International Journal of Modern Physics D

We consider three fundamental issues in quantum gravity: (a) the black hole information paradox (b) the unboundedness of entropy that can be stored inside a black hole horizon (c) the relation between the black hole horizon and the cosmological horizon. With help from the small corrections theorem, we convert each of these issues into a sharp conflict. We then argue that all three conflicts can be resolved by the following hypothesis: the vacuum wave functional of quantum gravity contains a “vecro” component made of virtual fluctuations of configurations of the same type that arise in the fuzzball structure of black hole microstates. Further, if we assume that causality holds to leading order in gently curved spacetime, then we must have such a vecro component in order to resolve the above conflicts. The term vecro stands for “Virtual Extended Compression-Resistant Object”, and characterizes the nature of the vacuum fluctuations that resolve the puzzles. It is interesting that puzzle (c) may relate the role of quantum gravity in black holes to observations in the sky.

The recent monumental detection of gravitational waves by LIGO, the subsequent detection by the LIGO/VIRGO observatories of a binary neutron star merger seen in the gravitational wave signal $GW170817$, the first photo of the event horizon of the supermassive black hole at the center of Andromeda galaxy released by the EHT telescope and the ongoing experiments on Relativistic Heavy Ion Collisions at the BNL and at the CERN, demonstrate that we are witnessing the second golden era of observational relativistic gravity. These new observational breakthroughs, although in the long run would influence our views regarding this Kosmos, in the short run, they suggest that relativistic dissipative fluids (or magnetofluids) and relativistic continuous media play an important role in astrophysical-and also subnuclear-scales. This realization brings into the frontiers of current research theories of irreversible thermodynamics of relativistic continuous media. Motivated by these considerations, we summarize the progress that has been made in the last few decades in the field of nonequilibrium thermodynamics of relativistic continuous media. For coherence and completeness purposes, we begin with a brief description of the balance laws for classical (Newtonian) continuous media and introduce the classical irreversible thermodynamics (CIT) and point out the role of the local-equilibrium postulate within this theory. Tangentially, we touch the program of rational thermodynamics (RT), the Clausius–Duhem inequality, the theory of constitutive relations and the emergence of the entropy principle in the description of continuous media. We discuss at some length, theories of non equilibrium thermodynamics that sprang out of a fundamental paper written by Müller in 1967, with emphasis on the principles of extended irreversible thermodynamics (EIT) and the rational extended irreversible thermodynamics (REIT). Subsequently, after a brief introduction to the equilibrium thermodynamics of relativistic fluids, we discuss the Israel–Stewart transient (or causal) thermodynamics and its main features. Moreover, we introduce the Liu–Müller–Ruggeri theory describing relativistic fluids. We analyze the structure and compare this theory to the class of dissipative relativistic fluid theories of divergent type developed in the late 1990 by Pennisi, Geroch and Lindblom. As far as theories of nonequilibrium thermodynamics of classical media are concerned, it is fair to state that substantial progress has been made and many predictions of the extended theories have been placed under experimental scrutiny. However, at the relativistic level, the situation is different. Although the efforts aiming to the development of a sensible theory (or theories) of nonequilibrium thermodynamics of relativistic fluids (or continuous media) spans less than a half-century, and even though enormous steps in the right direction have been taken, nevertheless as we shall see in this review, still a successful theory of relativistic dissipation is lacking.

In modern cosmology, the precision of the theoretical prediction is increasingly required. In cosmological $N$-body simulations, the effect of higher-order Lagrangian perturbation on the initial conditions appears in terms of statistical quantities of matter density field. We have considered the effect of third-order Lagrangian perturbation theory (3LPT) on the initial conditions, which can be applied to Gadget-2 code. Then, as statistical quantities, non-Gaussianity of matter density field has been compared between cases of different order perturbations for the initial conditions. Then, we demonstrate the validity of the initial conditions with second-order Lagrangian perturbation theory (2LPT).

We have considered a spatially flat, homogeneous and isotropic FLRW universe filled with a single fluid, known as logotropic dark fluid (LDF), whose pressure evolves through a logarithmic equation-of-state. We use the recent Pantheon SNIa and cosmic chronometer datasets to constrain the parameters of this model, the present fraction of DM ${\mathrm{\Omega }}_{m0}$ and the Hubble constant $H_0$. We find that the mean values of these parameters are ${\mathrm{\Omega }}_{m0}=0.288\pm 0.012$ and $H_0=69.652\pm 1.698\mathrm{\text{km/s/Mpc}}$ at the $1\sigma$ CL. We also find that the LDF model shows a smooth transition from the deceleration phase to acceleration phase of the universe in the recent past. We note that the redshift of this transition $z_t=0.706\pm 0.048$ ($1\sigma$ error) and is well consistent with the present observations. Interestingly, we find that the universe will settle down to a $\mathrm{\Lambda }$CDM model in future and there will not be any future singularity in the LDF model. Furthermore, we notice that there is no significant difference between the LDF and $\mathrm{\Lambda }$CDM models at the present epoch, but the difference (at the percent level) between these models is found as the redshift increases. We have also studied the generalized second law of thermodynamics at the dynamical apparent horizon for the LDF model with the Bekenstein and Viaggiu entropies.

The notions of two-dimensional area, Killing fields and matter flux are introduced in the setting of causal fermion systems. It is shown that for critical points of the causal action, the area change of two-dimensional surfaces under a Killing flow in null directions is proportional to the matter flux through these surfaces. This relation generalizes an equation in classical general relativity due to Ted Jacobson setting of causal fermion systems.

The purpose of this work is to examine the cosmic evolution in the presence of collisional matter (CM) with and without radiations in a modified Teleparallel theory involving a generic function $f(T,B)$ which depends on the scalar torsion $T$ and the boundary term associated to the divergence of torsion $2{\nabla }_{\mu }{T}^{\mu }=B$. We select seven novel $f(T,B)$ models including power law, logarithmic models and exponential models, some of these reported in [S. Bahamonde, M. Zubair and G. Abbas, Phys. Dark Univ. 19 (2018) 78; S. Bahamonde and S. Capozziello, The Eur. Phys. J. C. 77 (2017) 107; C. Escamilla-Rivera and J. L. Said, Class. Quantum Grav. 37 (2020) 165002] and discuss the evolutionary scenario. The behavior of deceleration parameter $q(z)$, Hubble parameter $H(z)$, Equation-of-state (EoS) for dark energy (DE) and effective EoS is presented. $\mathrm{\Lambda }$CDM epoch and crossing of phantom divide line (approaching to phantom era) is observed in scenarios like noncollisional matter (NCM) with radiation, CM with and without radiation. Results are found to be adequate with recent cosmic observations.

In this work, we present a few simple cosmological models under the modified theory of gravity in the particular form of $f(R,{𝒯})=R+2f({𝒯})$, where $R$ is the Ricci Scalar and ${𝒯}$ is the trace of the energy–momentum tensor. Two special cosmological models are studied with (i) hyperbolic scale factor and (ii) specific form of the Hubble parameter. The models are observed to predict relevant cosmological parameters closer to the observational values. Both the models reduce to overlap with the $\mathrm{\Lambda }$CDM model at late times. We have discussed some interesting results related to wormhole solutions as evolved from our model. The possible occurrence of Big Trip in wormholes for the models are also discussed.

In this paper, the thermodynamics of Reissner–Nordström-anti de Sitter black hole surrounded by quintessence is studied and the impact of the Gauss–Bonnet term is measured. The modified entropy, first law of thermodynamics and corresponding Smarr relation are derived due to the combined action of the Gauss–Bonnet term and quintessence fluid. We study the so-called black hole chemistry from the analysis of the corresponding equation-of-state, conjugate potential and the critical points in the extended phase space. To study the phase transitions, we plotted $P-v$, $T-v$ and $G-T$ diagrams and analyzed the conditions for the coexistence of phases.

Nonlinear electrodynamics with two-dimensional parameters is studied. The range of electromagnetic fields, when principles of causality, unitarity and the classical stability hold, are obtained. A singularity of the electric field at the center of charges is absent within our model and there are corrections to the Coulomb law as $r\to \infty$. The universe inflation takes place in the background of stochastic magnetic fields. The second stage of the universe evolution is the radiation era so that the graceful exit exists. We estimated the spectral index, the tensor-to-scalar ratio, and the running of the spectral index that are in a rough accordance with the PLANCK and WMAP data.

In this paper, we study the vacuum bosonic currents in the geometry of a compactified cosmic string in the background of the de Sitter spacetime. The currents are induced by magnetic fluxes, one running along the cosmic string and another one enclosed by the compact dimension. To develop the analysis, we obtain the complete set of normalized bosonic wave functions obeying a quasiperiodicity condition. In this context, we calculate the azimuthal and axial current densities and we show that these quantities are explicitly decomposed into two contributions: one originating from the geometry of a straight uncompactified cosmic string and the other induced by the compactification. We also compare the results with the literature in the case of a massive fermionic field in the same geometry.

The standard $\mathrm{\Lambda }$CDM model of cosmology is formulated as a simple modified gravity coupled to a single scalar field (“darkon”) possessing a nontrivial hidden nonlinear Noether symmetry. The main ingredient in the construction is the use of the formalism of non-Riemannian spacetime volume-elements. The associated Noether conserved current produces stress–energy tensor consisting of two additive parts — dynamically generated dark energy and dark matter components noninteracting among themselves. Noether symmetry breaking via an additional scalar “darkon” potential introduces naturally an interaction between dark energy and dark matter. The correspondence between the $\mathrm{\Lambda }$CDM model and the present “darkon” Noether symmetry is exhibited up to linear order with respect to gravity-matter perturbations. With the Cosmic Chronometers (CC) and the Redshift Space Distortion (RSD) datasets, we study an example for the “darkon” potential that breaks the Noether symmetry and we show that the preservation of this symmetry yields a better fit.

In this paper, we investigate the expected constraints on the Hubble constant from the gravitational-wave standard sirens, in a cosmological-model-independent way. In the framework of the well-known Hubble law, the GW signal from each detected binary merger in the local universe ($z<0.10$) provides a measurement of luminosity distance $D_L$ and thus the Hubble constant $H_0$. Focusing on the simulated data of gravitational waves from the third-generation gravitational wave detector (the Einstein Telescope, ET), combined with the redshifts determined from electromagnetic counter parts and host galaxies, one can expect the Hubble constant to be constrained at the precision of $\sim 10^{-2}$ with 20 well-observed binary neutron star (BNS) mergers. Additional standard-siren measurements from other types of future gravitational-wave sources (NS-BH and BBH) will provide more precision constraints of this important cosmological parameter. Therefore, we obtain that future measurements of the luminosity distances of gravitational waves sources will be much more competitive than the current analysis, which makes it expectable more vigorous and convincing constraints on the Hubble constant in a cosmological-model-independent way.

We propose that the transient vacuum fluctuations of quantum fields can be represented by transient vacuum quanta where both the lifetime and spatial extent are given by the Heisenberg Uncertainty Principle, HUP. Accordingly, a quantum field fluctuation at any spatial site results in the creation of a transient HUP-limited vacuum pair, corresponding to the particle and the antiparticle associated with that quantum field. All HUP-limited vacuum pairs, both massive and massless, are found to be on-shell particles, and are indistinguishable from each other except for differences related to their exchange symmetry. We find that the net vacuum energy density of all HUP-limited vacuum pairs is identically zero at any energy where both bosonic and fermionic transient vacuum pairs are present. The net vacuum energy density, from zero energy to the Planck limit, is found to be proportional to that of the rest mass of the lightest fermion, the lowest mass neutrino eigenstate. This net energy density is equal to the observed dark energy density if the mass of the lightest neutrino eigenstate is 3.2meV. The model predicts the absolute values of the masses of the three neutrino mass eigenstates and of the three neutrino flavor eigenstates for both normal and inverted ordering. The sum of the neutrino masses is found to be below the Planck satellite upper limit of 120meV, and the neutrino mass sum for normal ordering is below the most stringent upper bound of 78meV, indicating a preference for normal ordering.

In this paper, we find a four-dimensional metric for a large black hole immersed in dark matter. Specifically, we look for and find a static spherically symmetric black hole solution to the Einstein equations which gives, in the Newtonian limit, the rotation curves of galaxies, including the flat region and the baryonic Tully–Fisher relation, and which has a regular horizon. We obtain as well the energy–momentum tensor of the dark matter sourcing this spacetime and it turns, in special, to satisfy the four energy conditions (dominant, weak, null and strong) everywhere outside the horizon. This black-hole-dark-matter system represents a successful simplified model for galaxies, opens a new area for exploring the relativistic regime of dark matter, and shows that the theory of General Relativity together with dark matter can account for the rotation curves of galaxies.

In this paper, we investigate the combined effects of the cloud of strings and quintessence on the thermodynamics of a Reissner–Nordström–de Sitter black hole. Based on the equivalent thermodynamic quantities considering the correlation between the black hole horizon and the cosmological horizon, we extensively discuss the phase transitions of the spacetime. Our analysis proves that similar to the case in AdS spacetime, second-order phase transitions could take place under certain conditions, with the absence of first-order phase transition in the charged de Sitter (dS) black holes with cloud of string and quintessence. The effects of different thermodynamic quantities on the phase transitions are also quantitatively discussed, which provides a new approach to study the thermodynamic qualities of unstable dS spacetime. Focusing on the entropy force generated by the interaction between the black hole horizon and the cosmological horizon, as well as the Lennard–Jones force between two particles, our results demonstrate the strong degeneracy between the entropy force of the two horizons and the ratio of the horizon positions, which follows the surprisingly similar law given the relation between the Lennard–Jones force and the ratio of two particle positions. Therefore, the study of the entropy force between two horizons is not only beneficial to the deep exploration of the three modes of cosmic evolution, but also helpful to understand the correlation between the microstates of particles in black holes and those in ordinary thermodynamic systems.