arXiv subjects

B. C. Paul

Publications and source records attributed to B. C. Paul.

At least 19 recordsLinked to original sources

arXiv2026

Emergent Universe Scenario in the Modified Chaplygin gas : Towards an Exact Solution and Observational Constraints

The Modified Chaplygin Gas (MCG) model is revisited as a unified framework for describing cosmic evolution. Due to the nonlinear field equations, exact solutions in cosmic time are generally unavailable; hence, a first-order approximation is employed to obtain the scale factor and flip time. The effective equation-of-state parameter evolves from a positive value in the early universe to $w_{\mathrm{eff}}\approx -1$ at late times, yielding a transition from deceleration to acceleration and approaching the $Λ$CDM limit. In the phantom regime, the model admits an emergent solution with a finite minimum scale factor and a late-time de~Sitter--like phase. The emergent solution is geodesically complete and free from curvature singularities, as confirmed by the regular Kretschmann scalar. CMB constraints for all considered cases remain consistent with observations within the $1σ$ confidence level, while the combined Hubble-$57$ and CMB analysis provides improved parameter constraints. The solutions are further verified independently using the Raychaudhuri equation, providing an additional consistency check. Overall, the results establish the MCG model as a self-consistent and observationally viable framework for cosmic evolution within general relativity.

gr-qc↗

arXiv2026

Higher-Dimensional Cosmology in the Framework of Generalized Chaplygin Gas: An Approximation Method

We present a higher-dimensional cosmological model with a generalized Chaplygin-type gas to explain the late-time acceleration of the Universe. The model admits dimensional reduction and reproduces the standard $4D$ Chaplygin cosmology for $d=0$. Using Hubble-$57$ data, we obtain observational constraints on the model parameters. Due to the highly nonlinear nature of the field equations, the model successfully describes both matter-dominated and accelerating phases of the Universe. By adopting a first-order approximation, we derive exact time-dependent solutions for the scale factors in both ordinary and extra dimensions, along with an exact expression for the flip time. The model approaches the $Λ$CDM scenario at large scale factors and exhibits the desired transition from deceleration to acceleration. The evolution of the deceleration parameter, effective EoS parameter, and jerk parameter is also discussed. The constrained value of the Chaplygin parameter $α\ll 1$ disfavors the pure Chaplygin gas model $(α= 1)$ and suggests that smaller values of $α$ are favored at late times.

gr-qc↗

arXiv2025

Cosmological models with Bounce scenario in f(Q,C)-gravity

We present a bounce universe in modified $f(Q,C)$ gravity considering linear as well as exponential form of gravity. Bounce cosmological models are introduced to remove the singularity problem of the early universe. A new quadratic boundary term ($C^2$), which is added in the modified gravity to study different features of the universe in the framework of bouncing cosmology. Both power law expansion and exponential emergent universe are explored in linear modified gravity. The energy conditions and stability of cosmological bounce are investigated. We also compared power law expansion in linear and exponential form of modified gravity.

gr-qc↗

arXiv2025

Dynamical analysis of bulk viscous cosmological model in $F(T)$ gravity

Bulk viscous cosmological models is presented in the teleparallel ($F(T)$, where $T$ denotes torsion) gravity. In the teleparallel gravity, the Lagrangian of the gravitational action contains a general function $F(T)= T+ f(T)=(1+ γ) T+α(-T)^n$, where $γ$, $n$ and $α$ are dimensional constants. Cosmological solutions in Eckart theory and Truncated Israel Stewart theory are talked about in $F(T)=(1+ γ) T+α(-T)^n$ gravity form which is one of the most generalized gravity form in the torsional gravity. The substantial and geometrical prospective of the cosmological models in Eckart theory and Truncated Israel Stewart theory in $F(T)$ gravity are deliberated for flat Friedmann-Robertson-Walker space time. Dynamical analysis of the fixed points of exponential expansion in $F(T)$ with bulk viscous cosmological models are studied here. The characteristics of the various cosmological parameters such as bulk viscous pressure, energy density, scale factor, Hubble parameter and entropy evolution are studied in Power law and Exponential models. Stability analysis of the exponential models are also argued with cosmic growth in the relevant theories by using directional plots. The cosmological models are supported by observations results.

gr-qc↗

arXiv2025

Uniform rate inflation in $f(T, \mathcal{T})$-gravity

We present uniform rate inflation in a modified $f(T, \mathcal{T}) $ gravity. It is found that early inflation can be realized even with a scalar field field with a quadratic potential and a negative cosmological constant. We construct inflationary model for the inflaton field without slow-roll approximation. The inflaton field rolling at a constant speed permits a universe with sufficient inflation which encompasses the present universe fairly well. The initial value of the scalar satisfies a lower limit which is sufficiently large compared to the field required for chaotic inflation. The Planck prediction for cosmological perturbations are estimated. It is found that the cosmological model is stable.

gr-qc↗

arXiv2023

Non-singular flat universes in braneworld and Loop Quantum Cosmology

In this paper we take matter source with non-linear Equation of state (EoS) that has produced non-singular Emergent cosmology for spatially flat universe in General Relativity and minimally coupled scalar field with two different potentials that produce an inflationary emergent universe for positive spatial curvature in the relativistic context. We study all these three cases both in the context of Randall-Sundrum braneworld and effective Loop quantum cosmology (LQC) for zero spatial curvature that is observationally favoured and in the absence of any effective cosmological constant term. We solve the modified Friedmann equation in each case to obtain the time evolution of the scale factor and use it to check whether the initial singularity can be averted. In almost all the cases we find the initial singularity is absent. We study the nature of the slow roll inflation in the cases where we obtain inflationary emergent universes. The inflationary scenario is found to be improved than in a standard relatvistic context and we compare the improved scenario for both the braneworld and LQC models. Interestingly, we also obtain bouncing and cyclic universes from our analysis in some cases. We find that the initial singularity can be averted for a spatially flat universe with specific choice of matter EoS or scalar field potential, which do not violate the Null Energy condition in most cases, taking into account effective high energy (curvature) corrections with or without extra dimensions.

gr-qc↗

arXiv2023

Stable Gravastar model in Cylindrically Symmetric Space-time

We represent a class of new Gravastar solutions as obtained by Mazur and Mottola in a gravitational Bose-Einstein condensate (GBEC) in a cylindrical symmetric space-time. A stable gravastar with three distinct regions namely, (i) Interior de-Sitter space, (ii) Intermediate thin shell with a slice of finite length and (iii) exterior vacuum region. The interior region is characterised by positive energy density and negative pressure $(p=-ρ)$, which exerts a repulsive outward force at all points on the thin shell. The thin shell separating the interior and exterior is supposed to be consisting of ultra-relativistic stiff fluid having equation of state $p=ρ$, which satisfies the Zel'dovich's criteria. This thin shell, which is considered as the critical surface for the quantum phase transition, replaces both the classical de-Sitter and Schwarzschild event horizons. The new solution is free from any singularities as well as any information paradox. The energy density, total energy, proper length, mass and entropy of the shell region are explored in this model and the gravastar model is stable and physically viable.

gr-qc↗

arXiv2022

Interacting and Non-interacting Reńyi Holographic Dark Energy Models in DGP Braneworld

We investigate both the interacting and non-interacting Rényi Holographic Dark Energy (RHDE) models in DGP brane world framework. Cosmological parameters and their evolutions are probed to obtain realistic cosmological models. We note that both the models accommodate the present accelerating phase of expansion with the observed dark energy density. Classical stability of the cosmological model and $Om$- diagnostic are also studied to test the suitability of the cosmological models obtained in the presence of RHDE in DGP braneworld.

gr-qc↗

arXiv2021

Rényi Holographic Dark Energy Models In Multidimensional Universe

Rényi Holographic Dark Energy (RHDE) Models has been studied in different dimensions considering a flat FRW metric, subsequently exploring the nature of evolution of different cosmological parameters. It is seen that the number of dimensions affect the evolution of the cosmological parameters and the transition of the universe from decelerating to accelerating phase. The statefinder diagnosis has been performed and it is seen that the model corresponds to the $ΛCDM$ model for the present and future universe.

gr-qc↗

arXiv2021

Accelerating Universe in Higher Dimensional Space Time : an alternative approach

We have discussed here a higher dimensional cosmological model and explained the recent acceleration with a Chaplygin type of gas. Dimensional reduction of extra space is possible in this case. Our solutions are general in nature because all the well known results of 4D Chaplygin driven cosmology are recovered when $d = 0$. We have drawn the best fit graph using the data obtained by the differential age method (CC) and it is seen that the graph favours only one extra dimension. That means the Chaplygin gas is apparently dominated by a 5D world. Relevant to point out that the final equation in this case are highly nonlinear in nature. Naturally it is not possible to obtain explicit solution of the 4D scale factor with time. To circumvent this difficulty, we consider a first order approximation of the key equation which has made it possible to get time explicit solution of 4D scale factor in exact form as well as the expression of extra dimensions. It may be pointed out that for large four dimensional scale factor this solution mimics $Λ$CDM model. An analysis of flip time is also studied both analytically and graphically in some detail. It clearly shows that early \emph{flip} occurs for higher dimensions. It is also seen that the rate of dimensional reduction is faster for higher dimensions. So we may conclude that the effect of compactification of extra dimension helps the acceleration.

gr-qc↗

arXiv2021

Comprehensive broadband study of accreting neutron stars with Suzaku: Is there a bi-modality in the X-ray spectrum?

We present a broadband spectral analysis of accreting neutron stars using data from XIS and PIN onboard \emph{Suzaku}. From spectral fits of these sources with a single continuum model including a powerlaw and high energy cut-off, cyclotron lines (where required), we studied the correlation between various spectral parameters. Among 39 sources we studied, 16 are those where the existence of a cyclotron line is known in literature, and 29 need a cutoff energy. Among these 29 sources, 18 have cutoff energy bunched in a range of 3-10 keV while for 11 sources, it spreads over 12-25 keV. This bi-modal behaviour is not based on the specific nature of the systems being a Be XRB or supergiant HMXB, nor on different beaming patterns characterizing their X-ray emission (as inferred from simultaneous study of their pulse profiles). The broadband coverage of \emph{Suzaku} also shows that the cutoff energies saturate for higher values of cyclotron line energies - consistent with previous works in literature - for both the groups and the width of the cyclotron line show a weak correlation with the cyclotron line energy. We also find an anticorrelation with luminosity for both spectral index and folding energy, respectively. Unlike previous works, we did not detect any anticorrelation between X-ray luminosity and EW of K$α$ lines. Finally, we show that the EW and flux of the iron K$α$ line are smaller in SFXTs than classical NS-HMXBs. We discuss these findings in terms of different properties of stellar winds and accretion mechanisms.

astro-ph.HE↗

arXiv2020

Anisotropic Compact Objects in Modified $f(R,T)$ gravity

We obtain a class of anisotropic spherically symmetric relativistic solutions of compact objects in hydrostatic equilibrium in the $f(R,T) =R+2χT$ modified gravity, where $R$ is the Ricci scalar, $T$ is the trace of the energy momentum tensor and $χ$ is a dimensionless coupling parameter. The matter Lagrangian is $L_{m}=- \frac{1}{3}(2p_{t}+p_{r})$, where $p_r$ and $p_t$ represents the radial and tangential pressures. Compact objects with dense nuclear matter is expected to be anisotropic. Stellar models are constructed for anisotropic neutron stars working in the modified Finch-Skea (FS) ansatz without preassuming an equation of state. The stellar models are investigate plotting physical quantities like energy density, anisotropy parameter, radial and tangential pressures in all particular cases. The stability of stellar models are checked using the causality conditions and adiabatic index. Using the observed mass of a compact star we obtain stellar models that predicts the radius of the star and EoS for matter inside the compact objects with different values of gravitational coupling constant $χ$. It is also found that a more massive star can be accommodated with $χ<0$. The stellar models obtained here obey the physical acceptability criteria which show consistency for a class of stable compact objects in modified $f(R, T)$ gravity.

physics.gen-ph↗

arXiv2020

Dynamical Wormholes in Higher Dimensions and the Emergent Universe

Dynamical wormholes in higher dimensions which admit flat emergent universe (EU) model is presented. The EU model is free from initial singularity with other observed features of the universe. The basic assumption of EU model was that the present universe emerged out from a static Einstein universe. EU model originates from a dynamical wormhole, its throat is the seed of Einstein Static universe. A class of cosmological solutions in a higher dimensional flat universe is presented. A new shape function for closed universe is determined. The non-linear equation of state (EoS) corresponds to three types of cosmic fluids. The EoS parameters determine the cosmic fluids. The space-time dimensions determines the rate of change of a particular fluid that varies with the scale factor of a dynamically evolving universe with non-interacting fluids. Considering interaction at time $t > t_0$, among the three types of fluids it is possible to describe the observed universe satisfactorily. In a higher dimensional universe it is found that near the throat null energy condition (NEC) is violated, but away from the throat NEC is found to obey admitting the observed universe for a flat case. Another interesting aspect of the EU model is that it permits late accelerating phase. However, in asymptotic closed or open universe, flat emergent universe can be accommodated with NEC which is obeyed right from the throat to the present epoch. The tension at the throat of the wormhole is estimated which is found to depend on the initial size of the Einstein static universe and dimensions of the universe. It is interesting to note that NEC is not violated to accommodate dynamical wormholes for closed or open universe. Although exotic matter is required at the throat for the flat universe, no exotic matter is required for closed or open universe which encompass the emergent universe.

gr-qc↗

arXiv2020

On the possibility of traversable wormhole formation in the Galactic halo in the presence of scalar field

In the paper we obtain traversable wormhole (TW) solutions in the Einstein gravity with a functional form of the dark matter in the galactic halo. The dark matter model is pseudo isothermal which is derived from modified gravity. For a given central density the possibility of TW is explored determining the shape functions. The null energy condition (NEC) and the weak energy conditions in the model is probed. We also study the existence of TWs considering homogeneous scalar field in addition to the dark matter halo in the galaxies. An interesting observation is that TW solutions exist even if NEC is not violated in the presence of scalar field.

physics.gen-ph↗

arXiv2020

Emergent Universe Scenario in Modified Gauss-Bonnet Gravity

We present modified Gauss-Bonnet gravity without matter in four dimensions which accommodates flat emergent universe (EU) obtained in Einstein's general theory of gravity with a non-linear equation of state. The EU model is interesting which is free from big-bang singularity with other observed features of the universe. It is assumed that the present universe emerged out from a static Einstein universe phase exists in the infinite past. To obtain a flat EU model we reconstructed mimetic modified $f(G)$-gravity ($G$ representing Gauss-Bonnet terms) without matter. The functional form of f(G)-gravity is determined which accommodates the early inflation and late accelerating phases without matter.

astro-ph.CO↗

arXiv2015

Is 4U 0114+65 an eclipsing HMXB?

We present the pulsation and spectral characteristics of the HMXB 4U 0114+65 during a \emph{Suzaku} observation covering the part of the orbit that included the previously known low intensity emission of the source (dip) and the egress from this state. This dip has been interpreted in previous works as an X-ray eclipse. Notably, in this Suzaku observation, the count rate during and outside the dip vary by a factor of only 2-4 at odds with the eclipses of other HMXBs, where the intensity drops upto two orders of magnitude. The orbital intensity profile of 4U 0114+65 is characterized by a narrow dip in the RXTE-ASM (2-12 \rm{keV}) light curve and a shallower one in the Swift-BAT (15-50 \rm{keV}), which is different from eclipse ingress/egress behaviour of other HMXBs. The time-resolved spectral analysis reveal moderate absorption column density (N$_{H}$ - 2-20 $\times$ $10^{22}$ atoms $cm^{-2}$) and a relatively low equivalent width ($\sim$ 30 \rm{eV} \& 12 \rm{eV} of the iron K$_α$ and K$_β$ lines respectively) as opposed to the typical X-ray spectra of HMXBs during eclipse where the equivalent width is $\sim$ 1 \rm{keV}. Both XIS and PIN data show clear pulsations during the dip, which we have further confirmed using the entire archival data of the IBIS/ISGRI and JEM-X instruments onboard \emph{INTEGRAL}. The results we presented question the previous interpretation of the dip in the light curve of 4U 0114+65 as an X-ray eclipse. We thus discuss alternative interpretations of the periodic dip in the light curve of 4U 0114+65.

astro-ph.HE↗

arXiv2015

Emergent Universe with Interacting fluids and Generalized Second Law of Thermodynamics

We investigate the emergent universe scenario in the presence of interacting fluids. The non-linear equation of state (EoS) considered in the general theory of relativity for obtaining emergent universe is effectively a cosmological model with a composition of three fluids. In this paper we consider two models to realize viable cosmological scenarios, {\it viz.}, (i) a two-fluid model with interaction of a pressureless fluid with the fluid having the non-linear EoS needed for the emergent universe, and (ii) a three-fluid model with interaction among the three fluids which originate from the EoS of the emergent universe. It is found that realistic cosmological models in accordance with observations are not ruled out for both the above cases. We further show that the generalized second law of thermodynamics is found to hold good in the emergent universe with interacting fluids.

gr-qc↗

arXiv2014

Variations in the pulsation and spectral characteristics of OAO 1657-415

We present broad-band pulsation and spectral characteristics of the accreting X-ray pulsar OAO 1657--415 with a 2.2 d long \emph{Suzaku} observation carried out covering its orbital phase range $\sim$ 0.12--0.34, with respect to the mid-eclipse. During the last third of the observation, the X-ray count rate in both the X-ray Imaging Spectrometer (XIS) and the HXD-PIN instruments increased by a factor of more than 10. During this observation, the hardness ratio also changed by a factor of more than 5, uncorrelated with the intensity variations. In two segments of the observation, lasting for $\sim$ 30--50 ks, the hardness ratio is very high. In these segments, the spectrum shows a large absorption column density and correspondingly large equivalent widths of the iron fluorescence lines. We found no conclusive evidence for the presence of a cyclotron line in the broad-band X-ray spectrum with \emph{Suzaku}. The pulse profile, especially in the XIS energy band shows evolution with time but not so with energy. We discuss the nature of the intensity variations, and variations of the absorption column density and emission lines during the duration of the observation as would be expected due to a clumpy stellar wind of the supergiant companion star. These results indicate that OAO 1657--415 has characteristics intermediate to the normal supergiant systems and the systems that show fast X-ray transient phenomena.

astro-ph.HE↗