International Journal of Modern Physics D

In this paper, we investigate, through a Bayesian study, the ability of a local low matter density ΩM, in discrepancy with the value usually inferred from the CMB angular power spectrum, to accommodate observations from local probes without being in tension with the local values of the Hubble constant H0 or the matter fluctuation σ8 parameters. For that, we combine multiple local probes, with the criteria that they either can constrain the matter density parameter independently from the CMB constraints, or can help in doing so after making their relevant observations more model independent by relaxing their relevant calibration parameters. We assume however, either a dynamical dark energy model, or the standard ΛCDM model, when computing the corresponding theoretical observables. We also add the latest Baryonic Acoustic Oscillations (BAO) measurements from the DESI year one release to some of our MCMC runs. We found that, within ΛCDM model, for different combinations of our probes, we can accommodate a low matter density along with the H0 and σ8 values usually obtained from local probes, providing we promote the sound drag rs component in BAO calculations to a free parameter, and that even if we combine with the Pantheon + Supernova sample, an addition that was found in previous work to mitigate our concordance. Assuming w0waCDM, we also found that relaxing rs allow us to accommodate ΩM, H0 and σ8 within their local values, with still however a preference for w0wa values far from ΛCDM. However, when including Pantheon+ Supernova sample, we found that the latter preference for high matter density pushes σ8 to values much smaller than the ones usually obtained from local probes, mitigating by then a low matter density solution to the two common tensions. We conclude therefore that a low matter density value helps in preserving the concordance within ΛCDM model, even with recent BAO DESI measurements or extended to high redshift supernovae sample, while the dynamical dark energy model ultimately fails to solve all tensions at once within our low density matter hypothesis.

In this paper, we explore the well-known mass deficit/surplus phenomenon in General Relativity to suggest that it could play a part in the dark matter conundrum. Specifically in collapses and condensations of matter associated with negative intrinsic curvature of the foliation associated with the asymptotic boundary conditions, the external (ADM) mass can vastly exceed the integrated local energy over the internal volume. This can be phrased in terms of a deficit of volume for a given surface area (with respect to zero curvature). We explore the phenomenon in the context of generalizations of the Oppenheimer–Snyder models and other “cut and paste” models, the Lemaitre–Bondi–Tolman metric and several others. We produce constructions where the internal object is contracting or expanding, has a life time different from the asymptotic Universe, as well as a volume different than the excavated volume from the Universe. These are purely relativistic constructions and they could play a role in the puzzle of dark matter: attraction without visible or indeed any matter.

In this paper, we argue that black holes microstates leave an imprint on the gravitational vacuum through their virtual fluctuations. This imprint yields a power law fall off — rather than an exponential fall off — for the entanglement of planck scale fluctuations at different points. These entanglements generate an extra energy when space stretches too fast, since causality prevents a relaxation of these entanglements to their vacuum values. We obtain semiclassical dynamics for slow processes like star formation, but a radical departure from semiclassicality when a black hole horizon forms even though curvatures remain low everywhere. This resolution of the information puzzle also implies an extra energy source at the scale of the cosmological horizon, which may explain the mysteries of dark energy and the Hubble tension.

The recent fit of cosmological parameters by the Dark Energy Spectroscopic Instrument (DESI) collaboration will have a significant impact on our understanding of the universe. Given its importance, we conduct several consistency checks and draw conclusions from the fit. Specifically, we focus on the following key issues relevant to cosmology: (i) the acceleration of the universe’s expansion, which, according to the fit, differs over cosmological time compared to the standard cosmological model; (ii) the age of the universe, which appears slightly shorter than the age of the oldest stars; and (iii) the solution of the scale factor, both numerically and in an approximate analytical form.

Ground-based gravitational wave (GW) detectors primarily target several types of astrophysical sources. These include coalescence of compact binaries, such as stellar binary black holes, binary neutron stars, and neutron star-black hole binaries, as well as the stochastic GW background from numerous unresolved inspiralling and merging compact binaries across the universe. Other potential sources, such as core-collapse supernovae and rapidly rotating, nonspherical pulsars, are also anticipated, although their GW signals remain undetected. To date, the Laser Interferometer Gravitational wave Observatory (LIGO), Virgo and KAGRA (LVK collaboration) have successfully identified over 200 compact binary mergers. The GW signals from these mergers enable precise measurements of various system parameters, including distances, component masses, total masses, and spin magnitudes. Population studies of these mergers offer valuable insights into their formation mechanisms and evolution pathways. They also provide stringent constraints on fundamental physics such as neutron star equation of state and physical processes such as common envelope evolution that occur during their formation. Mergers involving neutron components can produce short Gamma-Ray burst and/or kilonova phenomena that can be observable through time-domain surveys and follow-up electromagnetic observations. When combined with the electromagnetic counterparts (if detected) and/or the information on host galaxies, these GW events can serve as bright or dark standard sirens to measure cosmological parameters with high precision. In this paper, we provide a brief review of the current status of compact binary coalescence studies and the detection of compact binary mergers by ground-based GW detectors. We focus on the formation and evolution of GW sources and discuss future prospects for development in this field.

For several independent reasons, the idea that notorious sources of entropy could exist in the Universe has been recently revived. By taking advantage of a new framework accounting for nonequilibrium processes in cosmology, we explicitly investigate the cosmological dynamics as a function of the entropy production, focusing on the stability of the system. An exhaustive investigation is performed. As the main physical conclusion, we show that for a wide class of entropy source terms, the fluid dynamics converges toward an effective cosmological constant. Constraints on the associated entropic force are also obtained.

In this paper, we consider the status of the classification of the vacuum, stationary and asymptotically flat black holes in scalar–tensor gravity. Contrary to the similar problem in general relativity, the black hole classification in scalar–tensor theories is much more challenging due to the very complicated character of the field equations and the very complex mathematical structure of the scalar–tensor gravity as a whole. We review most of the known no-hair results, and where possible new ones, as well demonstrate some of the difficulties that appear in our attempts to classify the black holes within scalar–tensor gravity. The proofs of the theorems and the underlying mathematical techniques are given in sufficient detail. To make the review self-contained, we also present the vacuum black hole uniqueness theorems in general relativity and their proofs.

It has been now widely recognized that gravitational lensing is an indispensable research method in various areas of astronomy. In our previous review, we gave a pedagogical introduction of the subject and some applications in cosmology by that time. Since then, progresses in observations as well as theories of gravitational lensing have been remarkable and various works have been conducted. In this review, we highlight some of these developments.

In this paper, we study the properties of the gravastar, which is supported by Chaplygin gas. It is well known that Gravastar is a very lucrative alternative for black holes that do not suffer from the pathology of singularity. We have used a Chaplygin gas for the interior and shell as it can take the form of dark energy and a stiff matter phase. We also discussed the associated scalar field from which the Chaplygin gas can be derived. We find gravastar solutions in the f(Q,T) framework. We have also given physical motivation to how the Chaplygin gas can form during the stars’ collapse from residue scalar fields. We have calculated and plotted all the relevant physical properties, like proper length, entropy, energy, etc., for such a gravastar. We have also used the Israel junction condition to find the potential across the thin shell with respect to various regular black holes like Bardeen, Hayward, and holonomy-corrected black holes. We have calculated the speed of sound and hence commented on the stability of the gravastar for all four cases. We have also calculated the deflection angle for all such exteriors and given phenomenological predictions of how this model can be tested via next-generation radio telescopes. Finally, we conclude this paper by pointing out the future scope of our research.

The continual production of gravitons during inflation endows loop corrections with secular logarithms which grow nonperturbatively large during a prolonged period of inflation. The physics behind these effects is reviewed, along with a catalog of the examples which have so far been found. Resummation can be accomplished by combining a variant of Starobinsky’s stochastic formalism with a variant of the renormalization group. The issue of gauge independence is also addressed.

It is known that in the presence of Euclidean wormholes the vacuum state is unstable, which, in general, leads to a number of phase transitions in the early Universe. According to the Kibble scenario during the phase transitions, defects of the domain-wall-type were formed. The specific feature of Euclidean wormholes is that a part of defects possesses negative energy and violates the energy conditions. The macroscopic character of such defects allows already the formation of a stable Lorentzian wormhole. This gives reason to consider relic wormholes as realistic astrophysical objects.

In this paper, we revisit a variational principle introduced by Padmanabhan for describing gravitation using a field action composed solely of a boundary term. We demonstrate that this procedure can also be applied to derive Maxwell’s and Yang–Mills equations.

The inability of sound disturbances to escape from a fluid moving faster than sound is known as an acoustic black hole. These black holes either create or encourage this radiation, which is called phono-Hawking. On the other hand, the event horizon is a sound black hole’s boundary. Our goal is to use the spatial–spatial quatification provided by inspired noncommutativity to observe the quantum influence on sonor black hole features. The stuff characterizing the fluid is affected by such a distortion.

The purpose of this review is to provide a pedagogical development of the Unruh effect and the thermofield double state. In Sec. 2, we construct Rindler spacetime and analyze the perspective of an observer undergoing constant acceleration in Minkowski spacetime, which motivates the establishment of the relationship between the Fourier modes in both geometries using the Bogoliubov–Valatin transformation. In Sec. 3, we explore the underlying physics leading to the Unruh effect, its analogy with the thermal radiation observed around a Schwarzschild black hole, and its manifestation through the coupling of a particle detector to the scalar field. Finally, in Sec. 4, we derive the thermofield double state by conducting a Euclidean analysis of the field and geometry.