This paper presents the Horizon Model (HM) of cosmology, designed to resolve the cosmological constant problem by equating the vacuum energy density with that of the observable universe. Grounded in quantum information theory, HM proposes the first element of reality emerging from the Big Bang singularity as a Planck-sized qubit. The model views the Big Bang as the opening of a white hole, with spacetime

In this work, we explore the de Broglie-Bohm quantum cosmology for a stiff matter, \(p=\rho \), anisotropic n-dimensional Universe. One begins by considering a Gaussian wave function for the Universe, which depends on the momenta parameters \(q_1\) and \(q_2\), in addition to the dispersion parameters \(\sigma _1\) and \(\sigma _2\). Our solutions show that the extra dimensions are stabilized through

In previous papers of this series we analysed the reduced phase space approach to perturbations of Einstein–Maxwell theory to second order around spherically symmetric backgrounds in the Gullstrand Painlevé gauge and confirmed consistency with previous approaches. In the analysis, we defined the gauge invariant variables non-perturbatively and took backreaction effects explicitly into account. In this

The remarkable connection between black holes and thermodynamics provides the most significant clues that we currently possess to the nature of black holes in a quantum theory of gravity. The key clue is the formula for the entropy of a black hole. I briefly review some recent work that provides an expression for a dynamical correction to the entropy of a black hole and briefly discuss some of the

The manner in which one approaches the interface between gravitation and quantum theory is influenced by one’s posture regarding quantum mechanics and the issues that revolve about its interpretational problems. We discuss here the way in which these issues occur in the inflationary cosmology setting, the serious confusions that ensue, and one path that we have used to deal with the problem which perhaps

In this work, we introduce a new fractional derivative that modifies the conventional Riemann-Liouville operator to obtain a set of fractional Einstein field equations within a 2+1 dimensional spacetime by assuming a static and circularly symmetric metric. The main reason for introducing this new derivative stems from addressing the divergence encountered during the construction of Christoffel symbols

Numerical evolutions of a system of equations close to null infinity in the geometric background of the inversion-Minkowski spacetime are performed. The evolved equations correspond to the linearisation of a second order metric formulation of the conformal Einstein field equations (CEFEs). These linear equations were first presented in [1] and the main purpose of this paper is to illustrate, through

Thanks to the rapid progress in gravitational wave astronomy/cosmology, primordial black holes (PBHs) have become one of the hot topics in cosmology. It has made projections of detecting signatures of PBHs feasible. In parallel, it has become clear that there exist a number of ways to produce PBHs from inflation. In this talk, I will review the PBH formation from inflation and the associated gravitational

In the present study, we deal with the cosmological evolution under autonomous and non-autonomous dynamical systems in the framework of covariant f(T) gravity where T is the scalar torsion. This work first investigates how the dynamical system survey can provide significant explanation near the cosmological finite-time singularities and then extracts the corresponding f(T) models. By studying the de

We compute the renormalized expectation value of the stress-energy tensor operator for a quantum scalar field propagating on three-dimensional global anti-de Sitter space-time. The scalar field has general mass and nonminimal coupling to the Ricci scalar curvature, and is subject to Dirichlet, Neumann or Robin boundary conditions at the space-time boundary. We consider both vacuum and thermal states

In this numerical work, we deal with two distinct problems concerning the propagation of waves in cosmological backgrounds. In both cases, we employ a spacetime foliation given in terms of compactified hyperboloidal slices. These slices intersect , so our method is well-suited to study the long-time behaviour of waves. Moreover, our construction is adapted to the presence of the time–dependent scale

In this paper, we study the tunneling of fermion particle for slowly rotating Kerr–Newman–de Sitter-like (KNdS-like) black hole in bumblebee gravity model by applying the generalized uncertainty principle (GUP). The Hawking temperature of slowly rotating KNdS-like black hole has been modified under GUP. The quantum gravity effect reduces the rise of Hawking temperature of slowly rotating KNdS-like

Personal reflections (this is not a scholarly history but my own memories and work on this topic). I apologize beforehand to everyone whose work I do not mention. Note that there is lots of such work, much brilliant. [This document is a transcription of the slides used at the conference, and as a result is rather rough as a document.]

Using the restricted phase space (RPST) formalism, we perform a comparative study of 4D dyonic AdS black hole thermodynamics in Gibbs–Boltzmann statistics and Rényi statistics. In RPST formalism, instead of pressure and volume, one considers central charge C and chemical potential \(\mu \) as thermodynamic variables. Inclusion of the magnetic charge \(\tilde{Q}_m\) gives rise to a richer phase structure

A brief overview of the “Black hole spectroscopy program” status is presented. Albeit given from a personal angle, it constitutes an attempt to convey the impressive progress achieved within the field in the last few years. Modeling and observational aspects are touched upon, although both from an observationally-oriented perspective. Particular emphasis is given to recent advancements within general

General relativity, as a diffeomorphism-invariant theory, allows the description of physical phenomena in a wide variety of coordinate systems. In the presence of boundaries, such as event horizons and null infinity, time coordinates must be carefully adapted to the global causal structure of spacetime to ensure a computationally efficient description. Horizon-penetrating time is used to describe the

Inertial dragging is a well-known effect described within the framework of General Relativity. Notwithstanding, some fundamental questions related to its nature still deserve attention. One of these, rooted in Mach’s principle, wonders whether the inertial mass of particles could be due to the relative motion with respect to other particles. To tackle this question, we study the inertial dragging resulting

The black hole solutions to Einstein’s vacuum equations in four dimensions contain just one example: the Kerr black hole. Over the past two decades, we have understood that higher-dimensional black holes are far more plentiful. I give a straightforward account of the reasons for this abundance based on three key ideas: (i) Horizons in \(D>4\) can be long. (ii) Long horizons are flexible. (iii) Long

We consider a spherically symmetric homogeneous perfect fluid undergoing a gravitational collapse to singularity in the framework of higher-dimensional Rastall gravity in the cases of vanishing and nonvanishing cosmological constants. The possible final states of the collapse in any finite dimension are black hole and naked singularity, but the naked singularity formation becomes less favored when

This research analyses the spacetime geometry of the static black bottle by studying the geodesic motion of photons. Geodesic equations are found using the Hamilton–Jacobi formalism. The geodesics are then classified based on a set of appropriate conserved physical quantities. Effective potentials are used to visualise the allowed orbits. The classifications also vary based on the acceleration parameter

Neutron stars (NSs), superdense objects with exceptionally strong gravitational fields, provide an ideal laboratory for probing general relativity (GR) in the high-curvature regime. They also present an exciting opportunity to explore new gravitational physics beyond the traditional framework of GR. Thus, investigating modified theories of gravity in the context of superdense stars is intriguing and

This work revisits the agegraphic dark energy (ADE) model by associating fractional entropy with the apparent horizon. This reveals the extent to which quantum-gravitational effects deform the horizon. The thermodynamic-gravity conjecture modifies the entropy expression, leading to changes in the energy density of ADE and the Friedmann equations. Based on this relationship, we have utilized modified

We analytically study cosmological evolution in a flat FLRW spacetime in the context of modified STEGR gravity or f(Q), using an exponential two-parameter model which represents a smooth perturbative expansion around the \(\Lambda \)CDM model. The cosmological analysis is carried out by calculating the Hubble parameter as a function of redshift, for selected values of the parameters. The Hubble parameter

In this work, we examine head-on collisions, produced by other work, of \(\ell \)-boson stars, potential candidates for dark matter compact objects. We begin with a review of the general properties and features of these stars, leveraging results from prior studies to analyze the gravitational wave signals generated by such collisions. Considering a maximum distance of 100 Mpc for potential events,

It has now been widely accepted that many inflation models can account for the formation of primordial black holes (PBHs). In particular, it has been fully realized that the PBH formation depends crucially on the tails of the probability distribution function of the curvature perturbation. I will review some models of inflation that give rise to the PBH formation, and their observational implications

In this paper, we consider the coupling of the metric-affine bumblebee gravity to the Abelian gauge field and obtain the effective model corresponding to the weak gravity limit of this theory. The effective bumblebee theory displays new unconventional couplings between the bumblebee field and its field strength, and the U(1) gauge field along with its respective field strength, as a result of the non-metricity

In this contribution we present an overview of our work on the numerical simulation of the perturbation of a black hole space-time by incoming gravitational waves. The formulation we use is based on Friedrich’s general conformal equations which have the unique property that they allow access to the asymptotic region of an asymptotically regular space-time. In our approach we set up an initial boundary

Semiclassical gravity couples classical gravity to the quantized matter in meanfield approximation. The meanfield coupling is problematic for two reasons. First, it ignores the quantum fluctuation of matter distribution. Second, it violates the linearity of the quantum dynamics. The first problem can be be mitigated by allowing stochastic fluctuations of the geometry but the second problem lies deep

In this work, we investigate a static spherically symmetric Reissner–Nordström event horizon and a naked singularity by analyzing the behavior of a gyroscope transported along a timelike curve. We find that the precession frequency remains finite for both the black hole and the naked singularity as long as the trajectory is timelike. Our objective is to quantify the differences in the gyroscope’s precession

In this paper, we study the Joule-Thomson Expansion process for two types of black holes: AdS-Einstein-Maxwell-power-Yang-Mills (AEMPYM) and AdS-Kerr-Sen (AKS). Our study focuses on understanding how various parameters influence the Joule-Thomson Coefficient, the inversion curve, and the ratio of minimum inversion temperature to critical temperature. For the AKS black hole, we observe that the isenthalpic

I present a series of conjectures aiming to describe the general dynamics of the Einstein equations of classical general relativity in the vicinity of extremal black holes. I will reflect upon how these conjectures transcend older paradigms concerning extremality and near-extremality, in particular, the so-called “third law of black hole thermodynamics”, which viewed extremality as an unattainable

In this work we study the gravitational collapse procedure in generalized Vaidya spacetime with Bose–Einstein condensate dark matter density profile. We use the generalized Vaidya metric to simulate the spacetime of a big star and subsequently obtain the field equations. Then we proceed to determine the star system’s mass parameter by solving the field equations. Then the gravitational collapse mechanism

We discuss the current status of black holes within quantum gravity approaches based on quantum field theory, and particularly asymptotic safety. We highlight key open questions in the field, including the difficulty of deriving global solutions from the effective action, the role of matter in constraining the set of all possible alternatives to classical black holes, the importance of the formation

We first present an overview of the Schwarzschild vacuum spacetime within general relativity, with particular emphasis on the role of scalar polynomial invariants and the null frame approach (and the related Cartan invariants), that justifies the conventional interpretation of the Schwarzschild geometry as a black hole spacetime admitting a horizon (at \(r=2M\) in Schwarzschild coordinates) shielding

In this contribution, we obtain classical tests of general relativity using the Hartle-Thorne metric endowed with magnetic dipole and electric charge. This metric represents the approximate stationary spacetime of a massive object with the other characteristics mentioned. These tests are light deflection, time delay, periastron precession, and gravitational redshift. We also provide numerical estimates

We establish a relationship between the equations that constitute the so-called good-bad-ugly model, whose nonlinearities are known to mimic those present in the Einstein field equations in generalized harmonic gauge. This relationship between ugly fields and good and bad ones stems from the fact that one can write the equation for the rescaled derivative of an ugly along an incoming null direction

In the second century of General Relativity, building upon the exquisite foundation that analytical and perturbative studies have provided, the detailed understanding of the non-linear regime of gravity will increasingly take a prominent role. Fueled in part by computational advances as well as observational challenges, and drawing inspiration and tools from other areas in physics, new insights will

This study explores a covariant extension of the Proca framework, incorporating nonlinear terms for a massive spin-1 field while ensuring consistency and ghost-free behavior. Coupled with gravity, this effective extended Proca-Nuevo model provides viable cosmological solutions, constrained by Big Bang Nucleosynthesis observations. By enforcing \( |\Delta T_f / T_f| < 4.7 \times 10^{-4} \), the model

We study the interaction between gravitational waves and a quantum two-level system consisting of a spin 1/2 particle using the formalism of the proper detector frame. This approach highlights the effects of gravitational waves on both the particles and the observer, emphasizing that only relative measurements can be made. Specifically, within this framework, the gravitational field of the waves is

Mukkamala and Pereñiguez recently discovered a new master function for even-parity metric perturbations of the Schwarzschild spacetime. Remarkably, this function satisfies the Regge–Wheeler equation (instead of the Zerilli equation), which was previously understood to govern the odd-parity sector of the perturbation only. In this paper I follow up on their work. First, I identify a source term for

By considering the analytic, static and spherically symmetric solution for the Schwarzschild black holes immersed in dark matter fluid with non-zero tangential pressure (Jusufi in Eur Phys J C 83:103, 2023) and Hernquist-type density profiles (Cardoso in Phys Rev D 105:L061501, 2022), we compute the luminosity of accretion disk. We study the circular motion of test particles in accretion disk and calculate

This article delves into the observational signatures and theoretical underpinnings of rotating astrophysical objects, with a particular focus on superspinars -exotic objects characterized by preventing the formation of event horizons due to their high angular momentum. While solutions within General Relativity (Kerr superspinars) predict such objects, their classical forms harbor naked singularities

In this paper, we consider the spherically symmetric gravitational collapse of isotropic matter undergoing dissipation in the form of heat flux, with a generalized Vaidya exterior, in the context of f(R, T) gravity. Choosing \(f(R, T)=R+2\lambda T\), and applying the f(R, T) junction conditions on the field equations for the interior and exterior regions, we have obtained matching conditions of the

The conference Black Holes Inside and Out marked the 50th anniversary of Hawking’s seminal paper on black hole radiance. It was clear already from Hawking’s analysis that a proper quantum gravity theory would be essential for a more complete understanding of the evaporation process. This task was undertaken in loop quantum gravity (LQG) 2 decades ago and by now the literature on the subject is quite

Various generalizations of the scalar, axisymmetric “horizon hair” for extremal black holes have recently appeared in the literature. In this paper, we propose an expression for a non-axisymmetric gravitational “charge” and its potentially observable imprint at a finite distance from the horizon (Ori-coefficient) in extremal Kerr black hole backgrounds. Using a hyperboloidal foliation, we offer strong

In recent papers, Fuentealba, Henneaux, and Troessaert (FHT) gave definitions for supertranslation invariant Lorentz charges in the ADM Hamiltonian formalism and showed that their definitions match with the Chen, Wang, Yau (CWY) definitions of Lorentz charges at null infinity which are free from “supertranslation ambiguities”. In this brief note, motivated by the analysis of FHT, we write expressions

In this paper, we consider families of solutions for excited states of Proca stars in spherical symmetry. We focus on the first two excited configurations and perform a series of fully non-linear dynamical simulations in order to study their properties and stability. Our analysis reveals that excited Proca stars are always unstable against even very small perturbations, and their dynamical evolution

The interaction of black holes with classical fields can lead to many interesting phenomena such as black-hole superradiance and the superradiant instability. The existence of these effects has been shown to have implications for beyond Standard Model particles that could explain dark matter, namely ultralight bosonic fields. In this note I give a historical account of this topic and briefly go through

The saddle point approximation to formal quantum gravitational partition functions has yielded plausible computations of horizon entropy in various settings, but it stands on shaky ground. In this paper we visit some of that shaky ground, address some foundational questions, and describe efforts toward a more solid footing. We focus on the case of de Sitter horizon entropy which, it has been argued

We construct non-static adiabatic axisymmetric structures embedded in an asymptotically \(\Lambda \)CDM universe from given solutions of the Poisson’s equation of Newtonian gravity, using a particular form of the metric in isotropic coordinates. The approach is used in building of a razor-thin disk source made of perfect fluid for the McVittie metric, a system composite by a Plummer-type perfect fluid

Do compact objects other than black holes and neutron stars exist in the universe? Do all black holes conform with the predictions of Einstein’s General Relativity? Do classical black holes exist at all? Future gravitational-wave observations and black-hole imaging might shed light on these foundational questions and deepen our understanding of the dark cosmos.

In this article, we theoretically construct a (2 + 1)-dimensional rotating thin shell wormhole using the Darmois-Israel junction formalism. This thin shell wormhole whose validity has been checked by analyzing the energy conditions, specifically, the weak and null energy conditions, is actually constructed by cutting and pasting two rotating hairy black hole spacetimes in (2+1)-dimensions. We further

This study explores the dynamics and phase-space behavior of a multi-component dark energy model, where the dark sector consists of a minimally coupled canonical scalar field and the cosmological constant, using a dynamical system analysis setup for various types of potential for which a general parameterization of the scalar field potentials has been considered. Several fixed points with different

To consider the inevitable cosmic magnetic effects instead of the unknown dark sector of matter/energy on the inflation phase of the expanding universe some authors have proposed several extended exotic Einstein–Maxwell gravities which are addressed in this work. Some of these exotic models include directional interaction terms between the electromagnetic vector field and the metric tensor field. We

Entangled Relativity is a non-linear reformulation of Einstein’s General Theory of Relativity (General Relativity) that offers a more parsimonious formulation. This non-linear approach notably requires the simultaneous definition of matter fields, thus aligning more closely with Einstein’s principle of relativity of inertia than General Relativity does. Solutions for spherically charged black holes

We conduct studies on Levin’s taxonomy of periodic orbits for neutral test particles around a Reissner-Nordström naked singularity. It was known that naked singularities could harbor two distinct regions of time-like bound orbits and thus we expect periodic orbits to appear in both regions. It is possible for a pair of periodic orbits from both regions to possess the exact same angular momentum L and

General relativity and quantum field theory are the cornerstones of our understanding of physical processes, from subatomic to cosmic scales. While both theories work remarkably well in their tested domains, they show minimal overlap. However, our research challenges this separation by revealing that non-perturbative effects bridge these distinct domains. We introduce a novel mechanism wherein, at

It is well-known that the Kretschmann curvature diverges strongly at the classical singularity of the black hole interior. In this paper, we are therefore interested in whether such a strong divergence can be tamed quantum mechanically in the vicinity of the black hole singularity. For this purpose, we consider DeWitt-regular quantum black hole solutions of a self-adjoint Wheeler–DeWitt equation originating