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

This work provides a spacetime interpretation of the confluent Heun functions within black hole perturbation theory (BHPT) and explores their relationship to the hyperboloidal framework. In BHPT, the confluent Heun functions are solutions to the radial Teukolsky equation, but they are traditionally studied without an explicit reference to the underlying spacetime geometry. Here, we show that the distinct

Ehlers, Pirani, and Schild argued that measurements of null and timelike geodesics yield Weyl and projective connections, respectively, with compatibility in the lightlike limit giving an integrable Weyl connection. Their conclusions hold only for a 4-dim representation of the conformal connection on the null cone, and by restricting reparameterizations of timelike geodesics to yield a torsion-free

The question of black hole evaporation is reviewed in the framework of quantum field theory in curved spacetimes and semiclassical gravity. We highlight the importance of taking backreaction effects into account to have a consistent picture of the fate of gravitational collapse in this framework. We describe the difficulties of solving the backreaction semiclassical equations due to practical complications

We consider a free-falling observer who crosses the event horizon in the Schwarzschild background. In the course of this fall, he/she can receive signals from an object (like a star surface) that emits radiation. We study how the frequency received by an observer changes depending on the proper time on his/her trajectory. The scenarios are classified depending on whether the frequency is infinite,

The gravitational path-integral of Gauss–Bonnet gravity is investigated and the transition from one spacelike boundary configuration to another is analyzed. Of particular interest is the case of Neumann and Robin boundary conditions which is known to lead to a stable Universe in Einstein–Hilbert gravity in four spacetime dimensions. After setting up the variational problem and computing the necessary

We present a novel perspective on gravity-induced wave function reduction using Bohmian trajectories. This study examines the quantum motion of both point particles and objects, identifying critical parameters for the transition from quantum to classical regimes. By analyzing the system’s dynamics, we define the reduction time of the wave function through Bohmian trajectories, introducing a fresh viewpoint

We investigate the quantum fate of the classical singularities that occur by gravitational collapse of a dust cloud. For this purpose, we address the quantization of a model first proposed by Georges Lemaître in 1933. We find that the singularities can generically be avoided. This is a consequence of unitary evolution in the quantum theory, whereby the quantum dust cloud collapses, bounces at a minimal

In this paper, we consider a nonminimal coupling model between gravity and nonlinear electrodynamics with cosmological constant. This cosmological model is designed to account for both the inflationary epoch of the early universe and the current phase of accelerated cosmic expansion. The nonlinear electrodynamic fields provide a mechanism for a graceful exit from the inflationary period, preventing

In this work, we explore non-commutative effects in fractional classical and quantum schemes using the anisotropic Bianchi type I cosmological model coupled to a scalar field in the K-essence formalism. We introduce non-commutative variables considering that all minisuperspace variables \(q^{i}_{nc}\) do not commute, so the symplectic structure was modified, resulting in some changes concerning the

I briefly describe motivation for, and the current state of research into understanding the structure and dynamics of black hole “imposters”: objects that could be misidentified as Kerr black holes given the current precision of LIGO/Virgo gravitational wave observations, or EHT accretion disk measurements. I use the term “weak imposter” to describe an object which is a black hole, i.e. it has an event

Quantum black holes, a broad class of objects that refine the solutions of general relativity by incorporating semiclassical and/or quantum gravitational effects, have recently attracted renewed attention within the scientific community. This resurgence of interest is largely driven by advances in gravitational wave astronomy, which have opened the possibility of testing some of these models in the

Kerr–Vaidya metrics are the simplest dynamical axially-symmetric solutions, all of which violate the null energy condition and thus are consistent with the formation of a trapped region in finite time according to distant observers. We examine different classes of Kerr–Vaidya metrics, and find two which possess spherically-symmetric counterparts that are compatible with the finite formation time of

This study explores the feasibility of an effective Friedmann equation in removing the classical Big Bang initial singularity and replacing it with a non-singular bounce occurring at a critical energy density value. In a spatially flat, homogeneous, and isotropic universe, the effective theory is obtained by introducing a function parametrically dependent on the critical energy density. This function

This brief review, intended for high energy and astrophysics researchers, explores the implications of recent theoretical advances in string theory and the Swampland program for understanding bounds on the structure of positive potentials allowed in quantum gravity. This has a bearing on both inflationary models for the early universe as well as the fate of our universe. The paper includes a review

We investigate the topological implications of stable minimal surfaces existing in a static perfect fluid space while ensuring that the fluid satisfies certain energy conditions. Based on the main findings, the topology of the level set \(\{f=c\}\) (the boundary of a stellar model) is studied, where c is a positive constant and f is the static potential of a static perfect fluid space. Bounds for the

I consider the algebra of operators along the world line of an observer as a background independent algebra in quantum gravity.

We compute analytically differential curvature invariants for accelerating, rotating and charged black holes with a cosmological constant \(\varLambda \). Specifically, we compute novel closed-form expressions for the Karlhede and the Abdelqader-Lake invariants, for accelerating Kerr–Newman black holes in (anti-)de Sitter spacetime or subsets thereof with the aim of detecting physically relevant surfaces

A well-known soliton (bubble) solution of five-dimensional Kaluza–Klein General Relativity is modified by imposing mass on the scalar field. By forcing the scalar field to be short-range, the failure of the original bubble solution to satisfy the equivalence principle is remedied, and the bubble acquires gravitational mass. Most importantly, the mass is quantized, even in this classical setting, and

In this paper, we have put forwarded a detailed investigation on the cosmic evolution of Bianchi type III model within the realm of Born-Infeld f(R) gravity executing the Palatini approach. Using a very eminent tool known as Dynamical System Approach (DSA), we have curtailed the complexity of the non linear field equations and study the dynamics for the form \(f(R) =R-\beta / R^n\). The main focus

Abstract "Is it a goose, a duck or a swan?"—asked the alien. "I do not know; and to know we have to look closer."—said the earthling. "But even from here we can see it has webbed feet... so it is not a chicken." Graphical Abstract

Here, we will discuss some ideas for possible classical/semi-classical modifications of the black hole solutions in General Relativity (GR). These modifications/extensions include black holes in higher dimensions; black holes with additional gravitational fields, or fields beyond the Standard Model of Particle Physics; black holes in alternative classical theories of gravity and in semiclassical gravity;

We study the dynamics of a charged radiating star in general relativity. The junction conditions at the surface of the star lead to a restriction that connects the radial pressure to the heat flux. The master equation reduces to a nonlinear second order differential equation which determines the temporal evolution. The dynamical behaviour is studied via a phase plane analysis which reveals interesting

We present a review of concepts of thermodynamic of spacetime that allows for an understanding of the gravitational dynamics encoding in it, discussing also the recovery of Weyl transverse gravity instead of General Relativity. We also discuss how these tools can provide some hints in the search of quantum gravity phenomenology, by introducing a formalism to analyze low-energy quantum gravity modifications

Using Darmois-Israel-Sen junction conditions, and with help of Visser’s cut-and-paste method, we study the dynamics of thin-shell wormholes that are made of two conformally Killing gravity (a.k.a Harada gravity) black holes. We check the energy conditions for different values of the new parameter that Harada introduced, as alternative for dark energy. We examine the radial acceleration to reveal the

We explore black hole solutions and some of its physical properties in Einstein’s theory in 4D, modified by a cubic gravity term and in the presence of non-linear electrodynamics. In the context of Effective Field Theories (EFT) and under certain assumptions, these curvature and non-linear electromagnetic terms represent the first corrections to the Einstein-Maxwell theory. We obtain static and spherically

This study examines the compatibility of the generalized holographic equipartition proposed in Sheykhi (Phys Rev D 87(6):061501, 2013) with the maximization of horizon entropy in an (n + 1)-dimensional non-flat Friedmann–Robertson–Walker (FRW) universe. Here, the entropy associated with the apparent horizon is described by Kaniadakis entropy, as well as truncated Kaniadakis entropy, which is expanded

The status of the equivalence principle in modified symmetric teleparallel gravity is examined. In this theory, minimum length geodesics are distinct from autoparallel geodesics, that is, the “shortest” paths are not the “straightest” paths. We show that a standard argument that singles out metric geodesics in general relativity does not apply in modified symmetric teleparallel gravity. This is because

Compact stars have long served as a test bed of gravitational models and their coupling with stellar matter. In this work, we explore the behavior of an exponential model in f(T) gravity through the Tolman-Oppenheimer-Volkoff equation. This is performed for different envelope thicknesses. Finally, constraints on the models parameters are obtained, which are comparable to the results obtained using