arXiv subjects

Sarbari Guha

Publications and source records attributed to Sarbari Guha.

At least 19 recordsLinked to original sources

arXiv2026

Arrow of time problem in gravitational collapse

We investigate the arrow of time problem in the context of gravitational collapse of radiating stars in higher dimensions for both neutral and charged matter. The interior spacetime is described by a shear-free spherically symmetric metric filled with a dissipative fluid. The exterior spacetime of the radiating star is taken as the higher dimensional Vaidya metric. We establish that the arrow of time associated with gravitational entropy is opposite to the thermodynamic arrow of time for all dimensions. The physical consequences of our results are considered. Our result conforms with previous studies on shear-free spherical collapse, which suggests, avoidance of the naked singularity as the end state results in a wrong arrow of time, indicating a fundamental problem with the local application of the Weyl curvature hypothesis.

gr-qc↗

arXiv2025

Structure Scalars for Charged Dissipative Spherical Collapse in $f(R, T)$ Gravity

We examine the structure scalars constructed from the orthogonal splitting of the Riemann tensor for the spacetime metric describing the interior of a charged matter configuration undergoing dissipative collapse in the framework of $f(R,T)$ gravity (where $R$ and $T$ are the Ricci scalar and the trace of energy-momentum tensor, respectively), and also the way these quantities influence the various physical parameters of the collapsing matter. In absence of dissipation, the energy density inhomogeneity is found to be influenced by the structure scalar $X_{TF}$ and the mass-function of the collapsing matter. Further, the presence of charge affects the structure scalars and the total mass-energy content. The dependence of the various physical parameters like heat dissipation, energy density inhomogeneity, evolution of the expansion scalar, the shear scalar, effective homogeneous energy density, and pressure anisotropy on the structure scalars, have been clearly indicated along with a discussion on the complexity factor of the collapsing configuration. The $f(R,T)$ junction conditions have been presented, showing the matching conditions for the matter Lagrangian and their derivatives at the boundary. The energy conditions are also presented and the possibility of violation of the Strong Energy Condition has been discussed.

gr-qc↗

arXiv2025

From a collapsing radiating star to an evaporating black hole: A smooth transition from classical to quantum entropy

We present a robust mechanism where the geometrical free-gravitational entropy of an isolated astrophysical radiating star undergoing continual gravitational collapse, as measured by an external observer, makes a smooth transition to the Bekenstein-Hawking entropy at the onset of the horizon formation and in the late times of black hole evaporation. It is interesting to note that both in the classical regime and the semi classical evaporating black hole regime, the matter is radiated via the Vaidya exterior surrounding the radiating star, as well as the evaporating black hole. Our result, being independent of the interior matter dynamics of the collapsing star, clearly indicates that the Bekenstein-Hawking entropy and its non-extensive nature indeed originates from the Riemannian geometry, which dictates the free-gravity entropy in general relativity.

gr-qc↗

arXiv2025

Formation of Singularity and Apparent Horizon for Dissipative Collapse in $f(R,T)$ Theory of Gravity

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 $f(R, T)=R+2λT$, 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 matter-Lagrangian and its derivatives across the boundary. The time of formation of singularity and the time of formation of apparent horizon have been determined and constraints on the integration constants are examined for which the final singularity is hidden behind the horizon.

gr-qc↗

arXiv2025

Vaidya-like exterior solution and formation of singularity in $f(R, T)$ theory of gravity

In this paper, we study the gravitational collapse of a fluid ball undergoing dissipation in the form of heat flux, in the framework of $f(R,T)$ gravity. To apply the junction conditions in order to match the interior and the exterior spacetimes and analyze the dynamics of collapse, it is necessary to know the exterior metric for the radiating fireball, which is not yet available in the context of $f(R,T)$ gravity. Therefore, at the outset, we have determined the exterior Vaidya-like radiating metric in $f(R,T)$ gravity. Subsequently, we solved the $f(R,T)$ field equations and used the $f(R,T)$ junction conditions to match the exterior with the interior spacetime. The time of formation of the singularity and the time of formation of the apparent horizon have been determined. It is concluded that the final singularity is hidden behind the horizon.

gr-qc↗

arXiv2024

Memory Effect of Gravitational Wave Pulses in PP-Wave Spacetimes

In this paper, we study the gravitational memory effect in pp-wave spacetimes due to the passage of a pulse having the form of a ramp profile through this spacetime. We have analyzed the effect of this pulse on the evolution of nearby geodesics, and have determined analytical solutions of the geodesic equations in the Brinkmann coordinates. We have also examined the changes in the separation between a pair of geodesics and their velocity profiles. The separation (along $ x $ or $ y $-direction) increases monotonically from an initial constant value. In contrast, the relative velocity grows from zero and settles to a final non-zero constant value. These resulting changes are retained as memory after the pulse dies out. The nature of this memory is similar to that determined by earlier workers using Gaussian, square, and other pulse profiles, thereby validating the universality of gravitational wave memory.

gr-qc↗

arXiv2024

Arrow of time and gravitational entropy in collapse

We investigate the status of the gravitational arrow of time in the case of a spherical collapse of a fluid that conducts heat and radiates energy. In particular, we examine the results obtained by W. B. Bonnor in his 1985 paper where he found that the gravitational arrow of time was opposite to the thermodynamic arrow of time. The measure of gravitational epoch function $P$ used by Bonnor was given by the ratio of the Weyl square to the Ricci square. In this paper, we have assumed the measure of gravitational entropy $P_{1}$ to be given by the ratio of the Weyl scalar to the Kretschmann scalar. Our analysis indicates that Bonnor's result seems to be validated, i.e., the gravitational arrow and the thermodynamic arrow of time point in opposite directions. This strengthens the opinion that the Weyl proposal of gravitational entropy applies only to the universe as a whole (provided that we exclude the white holes).

gr-qc↗

arXiv2023

Memory Effect of Plane Gravitational Wave Pulses

A gravitational wave pulse, while passing through spacetime, brings about a change in the relative separation between free particles. This `memory effect' serves as one of the signatures of gravitational waves. In this paper, we consider some viable pulse profiles which are not yet analyzed by others (e.g., $u^{-4}$, $u^{-2}$, $ \frac{c}{(u^2 + au +b)^2} $), and examine the memory effect produced by these wave pulses in pp-wave spacetime. We choose to work in the Brinkmann coordinates to solve the geodesic equations. From the plots of the corresponding analytical solutions, we observe a non-zero separation between a pair of geodesics in each case, after the pulse dies out. The displacement memory effect either increases or decreases monotonically, whereas the velocity memory effect reaches saturation after an initial rise or drop.

gr-qc↗

arXiv2023

From entropy to gravitational entropy

The concept of entropy forms the backbone of the principles of thermodynamics. R.C. Tolman initiated the correlation between gravity and thermodynamics. The development of black hole thermodynamics and the generalized second law of thermodynamics led to Penrose's conjecture that the Weyl tensor should serve as a measure of the entropy of the free gravitational field. This entropy, known as gravitational entropy, reflects the degrees of freedom of the free gravitational field. The proposition of gravitational entropy provides a justification for the low entropy of the initial Universe. It was necessary to associate an entropy function with the free gravitational field even at the time of the big bang, so that a gravity-dominated evolution of the Universe may preserve the second law of thermodynamics. It also places the concept of black hole entropy in a proper context, which appears as a special case of gravitational entropy. However, a self-consistent notion of gravitational entropy has eluded us till today. Various proposals have been put forward, initially based on Penrose's Weyl Curvature Hypothesis, and subsequently modified to fit the needs of specific geometries and matter distributions. Such proposals were basically geometric in nature. A decade back a new definition of gravitational entropy was proposed from the considerations of the relativistic Gibbs equation and based on the square root of the Bel-Robinson tensor. However, even this proposal is valid only for a restricted class of spacetimes. A complete description of gravitational entropy encompassing black hole physics and cosmological dynamics is yet to emerge. In this article, we will try to gather an overview of the concept of gravitational entropy beginning with the initial attempts to correlate gravity with thermodynamics and follow it up with the development of the various proposals of gravitational entropy.

gr-qc↗

arXiv2023

On the propagation of gravitational waves in matter-filled Bianchi I universe

In this paper we apply the Regge-Wheeler formalism to study the propagation of axial and polar gravitational waves in matter-filled Bianchi I universe. Assuming that the expansion scalar $ Θ$, of the background space-time, is proportional to the shear scalar $ σ$, we solved the background field equations in the presence of matter (found to behave like a stiff fluid). We then derive the linearised perturbation equations for both the axial and polar modes. The analytical solutions in vacuum spacetime could be determined in an earlier paper \cite{GD1} in a relatively straightforward manner. However, here we find that in the presence of matter, they require more assumptions for their solution, and bear more involved forms. As compared to the axial modes, the polar perturbation equations contain far more complicated couplings among the perturbing terms. Thus we have to apply suitable assumptions to derive the analytical solutions for some of the cases of polar perturbations. In both the axial and polar cases, the radial and temporal solutions for the perturbations separate out as products. We find that the axial waves are damped owing to the background anisotropy, and can deform only the azimuthal velocity of the fluid. In contrast, the polar waves must trigger perturbations in the energy density, the pressure as well as in the non-azimuthal components of the fluid velocity. Similar behaviour is exhibited by axial and polar gravitational waves propagating in the Kantowski-Sachs universe \cite{GD2}. Our work is in contrast to the work done in \cite{SYK}, where the authors analysed anisotropic universes modelled by Kasner spacetime and Rindler wedges using the method of gauge-invariant perturbations in the RW gauge.

gr-qc↗

arXiv2022

Density perturbation and cosmological evolution in the presence of magnetic field in $f(R)$ gravity models

In this paper, we have investigated the density perturbations and cosmological evolution in the FLRW universe in presence of a cosmic magnetic field, which may be assumed to mimic primordial magnetic fields. Such magnetic fields have sufficient strength to influence galaxy formation and cluster dynamics, thereby leaving an imprint on the CMB anisotropies. We have considered the FLRW universe as a representative of the isotropic cosmological model in the 1+3 covariant formalism for $f(R)$ gravity. The propagation equations have been determined and analyzed, where we have assumed that the magnetic field is aligned uniformly along the $x$-direction, resulting in a diagonal shear tensor. Subsequently,the density perturbation evolution equations have been studied and the results have been interpreted. We have also indicated how these results change in the general relativistic case and briefly mentioned the expected change in higher-order gravity theories.

hep-th↗

arXiv2022

How appropriate are the gravitational entropy proposals for traversable wormholes?

In this paper we have examined the validity of some proposed definitions of gravitational entropy (GE) in the context of traversable wormhole solutions of the Einstein field equations. Here we have adopted two different proposals of GE and checked for their applicability in the case of these wormholes. The first one is the phenomenological approach proposed by Rudjord et al \cite{entropy1} and expanded by Romero et al in \cite{entropy2}, which is a purely geometric method of measuring gravitational entropy. The latter one is the Clifton-Ellis-Tavakol (CET) proposal \cite{CET} for the gravitational entropy which arises in relativistic thermodynamics and is based on the Bel-Robinson tensor, which represents the effective super-energy-momentum tensor of free gravitational fields. Considering some of the Lorentzian traversable wormholes along with the Brill solution for NUT wormholes and the AdS wormholes, we have evaluated the gravitational entropy for these systems. Incidentally, the application of the CET proposal can provide unique gravitational entropies for spacetimes of Petrov type D and N only, whereas the geometric method can be applied to almost every kind of spacetime, although it has no relation with thermodynamics. For any traversable wormhole to be physically realistic, it should have a viable GE. We found that the GE proposals do give us a consistent measure of GE in several of them. This means that the existence of a viable gravitational entropy strictly depends on its definition.

gr-qc↗

arXiv2021

Propagation of Axial and Polar Gravitational Waves in Kantowski-Sachs Universe

We apply the Regge-Wheeler formalism to study axial and polar gravitational waves in Kantowski-Sachs universe. The background field equations and the linearised perturbation equations for the modes are derived in presence of matter. To find analytical solutions, we analyze the propagation of waves in vacuum spacetime. The background field equations in absence of matter are solved by assuming the expansion scalar to be proportional to the shear scalar. Using the method of separation of variables, the axial perturbation parameter $ h_0(t, r) $ is obtained from its wave equation. The other perturbation $ h_1(t, r) $ is then determined from $ h_0(t, r) $. The anisotropy of the background spacetime is responsible for the damping of the axial waves. The polar perturbation equations are much more involved compared to their FLRW counterparts, as well as to the axial perturbations in Kantowski-Sachs background. In both the axial and polar cases, the radial and temporal solutions separate out as products. The temporal part of the polar perturbation solutions are plotted against time to obtain an order of magnitude estimate of the frequency of the propagating GWs and lies in the range 1000-2000 Hz. Using standard observational data for the GW strain we have placed constraints on the parameters in the polar perturbation solutions. The perturbation equations in presence of matter show that the axial waves can cause perturbations only in the azimuthal velocity of the fluid without deforming the matter field. But the polar waves must perturb the energy density, the pressure and also the non-azimuthal components of the fluid velocity.

gr-qc↗

arXiv2021

Dynamical conditions and causal transport of dissipative spherical collapse in $f(R,T)$ gravity

In this paper, we have investigated the non-adiabatic spherical gravitational collapse in the framework of the $f(R,T)$ theory of gravity with a locally anisotropic fluid that undergoes dissipation in the form of heat flux, free-streaming radiation, and shearing viscosity. The dynamical equations are analyzed in detail, both in the Newtonian and post-Newtonian regimes. Finally we couple the dynamical equations to the full causal transport equation in the context of Israel-Stewart theory of dissipative systems. This yields us a better understanding of the collapse dynamics and may be connected to various astrophysical consequences.

gr-qc↗

arXiv2021

An investigation on gravitational entropy of cosmological models

In this paper, we investigate the entropy of the free gravitational field for a given epoch for some well-known isotropic and anisotropic cosmologies. We use the definition of gravitational entropy proposed by Clifton, Ellis, and Tavakol, where the 2-index square root of the 4-index Bel-Robinson tensor is taken to be the energy momentum tensor for the free gravity. We examine whether, in the vicinity of the initial singularity, the ratio of the energy density of free gravity to that of matter density goes to zero, validating Penrose's conjecture on Weyl curvature. Whenever this is true, the gravitational entropy increases monotonically with time, leading to structure formation. For the models considered by us, we identify the conditions for which the Weyl curvature hypothesis is valid, and the assumptions under which it is validated, or otherwise.

gr-qc↗

arXiv2020

Axial Gravitational Waves in Bianchi I Universe

In this paper, we have studied the propagation of axial gravitational waves in Bianchi I universe using the Regge-Wheeler gauge. In this gauge, there are only two non-zero components of $ h_{μν} $ in the case of axial waves: $h_0(t,r)$ and $h_1(t,r)$. The field equations in absence of matter have been derived both for the unperturbed as well as axially perturbed metric. These field equations are solved simultaneously by assuming the expansion scalar $Θ$ to be proportional to the shear scalar $σ$ (so that $a= b^n$, where $a$, $b$ are the metric coefficients and $n$ is an arbitrary constant), and the wave equation for the perturbation parameter $h_0(t,r)$ have been derived. We used the method of separation of variables to solve for this parameter, and have subsequently determined $h_1(t,r)$. We then discuss a few special cases in order to interpret the results. We find that the anisotropy of the background spacetime is responsible for the damping of the gravitational waves as they propagate through this spacetime. The perturbations depend on the values of the angular momentum $l$. The field equations in the presence of matter reveal that the axially perturbed spacetime leads to perturbations only in the azimuthal velocity of the fluid leaving the matter field undisturbed.

gr-qc↗

arXiv2019

Thermodynamics of FRW Universe With Chaplygin Gas Models

In this paper we have examined the validity of the generalized second law of thermodynamics (GSLT) in an expanding Friedmann Robertson Walker (FRW) universe filled with different variants of Chaplygin gases. Assuming that the universe is a closed system bounded by the cosmological horizon, we first present the general prescription for the rate of change of total entropy on the boundary. In the subsequent part we have analyzed the validity of the generalised second law of thermodynamics on the cosmological apparent horizon and the cosmological event horizon for different Chaplygin gas models of the universe. The analysis is supported with the help of suitable graphs to clarify the status of the GSLT on the cosmological horizons. In the case of the cosmological apparent horizon we have found that some of these models always obey the GSLT, whereas the validity of GSLT on the cosmological event horizon of all these models depend on the choice of free parameters in the respective models.

physics.gen-ph↗

arXiv2019

On the gravitational entropy of accelerating black holes

In this paper we have examined the validity of a proposed definition of gravitational entropy in the context of accelerating black hole solutions of the Einstein field equations, which represent the realistic black hole solutions. We have adopted a phenomenological approach proposed in Rudjord et al [20] and expanded by Romero et al [21], in which the Weyl curvature hypothesis is tested against the expressions for the gravitational entropy. Considering the $C$-metric for the accelerating black holes, we have evaluated the gravitational entropy and the corresponding entropy density for four different types of black holes, namely, non-rotating black hole, non-rotating charged black hole, rotating black hole and rotating charged black hole. We end up by discussing the merits of such an analysis and the possible reason of failure in the particular case of rotating charged black hole and comment on the possible resolution of the problem.

gr-qc↗