arXiv subjects

B C Paul

Publications and source records attributed to B C Paul.

8 recordsLinked to original sources

arXiv2025

Observationally constrained emergent universe scenario with non-conventional late-time dynamics

In this paper, we attempt to explore the possibility of a obtaining a viable emergent universe scenario supported by a type of fluid known as the extended Chaplygin gas, which extends a modification to the equation of state of the well known modified Chaplygin gas by considering additional higher order barotropic fluid terms. We consider quadratic modification only. Such a fluid is capable of explaining the present cosmic acceleration and is a possible dark energy candidate. We construct a theoretical model of the emergent universe assuming it is dominated by such a fluid at late times. Our model results in non-conventional late-time behavior and deviates from the standard $Λ$-CDM model. Dark energy is found to cross the \textit{phantom} divide in the past and present besides exhibiting \textit{thawing} behaviour in the future, asymptotically leading to transition into a decelerating phase making dark energy a \textit{transient} phenomenon. The qualitative nature of variation of the cosmological parameters resulting from model parameters observationally constrained through Markov Chain Monte Carlo sampling of Pantheon+OHD data is interestingly found to resemble the DESI results. Also,the value of $H(z)$ at a redshift $z=2.34$ and present value of Hubble parameter fits much better than $Λ$-CDM with recent observations. This leads us to the realization that such a fluid is not only a probable candidate for dark energy, but also sources an emergent universe unlike modified Chaplygin gas and the initial singularity problem can be resolved in a flat universe within the standard relativistic context.

gr-qc↗

arXiv2023

Lorentzian wormholes in an emergent universe

A non-singular Emergent Universe (EU) scenario within the realm of standard Relativistic physics requires a generalization of the Equation of State (EoS) connecting the pressure and energy density. This generalized EoS is capable of describing a composition of exotic matter, dark energy and cosmological dust matter. Since the EU scenario is known to violate the Null Energy Condition, we investigate the possibility of presence of static, spherically symmetric and traversable Lorentzian wormholes in an EU. The obtained shape function is found to satisfy the criteria for wormhole formation, besides the violation of the NEC at the wormhole throat and ensuring traversability such that tidal forces are within desirable limits. Also, the wormhole is found to be stable through linear stability analysis. Most ${importantly}$, the numerical value of the emergent universe parameter $B$ as estimated by our wormhole model is in agreement with and lies within the range of values as constrained by observational data in a cosmological context. Also, the negative sign of the second EU parameter $A$ as obtained from our wormhole model is in agreement with the one required for describing an EU, which further indicates on the existence of such wormholes in an emergent universe ${without}$ accounting for any additional exotic matter field or any modification to the gravitational sector.

gr-qc↗

arXiv2022

Skyrme Fluid in Anisotropic Universe

Cosmological solutions are obtained in an anisotropic Kantowski-Sachs (KS) and Bianchi Type-I universes considering a cosmological constant with Skyrme fluid. Interestingly, the solutions obtained here in both the KS and Bianchi-I anisotropic universes are found isotropize at late time due to the presence of the Skyrme fluid even in the absence of $Λ$ term or any inflationary mechanism involving the inflaton field. A comparative study of both the anisotropic cosmological models are carried out here and found that Bianchi-I universe admits oscillatory solutions for a given matter configuration. We also note that the emergent universe model can be obtained with Skyrme fluid. The anisotropy, deceleration and jerk parameters have been studied along with the linear perturbative stability to explore efficacy of the model. Both the cosmological models are stable in the absence of cosmological constant besides their compatibility with observational data. Thus, we claim Skyrme fluid a possible source for isotropization of an anisotropic universe via accelerated expansion, which is capable of reproducing some observed features of the universe.

gr-qc↗

arXiv2021

Morris-Thorne Wormholes in $f(R,T)$ modified theory of gravity

We study static traversable wormholes obtained by Morris and Thorne in general relativity (GR) in the framework of a modified theory of gravity. The modified gravitational action $f(R,T)$ is a function of the Ricci scalar ($R$) and of the trace of the energy momentum tensor ($T$). For a modified gravity $f(R,T)=R+αR^{2}+λT$, where $α$ and $λ$ are constants, we obtain wormhole solutions (WH) with normal matter for a relevant shape functions. The energy conditions are checked at the throat and away from the throat of the WH. The coupling parameters $α$ and $λ$ in the gravitational action play an important role to accommodate the matter composition. For a given $λ$, WH solutions are found in the presence of exotic matter at the throat for $α<0$. It is shown that WH exists in the modified gravity without exotic matter when $α>0$. We consider two different shape functions to investigate the existence of WH in the presence of exotic or normal matter. A class of WH solutions exist with anisotropic fluid provided $λ\neq - 8 π$. In flat asymptotic regions on both sides of the throat with anisotropic source and $λ= - 8 π$, it does not permit WH as per no go theorem. As $λ\rightarrow 0$ in the gravitational action, all the energy conditions are obeyed with the hybrid shape function indicating existence of WH with normal matter.

gr-qc↗

arXiv2021

Study of Gravastars in Rastall Gravity

Gravastars have been considered as a feasible alternative to black holes in the past couple of decades. Stable models of gravastar have been studied in many of the alternative gravity theories besides standard General Relativity (GR). The Rastall theory of gravity is a popular alternative to GR, specially in the cosmological and astrophysical context. Here, we propose a stellar model under the Rastall gravity following Mazur-Mottola's \cite{Mazur2001,Mazur2004} conjecture. The gravastar consists of three regions, viz., ($I$) Interior region, ($II$) Intermediate shell region, and ($III$) Exterior region. The pressure within the interior core region is assumed with a constant negative matter-energy density which provides a repulsive force over the entire thin shell region. The shell is assumed to be made up of fluid of ultrarelativistic plasma which follows the Zel'dovich's conjecture of stiff fluid \cite{Zeldo1962,Zel'dovich1972}. It is also assumed that the pressure is proportional to the matter-energy density according to Zel'dovich's conjecture, which cancel the repulsive force exerted by the interior region. The exterior region is completely vacuum which is described by the Schwarzschild-de Sitter solution. Under all these specifications we obtain a set of exact and singularity-free solutions of the gravastar model presenting several physically valid features within the framework of Rastall gravity. The physical properties of the shell region namely, the energy density, proper length, total energy and entropy are explored. The stability of the gravastar model is investigated using the surface redshift against the shell thickness and maximizing the entropy of the shell within the framework of Rastall gravity.

gr-qc↗

arXiv2014

Observational Constraints on EoS parameters of Emergent Universe

We investigate emergent universe model using recent observational data for the background tests as well as for the growth tests. The background test data comprises of Hubble data, Baryon Acoustic Oscillation (BAO) data, cosmic microwave background (CMB) shift data, Union compilation data. The observational growth data are obtained from Wiggle-Z measurements and rms mass fluctuations data from Lyman-$α$ measurements at different red shifts. The flat emergent universe model obtained by Mukherjee {\it et. al.} is permitted with a non-linear equation of state (in short, EoS) ($p=Aρ-Bρ^{\frac{1}{2}}$), where $A$ and $B$ are constants. The observed cosmological data are used here to estimate the range of allowed values of EoS parameters numerically. The best-fit values of the EoS and growth parameters are determined making use of chi-square minimization technique. The analysis is carried out here considering the Wang-Steinhardt ansatz for growth index ($γ$) and growth function ($f$ defined as $f=Ω_{m}^γ (a)$). Subsequently, the best-fit values of the EoS parameters are used to study the evolution of the growth function $f$, growth index $γ$, state parameter $ω$ with red shift parameter $z$. The present value of the parameters, namely, $f$, $γ$, $ω$, $Ω_{m}$ are also estimated. The late accelerating phase of the universe in the model is accommodated satisfactorily.

gr-qc↗

arXiv2009

Emergent Universe Scenario in the Einstein-Gauss-Bonnet Gravity with Dilaton

We obtain cosmological solutions which admit emergent universe (EU) scenario in the framework of Einstein Gauss-Bonnet gravity coupled with a dilaton field in four dimensions. The coupling parameter of the Gauss-Bonnet terms and the dilaton in the theory are determined for obtaining an EU scenario. The corresponding dilaton potential which admits such scenario is determined. It is found that the Gauss- Bonnet (GB) terms coupled with a dilaton field plays an important role in describing the dynamics of the evolution of the early as well as the late universe. We note an interesting case where the GB term dominates initially in the asymptotic past regime, subsequently it decreases and thereafter its contribution in determining the dynamics of the evolution dominates once again. We note that the Einstein static universe solution permitted here is unstable which the asymptotic EU might follow. We also compare our EU model with supernova data.

hep-th↗

arXiv2008

Probability for Primordial Black Holes in Multidimensional Universe with Nonlinear Scalar Curvature Terms

We investigate multi-dimensional universe with nonlinear scalar curvature terms to evaluate the probability of creation of primordial black holes. For this we obtain Euclidean instanton solution in two different topologies: (a) $S^{D-1}$ - topology which does not accommodate primordial black holes and (b) $S^1\times S^{D-2}$-topology which accommodates a pair of black holes. The probability for quantum creation of an inflationary universe with a pair of black holes has been evaluated assuming a gravitational action which is described by a polynomial function of scalar curvature with or without a cosmological constant ($Λ$) using the framework of semiclassical approximation of Hartle-Hawking boundary conditions. We discuss here a class of new gravitational instantons solution in the $R^4$-theory which are relevant for cosmological model building.

astro-ph↗