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Ritabrata Biswas

Publications and source records attributed to Ritabrata Biswas.

At least 19 recordsLinked to original sources

A Brief Study of Dark Energy Accretion onto Schwarzschild Black Hole : Biswas-Roy-Biswas Type Redshift Parameterization is Chosen

In this letter, we have considered accretion of a particular type of Dark Energy model onto a Schwarzschild type black hole. Before using the model, the free parameters of the Dark Energy model have been constrained with differential ages data. A narrow peak on top of a wide plateau in two parameters' distributions indicates a well defined best fit value embedded within a broad region of near-degenerate solutions. This means the data strongly favours one specific parameter value but also permit a wide range with comparable likelihood. Physically, it reflects that the Dark Energy dynamics are locally constrained yet globally insensitive to small parameter variations. An increasing $\log_{10}\left[M(z)/M_{0}\right]$ since $z=3$ signifies that black holes have continuously grown through accretion and mergers within the standard hierarchical formation scenario. The precise rate of this growth depends on the radiative efficiency $ε$, the effective accretion parameter $λ_{\rm eff}$, and the cumulative impact of merger events.

gr-qc

Stability Analysis of Four $f(Q)$ Gravity Models : A Cosmological Review in the Background of Bianchi-I Anisotropy

With the non-metricity scalar $Q$ as the functional argument, several $f(Q)$ gravity models are found to be proposed which are perfectly able to mimic the late-time accelerated expansion as pointed out by the type Ia supernovae observations. Temperature fluctuation differences for two celestial hemispheres, Hubble tension, voids, dipole modulation, anisotropic inflation, etc. motivates us to think beyond the $Λ$CDM model and the cosmological principle. Bianchi-I model portrays an anisotropic universe imposing shear. $f(Q)$ model also enables us to produce early inflation to late de Sitter universe without the requirement of $Λ$CDM. Ambiguities regarding fine-tuning or coincidences can be avoided alongwith. So, this article finds different stationary points of cosmic evolution with $f(Q)$ models habilitating in Bianchi-I anisotropic universe. Depending on models' nature, fixed points with different categories are found. Perturbations are followed wherever are applicable. While pursuing cosmological implications towards these fixed points, some are found to be formed only for the consideration of $f(Q)$ gravity and Bianchi-I both. Besides different prediction towards early inflation to late-time expansion which are available in existing literature of dynamical system studies, occurances of ultra slow roll inflation is predicted. For particular $f(Q)$ model, shear is predicted to decay leaving behind a constant valued residue. This models a universe that gradually turns more homogeneous. In some other models, depending on initial conditions, a final isotropic leftover is marked as the future fate of anisotropic world. More than one stable points are marked for special cases and are cosmologically interpreted.

gr-qc

Constraining Redshift Parametrization Models with Recentmost Data : Impacts on an Accretion Disc around Finslerian Kiselev Black Hole

We investigate the evolution of black hole mass within a cosmological background modeled by a Modified Chaplygin Gas (MCG) under various dark energy equation of state parametrizations, including Linear, Logarithmic, CPL, JBP models. The logarithmic mass ratio $\log_{10}[M(z)/M_0]$ is found to be highly sensitive to the redshift-dependent evolution of $ω(z)$, with gentle slopes in Linear, Logarithmic and CPL models indicating quasi-static accretion and steep slopes in JBP corresponding to rapid late-time variations highlighting transient suppression or enhancement of accretion due to repulsive dark energy effects. Peaks, minima and amplitude offsets in the mass ratio reflect the dynamic interplay between horizon thermodynamics, the evolving pressure of the MCG and cosmic expansion, illustrating how the black hole mass growth is directly influenced by both the temporal evolution of dark energy and the effective gravitational potential of the surrounding cosmic fluid. Our results demonstrate that black hole accretion acts as a sensitive probe of the time-dependent cosmic pressure landscape and provides physical insights into the coupling between local strong gravity and global accelerated expansion.

gr-qc

A Brief Thermodynamic Study For Four Dimensional Einstein Gauss Bonnet Black Holes Using Fractalised Barrow Entropy

Higher dimensional Gauss-Bonnet gravity can be particularized to a four dimensional case either using the Glavan, D. and Lin, C. type \cite{glavan2020einstein} limiting method or by the Hordenski type \cite{gurses2007gauss} metric compactification procedure. Depending on ADM mass and Gauss Bonnet coupling parameter $α_{GB}$, a black hole solution is prescribed \cite{hennigar2020taking}. The phrase which is responsible for divergence at some finite values of radial coordinate stays inside a square root and thus softens the divergence. A quantum gravity affected surface formula for the black hole is chosen. Fractalized entropy thus formed is known as the Barrow Entropy\cite{ladghami2024barrow}. Global nature for such an entropy formula is discovered. Temperature is calculated with and without different quantum corrections. Stability with such temperature structures are analyzed. To support this, sign changes in specific heat, occurrences of double points in free energy, sign changes and jumps in derivatives of free energy etc are thoroughly discussed. Probable rise of phase transitions are pointed.

gr-qc

Some New Types of Well-Behaved Polynomial Redshift Parametrization of Dark Energy Equation of State

In this paper, we explore a new type of smooth and well-behaved polynomial redshift function that can avoid a future singularity. Using this function, we have proposed different redshift parametrizations of the dark energy equation of state, drawing motivation from different polynomial functions like conventional polynomial, Legendre polynomial, Laguerre polynomial, Chebyshev polynomial and Fibonacci polynomial. The main feature of these parametrizations is their well-behaved nature throughout the evolution of the universe, which was a matter of concern in most of the previous polynomial parametrizations of the dark energy equation of state (EoS). This form of parametrization may be considered as an extension of those forms with no divergence at any redshift value. A comprehensive observational data analysis is performed with the Hubble, BAO and DESI datasets to constrain the parameter space of the models. Confidence contours showing joint and marginalized posterior distribution with different combinations of datasets are generated using a Markov Chain Monte Carlo approach. We see that our improved parametrizations enable us to derive more stringent restrictions on the current dark energy EoS and its derivative, which improves performance. Finally, a machine learning analysis is performed using some suitable algorithms like ELR, PILR, ANN, SVR, ERFR and GBR to compare the models. Among all the tested polynomial bases, the Legendre basis demonstrated superior performance with the lowest test RMSE and reduced $χ^{2}$ value under the Modified Differential Evolution theoretical model, indicating exceptional physical accuracy and numerical stability.

gr-qc

Einstein Maxwell Scalar Black Hole: Thermodynamic Properties with Logarithmic Barrow Entropy

The thermodynamics of black holes (BHs) within the Einstein Maxwell Scalar (EMS) framework, incorporating Barrow entropy and its logarithmic corrections to analyze quantum gravity effects is investigated here. A static, spherically symmetric BH solution is obtained by coupling the scalar field nonminimally to the electromagnetic field through a scalar dependent function. The thermodynamic properties including temperature, specific heat, and Gibbs free energy are derived and explored in the context of Barrow-modified entropy. We identify phase transitions and critical behavior by analyzing PV criticality and uncover the influence of scalar and electric charges on stability. Furthermore, the Joule Thomson expansion is examined to understand the inversion behavior and thermodynamic responses under adiabatic expansion. Our findings suggest the presence of thermodynamic instabilities, remnants, and nontrivial critical phenomena, providing new insights into BH thermodynamics in modified gravity scenarios with quantum corrections.

gr-qc

Cosmology with Distinct Functions $f$ of the Non-metricity Scalar $Q$ : A Dynamical System Approach

Symmetric teleparallel gravity is one among the general relativistic trinity which deals with the non-metricity scalar $Q$. In the Einstein Hilbert action, a function of $Q$ is chosen to be the main contributory part of the Lagrangian and a modified theory of gravity is constructed. In literature, different structures of the function of $Q$ are found which sustain several astrophysical observations like Big Bang nucleosynthesis, late-time cosmic acceleration etc. Autonomous systems for each such models with different $f(Q)$ structures are constructed. Corresponding fixed points and their stability properties are studied. For every case, stable, unstable and saddle-type fixed points are found to exist. These points on the phase portraits are cosmologically analyzed. It is tried to justify which way the corresponding state may lead if the initial state is perturbed. A comparative study of different models is represented.

gr-qc

Accretion of Dark Energy Candidates Following Redshift Parametrization Type Equation of State : Horava-Lifshitz Gravity

In this article, accretion of particular dark energy candidates is studied. These dark energy models possess equation of state dependent on redshift and some free parameters. Central gravitating object for this accretion model is chosen to be the Kehagias Sfetsos black hole sitting in Horava Gravity. Four special redshift parametrization models, viz. Chevallier-Polarski-Linder, Jassal-Bagala-Padmanabhan, Barboza-Alcaniz and Barboza-Alcaniz-Zhu-Silva are picked to study different accretion properties. Formation of critical point and related radial infalling speed, sonic speed, different arbitrary parameters' values etc are obtained. Mass growth curves are plotted. The rate of growth is followed to depend on the dark matter's behavior as well as the free parameters of dark energy models. Future loss in mass is noted for some of the chosen dark energy models.

gr-qc

Viscous Dark Energy Accretion Activities : Sonic Speed, Angular Momentum and Mach Number Studies

In this present article, we study different accretion properties regarding viscous accretion of dark energy. Modified Chaplygin gas is chosen as the dark energy candidate. Viscosity is encountered with the help of Shakura-Sunyaev viscosity parameter. We study sonic speed vs radial distance curves. We compare between adiabatic and dark energy dominated cases and follow that sonic speed falls as we go nearer to the central gravitating object. As viscosity is imposed, a threshold drop in accretion sonic speed is followed. Average rate of fall in accretion sonic speed is increased with black hole's spin. This is signifying that this kind of accretion is weakening the overall matter/energy infall. Specific angular momentum to Keplerian angular momentum ratio is found to fall as we go far from the black hole. Accretion Mach number turns high as we go towards the inner region and high wind Mach number is not allowed as we are going out. Combining, we conclude that the system weakens the feeding process of accretion.

astro-ph.CO

Accretion onto a Quintessence Contaminated Rotating Black Hole : Violating the Lower Limit for Eta over s

Viscous accretion flow around a rotating supermassive black hole sitting in a quintessence tub is studied in this article. To introduce such a dark energy contaminated black hole's gravitational force, a new pseudo-Newtonian potential is used. This pseudo-Newtonian force can be calculated if we know the distance from the black hole's center, spin of the black hole and equation of state of the quintessence inside which the black hole is considered to lie. This force helps us to avoid complicated nonlinearity of general relativistic field equations. Transonic, viscous, continuous and Keplerian flow is assumed to take place. Fluid speed, sonic speed profile and specific angular momentum to Keplerian angular momentum ratio are found out for different values of spin parameter and quintessence parameter. Density variation is built and tallied with observations. Shear viscosity to entropy density ratio is constructed for our model and a comparison with theoretical lower limit is done.

gr-qc

Posing Constraints on the Free Parameters of a New Model of Dark Energy EoS : Responses Through Cosmological Behaviours

Since the late 1990's observations of type Ia Supernova, our universe is predicted to experience a late time cosmic acceleration. Theoretical support to this observation were intended to be built via proposition of a hypothetical fluid which staying inside the universe exerts negative pressure. Cosmologists have prescribed many candidates for this exotic fluid so far. In this alley, a popular method is to choose time dependent equation of state parameter $ω= \frac{p}ρ$ and to parametrize it as a function of redshift. Again some common families of such parametrizations are constructed among which different members justify different properties of observed universe. Mainly, these were model dependent studies which comprise free parameters to be constrained by different observations. In this present article, a new expression for redshift parametrization is considered and we have constrain its free parameters for two Hubble parameter vs redshift data sets. These data sets are obtained depending on two basic methodologies known as different ages method and baryonic acoustic oscillation method. Different confidence contours for our model are located under the constraints of said data sets. Besides, different thermodynamic parameters related to the evolution of our universe are analysed. It is notified that our model indicates towards a delayed dark matter model which mimics EoS = $-1(Λ)$ phenomena at the present epoch. Deceleration parameters behaviour's are studied. It is noticed that a possibility of future deceleration may occur for this new model. Outcomes for both the data sets are compared with each other.

gr-qc

Generalized Model of Interacting Dark Energy and Dark Matter : Phase Portrait Analysis of Evolving Universe

Main aim of this work is to give a suitable explanation of present accelerating universe through an acceptable interactive dynamical cosmological model. A three-fluid cosmological model is introduced in the background of Friedmann-Lemaître-Robertson-Walker asymptotically flat spacetime. This model consists of interactive dark matter and dark energy with baryonic matter taken as perfect fluid satisfying barotropic equation of state. We consider dust as the candidate of dark matter. A scalar field $ϕ$ represents dark energy with potential $V(ϕ)$. Einstein's field equations are utilised to construct a three-dimensional interactive autonomous system by choosing suitable interaction between dark energy and dark matter. We take the interaction kernel as $Q = 3β^{2γ} Hρ_d$. In order to explain the stability of this system, we obtained some suitable critical points. We analyse stability of obtained critical points to show the different phases of universe and cosmological implications. Surprisingly, we find some stable critical points which represent late time dark energy dominated era when a model parameter $α=-5.05$. In order to explain both the energy dominated era as well as the late-time acceleration of the universe at same time, we introduce a two-dimensional interactive autonomous system. After graphical analysis of two-dimensional system, we get several stable points which represent dark energy dominated era and the late-time cosmic acceleration both at the same time. We also show the variation in interaction at vicinity of phantom barrier ($ω_{eff}=-1$). From our work we can also predict the future phase evolution of the universe. No tendency for future deceleration is detected so far.

gr-qc

Search for Missing Links Between Two Extreme Wind Speed Profiles : Dark Energy Accretion and Adiabatic Fluid Accretion

In recent past, the progresses in accretion studies onto relativistically gravitating central objects like a Schwarzschild singularity reveal that the accretion flow must be transonic. For such cases, the radial inward speed gradient can be written as a numerator by denominator form among which the later vanishes somewhere in between infinite distance from the attracter to the event horizon of the same. For sustainability of a physical solutions, the numerator should vanish at the same radial distance where denominator does vanish. From this point, we obtain a second degree first order differential equation of radial inward speed and hence we obtain two branches of flow, namely accretion and wind. For adiabatic accretion case, the wind curve is formed to be more or less parallel to the radial distance axis as we move far from the central object. For dark energy accretion, this curve is parallel to the radial velocity axis. Here we face a question. Why there is no fluid speed profiles in between these two extremities. While searching the reasons, we follow that dark energy, if treated as an accreting object, should stay around the central \emph{compact star} and hence will contaminate the metric which propertises the compact star. In this research work, we have proposed a model with a rotating black hole embedded in quintessence where quintessence equation of state and spin parameters of the black hole are regulatory factors of the model. The resulting accretion and wind curves are studied. The Effect of negative pressure of dark energy is found to get catalyzed by the entry of the spin of the black hole. We tally our results with observations of accretion or outflow phenomenon near to different quasars.

gr-qc

Threshold Drop in Accretion Density if Dark Energy is Accreting onto a Supermassive Black Hole

Galactic structures are supposed to be formed out of dark matter clustering. Some examples of supermassive black holes in the central regions of high redshift galaxies say that the concerned supermassive black holes have completed their constructions in a time less than it generally should be. To justify such discrepancies, we are forced to model about existences of black hole mimickers and exotic phenomena acting near the supermassive black holes. Motivated by these we study the natures of exotic matters, especially dark energy near the black holes. We choose modified Chaplygin gas as dark energy candidate. Again, the descriptions of gravitational waves or the attenuations of them when they are tunnelling through cosmological distances help us to measure the shear viscosity of the medium through which the waves have been travelled. Delayed decaying models of dark matters also suggest that dark energy and viscosity may come up as a byproduct of such decays or interactions. We consider the viscous nature of the medium, i.e., the dark energy. To do so, we choose an alpha-disc model as proposed by Shakura and Sunyaev. We study the variations of densities through accretion and wind branches for a different amount of viscosity regulated by the Shakura-Sunyaev's alpha parameter, spin parameter and different properties of accreting fluids, viz, the properties of adiabatic fluid and modified Chaplygin gas. We compare these results with each other and some existing density profiles drawn from observational data-based simulations. We follow that our result supports the data observed till date. Specifically, we see the wind to get stronger for dark energy as accreting agent. Besides, we see the accretion to have a threshold drop if the viscosity is chosen along with the repulsive effects of dark energy.

gr-qc

Violation of Universal Lower Bound for the Shear Viscosity to Entropy Density Ratio in Dark Energy Dominated Accretion

The universal lower bound of the ratio of shear viscosity to entropy density is suggested by the string theory and gauge duality for any matter. We examined the ratio of shear viscosity to entropy density for viscous accretion flow towards a central gravitating object in the presence of dark energy. The ratio appears close to the universal lower bound for certain optically thin, hot accretion flows as they are embedded by strong magnetic field. Dark energy is a kind of exotic matter which has negative pressure. So dark energy creates repulsive force between the accreting particles, which indicates that shear viscosity of the flow becomes very low. Dark energy as accreting fluid has very high entropy density. The ratio should reach near to the lowest value for dark energy accretion. We wish to study what happens to the shear viscosity to entropy density ratio for viscous dark energy accretion flow.

gr-qc

Thermodynamics of Reissner-Nordström Black Holes in Higher Dimensions: Rainbow Gravity Background With General Uncertainty Principle

In this paper, we investigate the thermodynamic properties of Reissner-Nordström black holes embedded in higher $(d)$ dimensions in the framework of rainbow gravity incorporating the effects of the generalized uncertainty principle. We also examine all the properties graphically by varying the rainbow gravity parameter $η$ and the generalized uncertainty principle parameter $α$. We find the existence of remnant and critical mass of the concerned black hole. We calculate the local temperature, local internal energy and hence we analyse the thermal stability of the black hole by computing the local heat capacity. Further, we study the phase transitions of the aforesaid black hole solution under the effects of generalized uncertainty principle. From the analysis of the specific heat at the horizon, we observe that there are phase transitions for all dimensions but when we analyze the same, measured by the local observer, we find that there exist only two phase transitions.

gr-qc

Geometrothermodynamic Analysis and $P$-$V$ criticality of Higher Dimensional Charged Gauss-Bonnet Black Holes With First Order Entropy Correction

We consider a charged Gauss-Bonnet black hole in $d$-dimensional spacetime and examine the effect of thermal fluctuations on the thermodynamics of the concerned black hole. At first we take the first order logarithmic correction term in entropy and compute the thermodynamic potentials like Helmholtz free energy $F$, enthalpy $H$ and Gibbs free energy $G$ in the spherical, Ricci flat and hyperbolic topology of the black hole horizon, respectively. We also investigate the $P$-$V$ criticality and calculate the critical volume $V_c$, critical pressure $P_c$ and critical temperature $T_c$ using different equations when $P$-$V$ criticality appears. We show that there is no critical point without thermal fluctuations for this type of black hole. We find that the presence of logarithmic correction in it is necessary to have critical points and stable phases. Moreover, we study the stability of the black holes by employing the specific heat. Finally, we study the geometrothermodynamics and analyse the Ricci scalar of the Ruppeiner metric graphically for the same.

gr-qc

Thermodynamics of $d$-Dimensional Charged AdS (Anti-de Sitter) Black Holes: Hamiltonian Approach and Clapeyron Equation

The study of thermodynamics in the view of the Hamiltonian approach is a newest tool to analyze the thermodynamic properties of the black holes. In this letter, we investigate the thermodynamics of $d$-dimensional ($d>3$) asymptotically AntideSitter black holes. A thermodynamic representation based on symplectic geometry is introduced in this letter. We extend the thermodynamics of $d-$dimensional charged AntideSitter black holes in the views of a Hamiltonian approach. Firstly, we study the thermodynamics in reduced phase space and correlate with the Schwarzschild solution. Then we enhance it in the extended phase space. In an extended phase space the thermodynamic equations of state are stated as constraints. We apply the canonical transformation to analyze the thermodynamics of said type of black holes. We plot $P$-$v$ diagrams for different dimensions $d$ taking the temperatures $T T_c$ and analyze the natures of the graphs and the dependencies on $d$. In theses diagrams, we point out the regions of coexistence. We also examine the phase transition by applying "Maxwell's equal area law" of the said black holes. Here we find the regions of coexistence of two phases which are also depicted graphically. Finally, we derive the "Clapeyron equation" and investigate the latent heat of isothermal phase transition.

gr-qc