the Compton cloud. Similarities of light curves of several variability classes of GRS 1915+105 and IGR J17091-3624, some of which are already reported in the literature, motivated us to compute CE for IGR J17091-3624 as well. We find that they are similar to what were reported earlier for GRS 1915+105, even though masses of these objects could be different.

Two radiation regimes are considered. The first one is provided by electrons moving in a magnetic field in circles with non-relativistic velocity (Cyclotron radiation). The second one is provided by electrons moving in a magnetic field on a helix with ultra-relativistic longitudinal and non-relativistic transverse velocity components. The applicability of these models is considered by the example of the object Hercules X-1. The test based on polarization observations is suggested to distinguish between the cyclotron and relativistic dipole origin of features observed in X-ray spectra of X-ray sources.

Using numerical MHD simulations, we investigated the motion of a magnetized neutron star through a non-uniform interstellar medium (ISM), the interaction of the magnetosphere with the ISM and the influence of ISM density on the shape of the magnetosphere tail.

We investigate the back-reaction of magnetic enhancement due to a plasma in orbit about a Kerr black hole. While previous work had shown that the enhancement diverges at some point outside the horizon, we see that the back-reaction reduces the enhancement to some extent. The first order correction is seen to be valid in ranges of spin and r together.

We perform two dimensional MHD Cellular Automata simulations for a two-dimensional model of a magnetized flux tube with a background flow. Self Organized Criticality is reached by the system. The lifetime distribution and energy emission as a function of time are discussed regarding an application of the model to X-Ray emission of Gamma Ray Bursts.

The solution of the system of Boltzmann equation for plasma in magnetic field, with arbitrary degenerate electrons and nondegenerate nuclei, is obtained by Chapman-Enskog method. Three functions, generalising Sonin polynomials are used for obtaining an approximal solution. Fully ionized plasma is considered. For nondegenerate and strongly degenerate plasma the asymptotic analytic formulae are obtained, which are compared with different results of previous authors. Due to kinetic approach, our results are more exact than usually used coefficients, obtained by a simplified approach for the thermal conductivity in a plasma of a neutron star crust.

We show that the removal of angular momentum is possible in the presence of large scale magnetic stresses, arisen by fields much stronger than that required for magnetorotational instability, in geometrically thick, advective, sub-Keplerian accretion flows around black holes in steady-state, in the complete absence of alpha-viscosity. The efficiency of such angular momentum transfer via Maxwell stress, with the field well below its equipartition value, could be equivalent to that of alpha-viscosity, arisen via Reynolds stress, with α = 0.01 − 0.08. We find in our simpler vertically averaged advective disk model that stronger the magnetic field and/or larger the vertical-gradient of azimuthal component of magnetic field, stronger the rate of angular momentum transfer is, which in turn may lead to a faster rate of outflowing matter, which has important implications to describe the hard spectral states of black hole sources. When the generic origin of alpha-viscosity is still being explored, mechanism of efficient angular momentum transfer via magnetic stresses alone is very interesting.

The important result of the iron core collapse simulation is the formation of the convectively unstable regions in the center and in the region of the accretion. The large region near the center suitable for the convective insatiability exists during 10 ms. In the frame of 3D hydrodynamic simulations with taking into account self-gravity I illustrate the possibility of the developing of the large-scale convection during such short time. Another important result from the viewpoint supernova explosion is the huge neutrino energy in the degenerate convective bubble about 100 MeV in compare with the thermal neutrino 10 MeV from the neutrino-sphere. Large neutrino energy is important for the explanation of the supernova explosion.

We investigate the evolution of hydromagnetic perturbations in a small section of accretion disks. It is known that molecular viscosity is negligible in accretion disks. Hence, it has been argued that Magnetorotational Instability (MRI) is responsible for transporting matter in the presence of weak magnetic field. However, there are some shortcomings, which question effectiveness of MRI. Now the question arises, whether other hydromagnetic effects, e.g. transient growth (TG), can play an important role to bring nonlinearity in the system, even at weak magnetic fields. Otherwise, whether MRI or TG, which is primarily responsible to reveal nonlinearity to make the flow turbulent? Our results prove explicitly that the flows with high Reynolds number (R_e), which is the case of realistic astrophysical accretion disks, exhibit nonlinearity by best TG of perturbation modes faster than that by best modes producing MRI. For a fixed wavevector, MRI dominates over transient effects, only at low R_e, lower than its value expected to be in astrophysical accretion disks, and low magnetic fields. This seriously questions (overall) persuasiveness of MRI in astrophysical accretion disks.

General relativity and quantum theory are believed to be incompatible, but are they? Here it is revealed that there is a logical way for these theories to be extended to a unified theory. In a fresh approach, the graviton is defined as the quantum field particle that produces the dimensions of time and space, whereby gravitational effects are one consequence of this role. The concept is explained and evidence is revealed which supports the new direction. The approach leads to the derivation of the Einstein equation of general relativity and the unification equation, which predicts that the frequency of gravitons in the void is f_X0=1.48 × 10⁴² s⁻¹. Thus the graviton is a high-energy particle, quite the opposite to expectations of current particle theory, but in keeping with its role of producing: the dimensions of time and space, vacuum energy, expansion of the Universe, and gravitational effects. The predicted frequency is accurately supported by empirical data from cosmological measurements. Observations that have been attributed to dark matter and vacuum energy, are now explained by scattering of gravitons and diffraction patterns of gravitons. Diffraction minima have reduced energy density, thus producing microscopic regions of curvature of spacetime in galactic haloes. Based of these diffraction patterns, equations are derived which accurately predict the rapid speed of orbiting bodies and explain the flatness of rotation curves. The evidence supports the extension of quantum theory and general relativity to a general quantum theory.

We investigate the cosmological implications of a class of modified gravity based on torsion. Starting from the Teleparallel Equivalent of General Relativity (TEGR), in which the gravitational field is described by the torsion tensor instead of the curvature one, we extend the Lagrangian to arbitrary functions of the torsion scalar. In such a scenario the torsional modification can describe the inflationary evolution, as well as the late-time acceleration. The whole discussion can be enlightening as to what roads one could follow in order to modify the gravitational interaction.

Cosmological inflation is discussed in the framework of F(R, G) gravity where F is a generic function of the curvature scalar R and the Gauss-Bonnet topological invariant G. The main feature that emerges in this analysis is the fact that this kind of theory can exhaust all the curvature budget related to curvature invariants without considering derivatives of R, R_μν, $R_{\sigma \mu \nu }^{\lambda }$ etc. in the action. Cosmological dynamics results driven by two effective masses (lenghts) related to the R scalaron and the G scalaron working respectively at early and very early epochs of cosmic evolution. In this sense, a double inflationary scenario naturally emerges.

Higher-derivative gravity, i.e. the system defined by General Relativity’s Lagrangian augmented by curvature-squared terms, is a renormalizable gravity model, along with its matter couplings. This model has two free parameters, α and β, which couple the higher-order terms R² and $R_{\mu \nu }^2$, respectively. In this work we study the bending of light in the framework of higher-derivative gravity utilizing both classical and semiclassical approaches. We show that the Ricci-squared sector is associated to a repulsive interaction and, at the tree-level, yields dispersive propagation of photons yet in first order. Also, a comparison between the predicted results and experimental data allows us to set an upper bound on the coupling constant β.

We review modified gravity models which provide the unification of inflation with dark energy era. Special attention is paid to F(R) gravity. Universality of gravitational alternative for accelerated expansion is indicated, as the corresponding unification maybe achieved also in modified Gauss-Bonnet gravity, non-local gravity or modified gravity with extra scalars. The occurrence of finite-time singularities at dark energy and/or inflation era is discussed. The possibility of bouncing cosmology from extended gravity is also shown. Finally, dark matter may naturally enter this scenario as third party of unification, for instance, in mimetic generalization of modified gravity.

The place and physical significance of Poincaré gauge theory of gravity (PGTG) as necessary generalization of metric gravitation theory in the framework of gauge approach to gravitation is discussed. Isotropic cosmology built in the framework of PGTG based on general expression of gravitational Lagrangian including both a scalar curvature and quadratic in the curvature and torsion invariants with indefinite parameters is considered. The most important physical consequences connected with the change of gravitational interaction, with possible existence of limiting energy density and gravitational repulsion at extreme conditions, and also with the vacuum repulsion effect are discussed. The solution of the problem of cosmological singularity and dark energy problem in the frame of isotropic cosmology built on the base of PGTG is given.

We study a cosmological model where the Einstein tensor is non-minimally coupled to the dynamics of free scalar field. Using FRW metric, we find the behavior of scale factor for vacuum, matter and dark energy dominated eras, and focus on the inflationary behavior at early universe. We study the perturbation analysis to confront the inflation under consideration with observational results.

We construct new explicit vacuum solutions of quadratic metric-affine gravity. The approach of metric-affine gravity in using an independent affine connection produces a theory with 10+64 unknowns, which implies admitting torsion and possible nonmetricity. Our spacetimes are generalisations of classical pp-waves, four-dimensional Lorentzian spacetimes which admit a nonvanishing parallel spinor field. We generalize this definition to metric compatible spacetimes with pp-metric and purely axial torsion. It has been suggested that one can interpret that the axial component of torsion as the Hodge dual of the electromagnetic vector potential. We compare these solutions with our previous results and other solutions of classical models describing the interaction of gravitational and neutrino fields.

We briefly summarize our recent results on type N and III universal spacetimes.

The aim of this talk is to demonstrate that the Hamiltonian formalism of bigravity with de Rham, Gabadadze, Tolley (dRGT) potential reduced to minisuperspace is a flexible and powerful method to invent and to study cosmological models. We discuss here only the effective coupling of matter, i.e. the minimal interaction of it to a special combination of two dynamical metrics. It is shown that all the observables can be expressed as functions of the ratio of the two scale factors.

We give a brief summary of the formalism of invariants in general scalar-tensor and multiscalar-tensor gravities without derivative couplings. By rescaling of the metric and reparametrization of the scalar fields, the theory can be presented in different conformal frames and parametrizations. Due to this freedom in transformations, the scalar fields themselves do not carry independent physical meaning (in a generic parametrization). However, there are functions of the scalar fields and their derivatives which remain invariant under the transformations, providing a set of physical variables for the theory. We indicate how to construct such invariants and show how the observables like parametrized post-Newtonian parameters and characteristics of Friedmann-Lemaître-Robertson-Walker cosmology can be neatly expressed in terms of the invariants.

We review a class of weakly non-local higher derivative theories of gravity as a step forward with Einstein’s general relativity. These theories are unitary (ghost-free) and UV-finite at quantum level in even and odd dimensions. In D = 4 the beta functions have been explicitly evaluated showing quantum scale-invariance (all the beta functions vanish). It is trivial to show finiteness in odd dimensions. Moreover these results can be easily extended to any even dimension and it is possible, that the higher-dimensional theory can be made finite too. Therefore, we have the possibility for finite and UV-complete quantum gravity in any dimension.

An interesting cosmological history was proposed by Prigogine et al. who considered the Universe as a thermodynamically open system. This scenario is characterized by a process of matter creation, which corresponds to an irreversible energy flow from the gravitational field to the pressureless matter fluid. Here, we show that the gravitationally induced particle production may arise from a nonminimal curvature-matter coupling. By considering the equivalent scalar-tensor theory, the cosmological implications of the model are discussed. As all known natural systems tend to a state of thermodynamic equilibrium, and assuming the universe is not different in this respect, we also discuss the conditions to attain the equilibrium state.

In the context of Horndeski cosmologies, we consider a dynamical adjustment mechanism able to screen any value of the vacuum energy of the matter fields leading to a fixed de Sitter geometry. Thus, we present the most general scalar-tensor cosmological models without higher than second order derivatives in the field equation that have a fixed spatially flat de Sitter critical point for any kind of material content or vacuum energy. These models allow us to understand the current accelerated expansion of the universe as the result of the evolution towards the critical point when it is an attractor.

The Born-Infeld gravity is a modified gravity motivated by the quantum gravity, which has the ability to get rid of the big bang and other singularities. We consider the Palatini formalism of the Born-Infeld gravity and investigate the FRW cosmology by using an effective potential. It is shown that the big bang singularity does not always emerge but the scale factor can be bounced. Based on this result, we also study the black hole formation for spherical dust collapse, and find that the dust ball does not always collapse into a singularity.

The detection of gravitational waves and the corresponding determination of polarization modes is a powerful tool to discriminate between general relativity and alternative theories of gravity as for instance f(R) theories. Within the framework of the linearized approach, we investigate the polarization of gravitational waves in f(R) theories in the metric formalism. Besides the usual two transverse-traceless tensor modes present in general relativity, there are in general two additional scalar ones: a massive longitudinal mode and a massless transverse mode (the so-called breathing mode). This last mode has often been overlooked in the literature, and we show why it is in general not possible to impose a traceless condition on the wave solutions. Our findings are in agreement with the results found using the Newman-Penrose formalism, thus clarifying the inconsistencies in the literature.

Observations of several peculiar, under- and over-luminous type Ia supernovae (SNeIa) argue for exploding masses widely different from the Chandrasekhar-limit. We explore the modification to Einstein’s gravity in white dwarfs for the first time in the literature, which shows that depending on the (density dependent) modified gravity parameter α, chosen for the present purpose of representation, limiting mass of white dwarfs could be significantly sub- as well as super-Chandrasekhar. Hence, this unifies the apparently disjoint classes of SNeIa, establishing the importance of modified Einstein’s gravity in white dwarfs. Our discovery questions both the global validity of Einstein’s gravity and the uniqueness of Chandrasekhar’s limit.

We discuss the most general Finsler spacetime geometry obeying the cosmological symmetry group SO(4). On this background geometry we derive the equations of motion for the most general kinetic fluid obeying the same cosmological symmetry. For this purpose we propose a set of coordinates on the tangent bundle of the spacetime manifold which greatly simplifies the cosmological symmetry generators.

A brief summary of the anholonomic frame deformation method, AFDM, for generating exact solutions with generic off–diagonal metrics and generalized nonlinear connections in modified gravity theories, MGTs, is presented. We generalize the method to systems of nonlinear partial differential equations, PDEs, for constructing solutions describing generalized (effective) Einstein-Yang-Mills-Higgs, EYMH, interactions in two measure, f(R) and massive gravity theories. Finally, we speculate on possible applications of such generalized anisotropic and inhomogeneous solutions in modern acceleration cosmology.

Here, we present preliminary results from our studies of the variation of different facets of the speed of light in the local inertial frame in Palatini formalism. In these theories, different aspects of the speed of light such as c_ST (the space-time causal structure constant) and c_EM (the electromagnetic wave velocity), do not coincide in the local inertial frame and modify standard ΛCDM model coming from General Relativity. We apply our model to available data such as Union II or Planck 2013. For instance, we can fit the SN Ia distance modulus and CMB shift parameter in the absence of Dark Energy. Also we apply our model to the comoving Hubble radius and find that our favourable model leads to a larger particle horizon at higher redshifts and hence, can solve the horizon problem without the need for inflation.

Cosmological hysteresis has interesting and vivid implications in the scenario of a cyclic bouncy universe. This, purely thermodynamical in nature, is caused by the asymmetry in the equation of state parameter during expansion and contraction phase of the universe, due to the presence of a single scalar field. When applied to variants of modified gravity models this phenomenon leads to the increase in amplitude of the consecutive cycles of the universe, provided we have physical mechanisms to make the universe bounce and turnaround. This also shows that the conditions which creates a universe with an ever increasing expansion, depend on the signature of ∮ pdV and on model parameters.

When using Einstein’s equations, there exist a number of techniques for embedding nonlinear structures in cosmological backgrounds. These include Swiss cheese models, in which spherically symmetric vacua are patched onto Friedmann solutions, and lattice models, in which weak-field regions are joined together directly. In this talk we will consider how these methods work in f(R) theories of gravity. We will show that their existence places constraints on the large-scale expansion of the universe, and that it may not always be possible to consider the Friedmann solutions and weak-field solutions of a theory independently from each other.

Current acceleration of the cosmic expansion leads to coincidence as well as fine-tuning issues in the framework of general relativity. Dynamical scalar fields have been introduced in response of these problems, some of them invoking screening mechanisms for passing local tests of gravity. Recent lab experiments based on atom interferometry in a vacuum chamber have been proposed for testing modified gravity models. So far only analytical computations have been used to provide forecasts. We derive numerical solutions for chameleon models that take into account the effect of the vacuum chamber wall and its environment. With this realistic profile of the chameleon field in the chamber, we refine the forecasts that were derived analytically. We finally highlight specific effects due to the vacuum chamber that are potentially interesting for future experiments.

We show that pure quadratic gravity with quantum loop corrections yields a viable inflationary scenario. We also show that a large family of models in the Jordan frame, with softly-broken scale invariance, corresponds to the same theory with linear inflaton potential in the Einstein frame. It follows that all these quasi scale-invariant models have the same relation between the tensor-to-scalar ratio and the scalar spectral index, which is also consistent with the current data. Thus, they form a family of attractors, which is sharply distinct from the recently discovered α-attractors of Kallosh, Linde et al.

We study the propagation of a null fluid under the modified dynamics provided by the Palatini formulation of the Born-Infeld theory of gravity. Working in a Cartesian coordinate system, we obtain exact analytical solutions for the metric tensor, which describes the gravitational wave produced by the null fluid. The result is compared with the prediction of general relativity.

We develop the parameterized post-Keplerian approach for class of analytic f(R)-gravity models. Using the double binary pulsar system PSR J0737-3039 data we obtain restrictions on the parameters of this class of f(R)-models and show that f(R)-gravity is not ruled out by the observations in strong field regime.

A fundamental problem of general relativity is its conflict with quantum physics, i.e., the difficulty putting it into the quantum shoes. On its solution we see light shed by the cosmological constant problem, which we read as a low-energy manifestation of the conflict and a signpost to the reconciliation. We expect the reconciliation between gravity and quantum physics can solve the cosmological constant problem: The former should give a low-energy or an energy-independent solution to the latter, since the latter is not only a high-energy but also a low-energy problem. This requirement provides a physical criterion for evaluating possible solutions of the reconciliation, especially at low energies. Thus, we further expect the keys to the reconciliation can be found in the well-known low-energy physics rather than hiding in unconfirmed physics beyond our reach. Along this line of thought, we discuss possible keys to the reconciliation, such as the quantum treatment of the constrained degrees of freedom of gravity, the timeless problem, and a potential mismatch between average curvature and average metric.

Effective gravitational field theories with background fields break local Lorentz symmetry and diffeomorphism invariance. Examples include Chern-Simons gravity, massive gravity, and the Standard-Model Extension (SME). The physical properties and behavior of these theories depend greatly on whether the spacetime symmetry breaking is explicit or spontaneous. With explicit breaking, the background fields are fixed and nondynamical, and the resulting theories are fundamentally different from Einstein’s General Relativity (GR). However, when the symmetry breaking is spontaneous, the background fields are dynamical in origin, and many of the usual features of Einstein’s GR still apply.

We consider singularities of static spherically symmetric objects in minimal dilatonic gravity. They are only partially studied and purely understood even in the simplest models of extended gravity. We introduce the proper form of the structure equations and derive a set of all singularities, which turn to form several types of sub-manifolds of the phase space. We also introduce for the first time the Lyapunov function for the corresponding system, its equation, and its basic properties. The dependence on the mass of the dark scalar is discussed.

The one-loop long distance quantum corrections to the Newtonian potential imply tiny but observable effects in the restricted three-body problem of celestial mechanics, i.e., both at the Lagrangian points of stable equilibrium and at those of unstable equilibrium the Newtonian values of planetoid’s coordinates are changed by a few millimetres in the Earth-Moon system. First, we find that the equations governing the position of both noncollinear and collinear quantum libration points are algebraic fifth degree and ninth degree equations, respectively. Second, we discuss the prospects to measure, with the help of laser ranging, the above departure from the equilateral triangle picture, which is a challenging task. On the other hand, a modern version of the planetoid is the solar sail, and much progress has been made, in recent years, on the displaced periodic orbits of solar sails at all libration points. By taking into account the quantum corrections to the Newtonian potential, displaced periodic orbits of the solar sail at libration points are again found to exist.

An assessment is made of recent attempts to evaluate how quantum gravity may affect the anisotropy spectrum of the cosmic microwave background. A perturbative scheme for the solution of the Wheeler-DeWitt equation has been found to allow for enhancement of power at large scales, whereas the alternative predicts a suppression of power at large scales. Both effects are corrections which, although conceptually interesting, turn out to be too small to be detected. Another scheme relies upon a Born-Oppenheimer analysis: by using a perturbative approach to the nonlinear ordinary differential equation obeyed by the two-point function for scalar fluctuations, a new family of power spectra has been obtained and studied by the authors.

The existence and stability of the Einstein static solution have been built in the Einstein-Cartan gravity. We show that this solution in the presence of perfect fluid with spin density satisfying the Weyssenhoff restriction is cyclically stable around a center equilibrium point. Thus, study of this solution is interesting because it supports non-singular emergent cosmological models in which the early universe oscillates indeterminately about an initial Einstein static solution and is thus past eternal.

In this paper I present the most recent developments in the formulation of a tomographic approach to quantum and classical cosmology. I introduce quantum tomography, in order to show how the quantum tomographical techniques can be applied to reconstruct a quantum state, then I discuss these techniques can be applied to quantum and classical cosmology. It follows that the information about the initial state of the universe should be obtained by analyzing the local deviations from the Hubble law, but more realistically one should analyse the spectrum of cosmological perturbations and the recent maps of the cosmical background radiation.

In this paper we consider the Gauss-Bonnet action in Weyl-Cartan geometry. The role of Gauss-Bonnet action in Weyl and also Cartan gravity is known for decades and we know that this combinations lead to a gravity theory plus some gauge fields. We first show that the Weyl-Cartan theory gives rise to a gravitational theory minimally coupled to two vector fields, one of which is massless and the other is massive. The parameters will be then constraint to have a healthy theory. By generalizing the action through introducing a scalar filed coupled to Gauss-Bonnet action we will show that the axion inflation model can be produced in this model. The theory predicts a coupling between the axion field and the canonical vector kinetic term as well as the usual axion-vector field coupling. We will also consider the cosmological implications of the axion model.

We modify the Galileon theory minimally coupled to general relativity by breaking the Lorentz invariance of the theory. To break the Lorentz invariance, we use the Lagrange multiplier method. The new term in the action implies that the gradient of the Galileon field is time-like, with unit norm. The theory can also be considered as an extension of the mimetic dark matter theory, by adding some derivative self interactions to the action, which keeps the equation of motion at most second order in time derivatives. Also the theory is similar to scalar Einstein-aether theory with healthy scalar aether self interactions. For pressure-less baryonic matter, we show that the universe experiences a decelerated expanding phase followed by a late time acceleration. The cosmological implications of a special coupling between the scalar field and the trace of the energy-momentum tensor are also explored.

Massive gravitational modes in effective field theories can be recovered by extending general relativity and taking into account generic functions of the curvature invariants, not necessarily linear in the Ricci scalar R. In particular, adopting the minimal extension of f(R) gravity, an effective field theory with massive modes is straightforwardly recovered. This approach allows to evade shortcomings like ghosts and discontinuities if a suitable choice of expansion parameters is performed.

Fourth-derivative gravity has two free parameters, α and β, which couple the curvature-squared terms R² and $R_{\mu \nu }^2$. Relativistic effects and short-range laboratory experiments can be used to provide upper limits to these constants. In this work we briefly review both types of experimental results in the context of higher-derivative gravity. The strictest limit follows from the second kind of test. Interestingly enough, the bound on β due to semiclassical light deflection at the solar limb is only one order of magnitude larger.

We consider generalised pp-waves with purely axial torsion, which we previously showed to be new vacuum solutions of quadratic metric-affine gravity. Our analysis shows that classical pp-waves of parallel Ricci curvature should not be viewed on their own. They are a particular representation of a wider class of solutions, namely generalised pp-waves of parallel Ricci curvature. We compare our pp-waves with purely axial torsion to solutions of Einstein-Weyl theory, the classical model describing the interaction of gravitational and massless neutrino fields.

New soliton solutions for thick branes in 4 + 1 dimensions are considered in this article. In particular, brane models based on the sine-Gordon (SG), φ⁴ and φ⁶ scalar fields are investigated; in some cases Z₂ symmetry is broken. Besides, these soliton solutions are responsible for supporting and stabilizing the thick branes. In these models, the origin of the symmetry breaking resides in the fact that the modified scalar field potential may have non-degenerate vacuua and these non-degenerate vacuua determine the cosmological constant on both sides of the brane. At last, in order to explore the particle motion in the neighborhood of the brane, the geodesic equations along the fifth dimension are studied.

Galilean Genesis is one of the alternative inflation models and originally constructed from the Galileon theory. This model has an interesting feature that our universe started from the Minkowski universe. We generalized Galilean Genesis models by using Horndeski theory. We studied the evolution of background and perturbations. We find this model solve the problems that inflation model solved in the same way. We can make a flat power spectrum of scalar perturbation by choosing the specific value of introduced parameter in this model. I will talk above and recent results.

We shall discuss equivalence of frames in Palatini f(ℛ)-theories at action level. A Palatini formulation of Brans-Dicke theories (equivalent to the purely metric ones) will also be discussed.

Cosmological rotation of linear polarization (cosmic birefringence or cosmic polarization rotation) provides an excellent probe to study new physics. We derived stringent limits on selected extensions of the Standard Model looking at the cosmological rotation of linear polarization for several datasets (Cosmic Microwave Background, Radio Galaxies, Radio Sources, Crab Nebula and Gamma-ray Bursts) corresponding at different energies for the photons and different distances of the sources.

We consider the construction of a transgression field theory invariant under the Poincare supergroup ${\mathcal{P}}_5$, modulo boundary terms. The transgression action is constructed from the lagrangian for five dimensional Chern–Simons supergravity through the introduction of two 1-forms gauge connections valuated in the corresponding Lie superalgebra. These conections are associated to linear and nonlinear realizations of the Poincaré supergroup. From a dimensional reduction of the transgression field theory we get a four dimensional gauged Wess–Zumino–Witten action whose fields are the original gauge fields of the Chern–Simons theory plus the set of tensorial and spinorials functions relating the gauge conections corresponding to the parameterized coordinates of the coset space ${\mathcal{P}}_5/SO(1,4)$ in the adjoint representation. The resulting action and its equations of motion are studied.

In this proceedings (based on Ref. 1), I will consider quantum aspects of a non-local, infinite-derivative scalar field theory - a toy model depiction of a covariant infinite-derivative, non-local extension of Einstein’s general relativity which has previously been shown to be free from ghosts around the Minkowski background. The graviton propagator in this theory gets an exponential suppression making it asymptotically free, thus providing strong prospects of resolving various classical and quantum divergences. In particular, I will find that at 1-loop, the 2-point function is still divergent, but once this amplitude is renormalized by adding appropriate counter terms, the ultraviolet (UV) behavior of all other 1-loop diagrams as well as the 2-loop, 2-point function remains well under control. I will go on to discuss how one may be able to generalize our computations and arguments to arbitrary loops.

Wormhole, domain wall, and boson star solutions supported by two ghost scalar fields are investigated within the framework of general relativity. Families of solutions are constructed numerically, depending on central values of the scalar fields. For such solutions we find masses of the objects under consideration and show the profiles of the scalar fields and of their energy density distributions.

Using exactly solvable models, it is shown that black hole singularities in different electrically charged configurations can be cured. Our solutions describe black hole space-times with a wormhole giving structure to the otherwise point-like singularity. We show that geodesic completeness is satisfied despite the existence of curvature divergences at the wormhole throat. In some cases, physical observers can go through the wormhole and in other cases the throat lies at an infinite affine distance.

In this proceedings we review a method for generating rotating wormhole solutions of the Einstein equations with ghost (phantom) matter. One of the solutions contains a ring singularity only on the south hemisphere. We analyze the geodesic motion around this solution and show that there is no geodesics touching the ring singularity of the wormhole in any way. This leads us to the famous Penrose’s cosmic censorship conjecture.

While General Relativity (GR) ranks undoubtedly among the best physics theories ever developed, it is also among those with the most striking implications. In particular, GR admits solutions which allow faster than light motion and consequently time travel. Here we shall consider a “pre-emptive” chronology protection mechanism that destabilises superluminal warp drives via quantum matter back-reaction and hence forbids even the conceptual possibility to use these solutions for building a time machine. This result will be considered both in standard quantum field theory in curved spacetime as well as in the case of a quantum field theory with Lorentz invariance breakdown at high energies. Some lessons and future perspectives will be finally discuss.

A spherically symmetric wormhole in Newtonian gravitation in curved space, enhanced with a connection between the mass density and the Ricci scalar, is presented. The wormhole inhabits a spherically symmetric curved space. The gravitational potential, gravitational field and the pressure that supports the fluid that permeates the Newtonian wormhole are computed. Particle dynamics and tidal effects in this geometry are studied.

Wormholes may arise as solutions of extensions of General Relativity without violation of the energy conditions. Working in a Palatini approach we consider classical geometries supporting such wormholes. It is shown that the resulting space-times represent explicit realizations of the concept of geon introduced by Wheeler, interpreted as self-consistent bodies generated by an electromagnetic field without sources.

We study the process of quantum tunnelling in self-interacting scalar field theories with non-minimal coupling to gravity. In these theories gravitational instantons can develop a neck – a feature prohibited in theories with minimal coupling, and describing the nucleation of geometries containing a wormhole. We also clarify the relationship of neck geometries to violations of the null energy condition.

We consider static spherically symmetric solutions in the scalar-tensor theory of gravity with a scalar field possessing the nonminimal kinetic coupling to the curvature. The lagrangian of the theory contains the term (εg^μν+ηG^μν)ϕ,μϕ,ν and represents a particular case of the general Horndeski lagrangian, which leads to second-order equations of motion. We use the Rinaldi approach to construct analytical solutions describing wormholes with nonminimal kinetic coupling. It is shown that wormholes exist only if ε = −1 (phantom case) and η > 0. The wormhole throat connects two anti-de Sitter spacetimes. The wormhole metric has a coordinate singularity at the throat. However, since all curvature invariants are regular, there is no curvature singularity there.

We find general parameterizations for generic off-diagonal spacetime metrics and matter sources in general relativity (GR) and two measure/bi-metric modified gravity theories, MGTs, when the field equations decouple with respect to nonholonomic frames of reference. This allows us to construct various classes of exact solutions when the coefficients of the fundamental geometric/physical objects depend on all spacetime coordinates via corresponding classes of generating and integration functions and/or constants. Such (modified) spacetimes display Killing and non-Killing symmetries, describe nonlinear vacuum configurations and effective polarizations of cosmological and interaction constants. Certain examples of exact locally anisotropic wormhole and generic off-diagonal cosmological solutions are analysed for MGTs of massive and/or modified f(R,T) gravity, two measure theory etc. We conclude that considering generic off-diagonal nonlinear parametric interactions in GR it is possible to mimic various effects in massive and/or modified gravity, or to distinguish certain classes of generic modified gravity solutions which cannot be encoded in GR.

We consider the characteristics of nonlinear energy conditions and of quantum extensions of these and the usual energy conditions. We show that they are satisfied by some quantum vacuum states that violate the usual energy conditions.

In the context of Gravity’s Rainbow, we compute the graviton one-loop contribution to a classical energy in a traversable wormhole background, by considering the equation of state p_r = ωρ. The investigation is evaluated by means of a variational approach with Gaussian trial wave functionals. However, instead of using a regularization/renormalization process, we use the distortion induced by Gravity’s Rainbow to handle the divergences.

We discuss rotating wormhole solutions in Einstein gravity in four and five dimensions, supported by a phantom scalar field. These solutions evolve from the corresponding static wormhole, when the throat attains a finite rotational velocity. With increasing rotational velocity the throat deforms. At a maximal value of the rotational velocity, an extremal black hole solution is encountered. The rotating Ellis wormholes acquire a finite mass. In four dimensions they possess bound orbits. In five dimensions, with both angular momenta set equal, a stability analysis shows that the unstable mode of the static wormholes solutions vanishes when the angular momentum exceeds a critical value.

So-called “Buchert averaging” is actually a coarse-graining procedure, where fine detail is “smeared out” due to limited spatio-temporal resolution. For technical reasons, (to be explained herein), “averaging” is not really an appropriate term, and I shall consistently describe the process as a “coarse-graining”. Because Einstein gravity is nonlinear the coarse-grained Einstein tensor is typically not equal to the Einstein tensor of the coarse-grained spacetime geometry. The discrepancy can be viewed as an “effective” stress-energy, and this “effective” stress-energy often violates the classical energy conditions. To keep otherwise messy technical issues firmly under control, I shall work with conformal-FLRW (CFLRW) cosmologies. These CFLRW-based models are particularly tractable, and are also particularly attractive observationally: the CMB is not distorted. In this CFLRW context one can prove some rigorous theorems regarding the interplay between Buchert coarse-graining, tracelessness of the effective stress-energy, and the classical energy conditions.

We investigate negative tension branes as stable thin shell wormholes in Reissner-Nordström-(anti) de Sitter spacetimes in d dimensional Einstein gravity. Imposing Z₂ symmetry, we construct and classify traversable static thin shell wormholes in spherical, planar (or cylindrical) and hyperbolic symmetries. In spherical geometry, we find the higher dimensional counterpart of Barceló and Visser’s wormholes, which are stable against spherically symmetric perturbations. We also find the classes of thin shell wormholes in planar and hyperbolic symmetries with a negative cosmological constant, which are stable against perturbations preserving symmetries. In most cases, stable wormholes are found with the combination of an electric charge and a negative cosmological constant. However, as special cases, we find stable wormholes even with vanishing cosmological constant in spherical symmetry and with vanishing electric charge in hyperbolic symmetry.

Gravitational and electromagnetic (EM) field of the Dirac electron is described by an ultra-extreme Kerr-Newman (KN) black hole (BH) solution which has the naked singular ring and two-sheeted topology. This space is regulated by the formation of a solitonic source which shares much in common with the known MIT- and SLAC-bag models, but has the important advantage, of being in accordance with gravitational and electromagnetic field of the external KN solution. The used field model is supersymmetric LG model based on three chiral fields forming a domain wall bubble interpolating between the external exact KN solution and a flat supersymmetric core, which regulates singular zone of the Kerr-Newman solution. We reduce Hamiltonian to a Bogomolnyi form and obtain the regular BPS-saturated solution as an oscillon with quantized angular momentum. The main features of the soliton indicate its similarity to the bag models.

We suggest a new scenario in which the Universe starts its evolution with a fractal topological structure. This structure is described by a gas of cosmological wormholes. It is shown that the polarization of such a gas in external fields possesses the spatial dispersion, which results in a modification of the Newton’s law. The dependence on scales is determined by the distribution of distances between throat ends. The observed in galaxies logarithmic correction confirms that the distribution has fractal properties. We also discus the possibility of restoring such a distribution from observations.

On a warped five-dimensional Friedmann-Lemaître-Robertson-Walker(FLRW) spacetime, dark energy can be induced by a U(1) scalar-gauge field on the brane. The standard model fields interact via the bulk Weyl tensor and causes brane fluctuations. Due to the warp factor, disturbances don’t fade away during the expansion of the universe, as is the case in 4D counterpart models. As a result, the late-time behavior could be significant deviate from the standard evolution of the universe. The effect is triggered by the time-dependent part of the warp factor with two branches. It turns out that one of the metric components becomes singular at the moment the warp factor develops a extremum. This behavior could have influence on the possibility of a transition from acceleration to deceleration or vice versa. The U(1) scalar gauge field, or self-gravitating cosmic string, builds up a huge mass per unit length in the bulk and can induce massive KK-modes felt on the brane. We find no conical residue in the late-time behavior of the space time, as is found in the 4D models.

Although the linear massive spin-2 theory is not restored to general relativity (GR) even in a massless limit, Vainshtein proposed the discontinuity can be resolved by taking into account nonlinear interactions. We consider whether the nonlinear ghost-free bigravity theory can be restored to GR both in the cosmological background and in the strong gravitational field based on a spherically symmetric configuration. In the cosmological background, the scalar graviton is stabilized by nonlinear interactions in the early stage of the Universe, thus the bigravity has the GR limit with neither a ghost instability nor a gradient instability. On the other hand, the bigravity cannot describe a static compact object beyond a critical value of gravitational field strength for a parameter space of coupling constants, at which the scalar graviton shows a singular behaviour.

We construct new finite energy regular solutions in Einstein-Yang-Mills-SU(2) theory. They are static, axially symmetric and approach at infinity the anti-de Sitter spacetime background. These configurations are characterized by a pair of integers (m, n), where m is related to the polar angle and n to the azimuthal angle, being related to the known flat space monopole-antimonopole chains and vortex rings. Generically, they describe composite configurations with several individual components, possesing a nonzero magnetic charge, even in the absence of a Higgs field. Such Yang-Mills configurations exist already in the probe limit, the AdS geometry supplying the attractive force needed to balance the repulsive force of Yang-Mills gauge interactions. The gravitating solutions are constructed by numerically solving the elliptic Einstein-DeTurck–Yang-Mills equations. The variation of the gravitational coupling constant α reveals the existence of two branches of gravitating solutions which bifurcate at some critical value of α. The lower energy branch connects to the solutions in the global AdS spacetime, while the upper branch is linked to the generalized Bartnik-McKinnon solutions in asymptotically flat spacetime.

I calculate the energy within the spherically symmetric sector of the ghost-free massive gravity theory by explicitly resolving the constraints. The energy is found to be positive for globally regular and asymptotically flat deformations of the flat space. It can also be negative and even unbounded from below, however, the corresponding solutions of the constraint equations are either not globally regular or not asymptotically flat. Therefore, the negative energy states cannot affect the positive energy sector.

We outline a geometric method of constructing generic off–diagonal and diagonal cosmological solutions of effective Einstein equations modeling modified gravity theories with two non–Riemannian volume forms and associated bimetric and/or biconnection geometric structures. Such solutions are determined by generating functions, effective sources and integration constants and characterized by nonholonomic frame torsion effects. In the physical Einstein frame, the constructions involve: (i) nonlinear re–parametrization symmetries of the generating functions and effective sources; (ii) effective potentials for the scalar field with possible two flat regions which allows unified description of locally anisotropic and/or isotropic early universe inflation related to acceleration cosmology and dark energy; (iii) there are “emergent universes” described by off–diagonal and diagonal solutions for certain nonholonomic phases and parametric cosmological evolution resulting in various inflationary phases; (iv) we can reproduce in two measure theories massive gravity effects.

Four-dimensional Einstein-Maxwell-Dilaton theory admits asymptotically flat extremal dyonic solutions for an infinite discrete sequence of the coupling constant values. The quantization condition arises as consequence of regularity of the dilaton function at the event horizon. These dyons satisfy the no-force condition and have flat reduced three spaces like true BPS configurations, but no supersymmetric embeddings are known except for some cases of lower values of the coupling sequence.

Now there are two basic observational techniques to investigate a gravitational potential at the Galactic Center, namely, a) monitoring the orbits of bright stars near the Galactic Center to reconstruct a gravitational potential; b) measuring a size and a shape of shadows around black hole giving an alternative possibility to evaluate black hole parameters in mm-band with VLBI-technique. At the moment one can use a small relativistic correction approach for stellar orbit analysis (however, in the future the approximation will not be not precise enough due to enormous progress of observational facilities) while now for smallest structure analysis in VLBI observations one really needs a strong gravitational field approximation. We discuss results of observations, their conventional interpretations, tensions between observations and models and possible hints for a new physics from the observational data and tensions between observations and interpretations. We will discuss an opportunity to use a Schwarzschild metric for data interpretation or we have to use more exotic models such as Yukawa potential, Reissner–Nordstrom or Schwarzschild–de-Sitter metrics for better fits.

We analyse archival XMM-Newton and Chandra observations of some dwarf MW satellites and characterized the X-ray source population by cross-correlating with available databases. We also investigate if intermediate-mass black holes are hosted in the center of these galaxies. In the most interesting case of UMI dwarf, we put an upper limit to the central compact object luminosity of ≃ 4 × 10³³ erg s⁻¹. As the target correlates in position also with a radio source, we estimated a black hole mass of $\simeq {2.76}_{-2.54}^{+32.00}\times {10}^6{\text{M}}_{\odot }$.

The origin of high velocity stars observed in the halo of our Galaxy is still unclear. In this work we test the hypothesis, raised by results of recent high precision N-body simulations, of strong acceleration of stars belonging to a massive globular cluster orbitally decayed in the central region of the host galaxy, where it suffers of a close interaction with a super massive black hole, which, for these test cases, we assumed 10⁸ M_⊙ in mass.

Among the sources of gravitational waves worth modelling there are Extreme Mass Ratio binaries. In order to understand the dynamics during their coalescence, we describe a special class of ballistic geodesics in Schwarzschild space-time. These orbits are in 1-1 correspondence with stable circular orbits. From these special orbits we derive analytic expressions for the source terms in the Regge-Wheeler radiative equations for a point-particle, and compute the gravitational waves emitted during the infall in an Extreme Mass Ratio black-hole binary coalescence. Results of these calculations and the waveforms obtained are then discussed, showing limits and applicability of the method, among which the recognition of a universal geodetic behaviour in the last phase of plunge.

Astrophysical black hole candidates are thought to be the Kerr black holes of general relativity, but the actual nature of these objects has still to be confirmed. The continuum-fitting and the iron line methods are currently the only available techniques to probe the spacetime geometry around these bodies and test the Kerr black hole paradigm. The continuum-fitting method is a robust approach, but the shape of the disk’s thermal spectrum is in general too simple to measure the spin and to constrain possible deviations from the Kerr solution at the same time. The iron line analysis is potentially a powerful technique, but at the moment we do not have high quality data and a robust astrophysical model.

Regular black holes are found among the exact solutions of general relativity coupled to Maxwell’s electromagnetism and electrically charged matter given by Guilfoyle. Some properties of the corresponding spacetimes are investigated.

We study non-perturbatively and self-consitently neutron and strange stars in f(R) theory of gravity, with f(R) = R + aR², the so-called R-squared gravity. Slowly rotating models as well as f-mode oscillations are thoroughly investigated. The results are compared to the general relativistic ones and the differences in the results are discussed. A high degree of equation of state independence and insensitivity to the gravitational theory of the adopted asteroseismology relations is observed.

We investigate the thermodynamic equilibrium states of a rotating thin shell, i.e., a ring, in the (2 + 1)-dimensional spacetime with a negative cosmological constant. The inner and outer regions with respect to the shell are given by pure anti-de Sitter (AdS) and the Bañados-Teitelbom-Zanelli (BTZ) spacetimes, respectively. The first law of thermodynamics of the thin shell, together with three equations of state for the pressure, the local inverse temperature and the thermodynamic angular velocity of the shell, yields the entropy of the shell, which is shown to depend only on its gravitational radii. When the shell is pushed to its own gravitational radius and its temperature is taken to be the Hawking temperature of the corresponding black hole, the entropy of the shell coincides with the Bekenstein-Hawking entropy.

We focus on Eddington-inspired Born-Infeld gravity as a modified theory of gravity, and consider the prospect for observational discriminant of it from general relativity. We are successful to show that the coupling constant in the theory can be determined in principle via the terrestrial measurements of the neutron skin thickness of ²⁰⁸Pb together with the astronomical observations of the radii of 0.5M_⊙ neutron stars.

The equilibrium and stability of anisotropic strange stars is analyzed. For this purpose, we consider that the anisotropy factor follows the equation σ = k p_r(1 − e^−λ); being k a constant, p_r the radial pressure and e^−λ a metric function. We found that the anisotropy yields considerable changes in some macroscopic properties of strange stars. Despite this, such as is determined in isotropic strange stars, the onset of the instability is marked by the maximum mass point. This indicates that stable and unstable equilibrium configurations can be recognize through the conditions dM/d_ρc > 0 and dM/d_ρc < 0, respectively.

Rapidly rotating compact objects are considered laboratories to test general relativity and theories beyond. We determined observables such as the mass, the angular momentum, the moment of inertia, or the quadrupole moment for neutron stars and black holes in dilatonic Einstein-Gauss-Bonnet theory, a theory motivated by string theory. We used several equations of state (EOS) for the neutron matter and considered the dependence of the observables on the EOS and on the Gauss-Bonnet coupling constant. While there is a considerable EOS dependence for the observables themselves, the relation between the scaled moments of inertia and the scaled quadrupole moments is almost independent of the EOS, when the scaled angular momentum is held fixed.

The Buchdahl bound states that the radius to mass ratio of a star is equal or greater than 9/4, on the assumption that the star is made of a perfect fluid, the density is a nonincreasing function of the radius and the exterior is the Schwarzschild solution. The bound is saturated by the infinite central pressure Schwarzschild interior solution. A generalization of this bound to electrically charged stars has been given by Andréasson. This Buchdahl-Andréasson bound is found through the assumption that the radial pressure plus twice the tangential pressure of the matter is less than the energy density. For zero electric charge one recovers the Buchdahl bound. A class of configurations that saturate the electrically charged Buchdahl-Andréasson bound are electrically charged shells. Another class of configurations is found here. Indeed, Guilfoyle’s electrically charged stars which have a very stiff equation of state, the Cooperstock-de la Cruz-Florides equation of state, also saturate the bound. When the electric charge is zero Guilfoyle’s stars reduce to the Schwarzschild incompressible stars.

We give a short review on recent progress in the theory of tidal deformability of a slowly spinning compact object. A rotating object immersed in a quadrupolar, electric tidal field can acquire some induced mass, spin, quadrupole, octupole and hexadecapole moments to second order in the spin. Angular momentum introduces couplings between electric and magnetic distortions and new classes of spin-induced, tidal Love numbers emerge. All tidal Love numbers of a Kerr black hole were proved to be exactly zero to first order in the spin and also to second order in the spin, at least in the axisymmetric case. The tidal Love numbers of a neutron star depend strongly on the equation of state. Spin-tidal couplings deteriorate some approximate universal relations that exist for neutron stars in the static case. For a binary system close to the merger, various components of the tidal field become relevant. Preliminary results suggest that spin-tidal couplings can introduce important corrections to the gravitational waveforms of spinning neutron-star binaries approaching the merger.

We argue that the formation of a Schwarzschild black hole via Datt-Oppenheimer-Snyder type gravitational collapse must be accompanied by a change in topology upon formation of the event horizon which physically separates matter in the interior from that of the exterior. While it is true that collapsing matter crossing the event horizon continues to fall towards the singularity of the Schwarzschild interior, this region does not in fact contain the matter originally responsible for the collapse. Rather, the latter occupies a distinct internal spacetime region with its own independent evolution. The existence of this additional component of the simplest black hole has a number of profound implications - Schwarzschild black holes are stable with constant mass; they each contain a self-contained mini-universe within their event horizons; and they live within a spacetime that is inherently double-sheeted.

New analytic solutions of the Lane-Emden equation in the presence of an anisotropic factor are derived.

A novel framework is presented that can be adapted to a wide class of generic spherically symmetric thin-shell wormholes. By using the Darmois–Israel formalism, we analyze the stability of arbitrary spherically symmetric thin-shell wormholes to linearized perturbations around static solutions. We demonstrate in full generality that the stability of the wormhole is equivalent to choosing suitable properties for the exotic material residing on the wormhole throat. As an application, we consider the thin-shell variant of the Ellis wormhole for the cases of a vanishing momentum flux and non-zero external fluxes.

A (d-1)-dimensional thin matter shell immersed in a d-dimensional Schwarzschild spacetime is studied from a thermodynamic point of view. By requiring the shell to be static and to have a well-defined local temperature, one can established final formulas for the the first law of thermodynamics. One can then choose a definite expression for the shell’s temperature. In the case the shell has the Hawking temperature and it is put at its own gravitational radius, it implies that it has the Bekenstein-Hawking entropy of a d-dimensional Schwarzschild black hole.

In this paper we study static spherically symmetric fluids by taking the anisotropic pressure into consideration. Then we obtain one new class of exact solutions for Lane-Emden equations.

The generalized McVittie solution, representing a central time-dependent mass in an expanding cosmological background, has been shown to be an exact solution of a self-gravitating subset of the Horndeski class of scalar field actions, and constitutes an important example of an analytic solution for hairy black holes. Following the analysis performed on its fixed-mass counterpart, we demonstrate that a time-dependent central mass may have a significant impact on the overall causal structure of the spacetime. The metric always has an event horizon at future cosmological time infinity in the appropriate limits, but the character of the horizon depends on the accretion and cosmological histories in the bulk. The possible limits are a black-hole horizon, a pair of black- and white-hole horizons separated by a bifurcating 2-sphere, or an entirely white-hole horizon. The last case is only possible if there is accretion onto the central mass.

We obtain exact scattering coefficients of linear field perturbations around black holes from monodromy data of the associated radial wave equation. We apply the method to Kerr-NUT-(A)dS black holes, whose wave equation is always separable, and apply the isomonodromic approach recently described by the authors in the literature. The non-trivial monodromy can be calculated by appropriate initial conditions on the isomonodromic τ-function.

We analyze the quasi-normal mode spectrum of realistic neutron stars by studying axial and polar modes. We consider different equations of state satisfying the 2M_⊙ constraint, most of them containing exotic matter. New phenomenological relations between the frequency and damping time of the modes are obtained. These new relations are independent of the equation of state and relate the quasi-normal mode spectrum to the global properties of the star. Hence they could be used to constrain the equation-of-state of neutron stars. Also, We present new universal relations between quasi-normal modes and I-LOVE-Q parameters for neutron stars including exotic mater.

A number of recent studies has focused on the implications of new physics at the Planck scale on the equilibrium of compact astrophysical objects such as white dwarf and neutron stars. Here we analyze the modification of the equilibrium configurations induced by the so-called Gravity’s Rainbow that account for Planck scale deformation of the space-time.

The junction of an interior Minkowski with an exterior non-extremal Reissner-Nordström spacetime separated by an electrical perfect fluid thin shell is studied. Using the Israel junction formalism, the properties of a perfect fluid static thin shell separating an interior Minkowski outside Reissner-Nordström spacetime are studied and the cases of pressure and tension static shells of matter appear naturally depending on the sub-region where the shell is considered.

The angular and frequency characteristics of the gravitational radiation emitted in collisions of massless particles is studied perturbatively in the context of classical General Relativity for small values of the ratio α ≡ 2r_S/b of the Schwarzschild radius over the impact parameter. The particles are described with their trajectories, while the contribution of the leading nonlinear terms of the gravitational action is also taken into account. The old quantum results are reproduced in the zero frequency limit ω ≪ 1/b. The radiation efficiency ∊ ≡ E_rad/2E outside a narrow cone of angle α in the forward and backward directions with respect to the initial particle trajectories is given by ∊ ~ α² and is dominated by radiation with characteristic frequency ω ~ O(1/r_S). The comparison with previous works and the known literature is presented.

Based on a recent proposal for the gravitational entropy of free gravitational fields, we investigate the thermodynamic properties of black hole formation through gravitational collapse in the framework of the semitetrad 1+1+2 covariant formalism. In the simplest case of an Oppenheimer-Snyder-Datt collapse we prove that the change in gravitational entropy outside a collapsing body is related to the variation of the surface area of the body itself, even before the formation of horizons. As a result, we are able to relate the Bekenstein-Hawking entropy of the black hole endstate to the variation of the vacuum gravitational entropy outside the collapsing body.

Finite-time thermodynamics provides the means to revisit ideal thermodynamic equilibrium processes in the light of reality and investigate the energetic “price of haste”, i.e. the consequences of carrying out a process in finite time, when perfect equilibrium cannot be awaited due to economic reasons or the nature of the process. Employing the formalism of geometric thermodynamics, a lower bound on the energy dissipated during a process is derived from the thermodynamic length of that process. The notion of length is hereby defined via a metric structure on the space of equilibrium thermodynamics, spanned by a set of thermodynamic variables describing the system. Since the aim of finite-time thermodynamics is to obtain realistic limitations on idealized scenarios, it is a useful tool to reassess the efficiency of thermodynamic processes. We examine its implications for black hole thermodynamics, in particular scenarios inspired by the Penrose process, a thought experiment by which work can be extracted from a rotating black hole. We consider a Kerr black hole which, by some mechanism, is losing mass and angular momentum. Thermodynamically speaking, such a process is described in the equilibrium phase space of the black hole, but in reality, it is neither reversible nor infinitely slow. We thus calculate the dissipated energy due to non-ideal finite-time effects.

Energy quantization of a black hole is usually done by the implementation of an area quantization scheme after expressing the energy as a function of area. However, this method fails to yield consistent results for local and asymptotic observers simultaneously. Here, we adopt a counter-intuitive top-down approach to construct the energy operator and its spectrum for equilibrium black hole horizons. The surface gravity appears as a parameter in the energy spectrum, rather than being quantized as a function of area. The energy spectrum explains both the observations of local and asymptotic observers. The analysis is performed on the basis of the quantum theory of black hole horizon within the realms of loop quantum gravity.

This short article reviews the recent theoretical and experimental activity on the realization of analog black holes using quantum fluids of light and the on-going quest for signatures of Hawking radiation in these systems. A brief comparison to other optical and condensed-matter analog models is made.

We discuss the main features and our ongoing of work in the field on analogue gravity in photon fluids.

We discuss the quantization of a spherical dust shell in a rigorous manner. Classically, the shell can collapse to form a black hole with a singularity. In the quantum theory, we construct a well-defined self-adjoint extension for the Hamilton operator. As a result, the evolution is unitary and the singularity is avoided. If we represent the shell initially by a narrow wave packet, it will first contract until it reaches the region where classically a black hole would form, but then re-expands to infinity. In a way, the state can be interpreted as a superposition of a black hole with a white hole.

We summarize recent results by the authors in Refs. 1, 2, 3 on the extraction of scattering amplitudes for scalar fields in Kerr/Kerr-de Sitter backgrounds. Analytical, closed forms are found in terms of the Painlevé V and VI transcendents for generic values of the physical parameters.

In this work we present an overview on the absorption features of Bardeen regular black holes In particular, we compute the absorption cross section of planar massless scalar waves – for arbitrary frequencies – showing that it approaches the area of the black hole in the small-frequency regime and the capture cross section in the high-frequency regime. We also compare with the case of Reissner-Nordström black holes.

An exact correspondence is shown between a new moving mirror trajectory in (1+1)D and a spacetime in (1+1)D in which a black hole forms from the collapse of a null shell. It is shown that the Bogolubov coefficients between the “in” and “out” states are identical and the exact Bogolubov coefficients are displayed. Generalization to the (3+1)D black hole case is discussed.

There is an exact correspondence between the most simple solution to Einstein’s equations describing the formation of a black hole and a particular moving mirror trajectory. In both cases the Bogolubov coefficients in 1+1 dimensions are identical and can be computed analytically. Particle creation is investigated by using wave packets. The entire particle creation history is computed, incorporating the early-time non-thermal emission due to the formation of the black hole (or the early-time acceleration of the moving mirror) and the evolution to a Planckian spectrum.

We reexamine the scattering coefficients of shallow water waves blocked by a stationary counter current over an obstacle. By considering series of background flows, we show that the most relevant parameter is F_max, the maximal value of the ratio of the flow velocity over the speed of low frequency waves. For subcritical flows, i.e., F_max < 1, there is no analogue Killing horizon and the mode amplification is strongly suppressed. Instead, when F_max ≿ 1.1, the amplification is enhanced at low frequency and the spectrum closely follows Hawking’s prediction. We further study subcritical flows close to that used in the Vancouver experiment. Our numerical analysis suggests that their observation of the “thermal nature of the mode conversion” is due to the relatively steep slope on the upstream side and the narrowness of the obstacle.

Current proposals for regularizing the classical singularity of black holes present long-lived trapping horizons, with enormous inaccessible evaporation lifetimes. We propose an alternative regularization model, inspired in condensed matter gravitational analogues, in which the collapse of a stellar object would result in a genuine time-symmetric bounce. In geometrical terms this amounts to the connection of a black-hole geometry with a white-hole geometry in a regular manner. The complete bouncing geometry is a solution of standard classical general relativity everywhere except in a transient region that necessarily extends beyond the gravitational radius. The duration of the bounce as seen by external observers is very brief. This motivates the search for new forms of stellar equilibrium.

It is (or should be) well-known that the Hawking flux that reaches spatial infinity is extremely sparse, and extremely thin, with the Hawking quanta, one-by-one, slowly dribbling out of the black hole. The typical time between quanta reaching infinity is much larger than the timescale set by the energy of the quanta. Among other things, this means that the Hawking evaporation of a black hole should be viewed as a sequential cascade of 2-body decays.

We discuss the imprints of the initial quantum state, describing a matter chunk thrown into a black hole, contained in the the black hole radiation. We show that such a matter, described through non vacuum states in QFT, leads to distortions of the Hawking radiation from being exactly thermal. Using these distortions, we can uncover a specific amount of information about the initial state apart from its classical charges. For a large class of initial states, some specific observables defined in the initial Hilbert space are completely determined from the resulting final spectrum. We identify the class of instates which can be fully reconstructed from the information contained in the distortions at the semiclassical level.

This work shows that the homogeneous black string and black p-branes of Gauss-Bonnet theory in ten dimensions are unstable. The perturbation that we consider is spherically symmetric and it is characterized by a total momentum k along the extended directions. There is a critical wavelength above which the instability is triggered, as it is the case for black strings and black p-branes in General Relativity. The master equation is solved by power series. We observe that the critical wavelength increases with the number of extended directions p, while the maximum exponential growth decreases as p increases.

We wish to universalize the kinematic property of Einstein gravity in 3 dimension for all odd dimensions. This uniquely picks out pure Lovelock gravitational equation involving only one Nth order term. This we do by defining a Lovelock analogues of Riemann and Ricci curvatures and then show that in all odd critical, d = 2N +1 dimensions, Lovelock Riemann is entirely given in terms of Ricci.

We study the generalization of the Kerr-Newmann black hole in 5D Einstein-Maxwell-Chern-Simons theory with free Chern-Simons coupling parameter. These black holes possess equal magnitude angular momenta and an event horizon of spherical topology. We focus on the extremal case with zero temperature. We find that, when the Chern-Simons coupling is greater than two times the supergravity case, new branches of black holes are found which violate uniqueness. In particular, a sequence of these black holes are non-static radially excited solutions with vanishing angular momentum. They approach the Reissner-Nordström solution as the excitation level increases.