Searcharxiv⌕ Search

arXiv subjects

Anirudh Pradhan

Publications and source records attributed to Anirudh Pradhan.

At least 73 records · Page 4Linked to original sources
arXiv2024

Power law cosmology in modified theory with thermodynamics analysis

In this paper, we consider a cosmological model in $ f(R, G) $ gravity in a flat space-time, where $ R $ is the Ricci scalar and $ G $ is the Gauss-Bonnet invariant. The function $ f(R, G) $ is taken as a linear combination of $ R $ and an exponential function of $ G $. We analyze the observational constraints under a power law cosmology which depends on two physical parameters: the Hubble constant $ H_0 $ and the deceleration parameter $ q $. We constrain these two dependent parameters using the latest 77 points of the OHD data, 1048 points of the Pantheon data, and the joint data OHD+Pantheon and compare the results with the $ Λ$CDM. Also, we speculate constraints using a simulated data set for the future JDEM (Joint Dark Energy Mission)/Omega, supernovae survey. We see that $ H_0 $ is in very close agreement with some of the latest results from the Planck Collaboration that assume the $ Λ$CDM model. Our work in power law cosmology better fits the Pantheon data than the earlier analysis \cite{Kumar:2011sw, Rani:2014sia}. However, the constraints obtained on $ H $ average, $ $ and $ q $ average, $ $ using the simulated data set for the future JDEM/Omega, supernovae survey are found to be inconsistent with the values obtained from the OHD and the Pantheon data. Additionally, we discuss statefinder diagnostics and see that the power law models approach the standard $Λ$CDM model ($ q\rightarrow -1 $). This model satisfies the Generalized Second Law of Thermodynamics. Finally, we conclude that the power law cosmology in $ f(R, G) $ gravity explains most of the distinguished attributes of evolution in cosmology.

gr-qc↗
arXiv2024

Is dark energy necessary for the sustainability of traversable wormholes?

In the standard approach to studying wormhole geometry, the presence of dark energy is unavoidable to ensure traversability. The dark energy provides the negative gravity effect to keep the throat open. The question we analyse is whether the same can be achieved without dark energy. It turns out that if we couple the trace of energy-momentum with the standard Einstein-Hilbert Lagrangian and utilise a sppecific equation of state then dark energy may be obviated. The Casimir stress energy is known to result in the violation of the null energy condition (NEC) on the energy momentum tensor. This phenomenon makes such an equation of state (EoS) an ideal candidate to generate traversable wormhole (WH) geometries. The laboratory proven phenomenon provides a mechanism to sustain an open WH throat without having to appeal to dark energy. We generate two classes of WH solutions with this in $f(R,T)$ gravity theory, where $R$ represents the Ricci scalar and $T$ is the trace of the stress-energy tensor. For the background geometry we choose a static and spherically symmetric metric, and derive the field equations for exact WH solutions. For the specific choice of the Casimir EoS relating the energy-momentum tensor components [ Kar and Sahdev: Phys. Rev D {\bf 52} 2030 (1995)] and different choices for redshift functions, we determine the WH geometry completely. The obtained WH solutions violate the NECs and all qualitative constraints demanded of physically realisable WHs are satisfied. This is demonstrated via graphical plots for a suitably chosen range of values of the $f(R,T)$ coupling parameter. Furthermore, our study involved an investigation into the repulsive effect of gravity, which revealed that its presence leads to a negative deflection angle for photons traveling along null geodesics.

gr-qc↗
arXiv2024

Investigating Stable Quark Stars in Rastall-Rainbow Gravity and Their Compatibility with Gravitational Wave Observations

We present a stable model for quark stars in Rastall-Rainbow (R-R) gravity. The structure of this configuration is obtained by utilizing an interacting quark matter equation of state. The R-R gravity theory is developed as a combination of two distinct theories, namely, the Rastall theory and the gravity's rainbow formalism. Depending on the model parameters ($\barλ, η, Σ$), the mass-radius relations are numerically computed for modified Tolman-Oppenheimer-Volkoff (TOV) equations with proper boundary conditions. The stability of equilibrium configuration has been checked through the static stability criterion, adiabatic index and the sound velocity. Our calculations predict larger maximum masses for quark stars, and the obtained results are compatible with accepted masses and radii values, including constraints from GW190814 and GW170817 events in all the studied cases.

gr-qc↗
arXiv2023

Effects of anisotropic pressure on interacting quark star structure

Perturbative Quantum Chromodynamics (pQCD) corrections and color superconductivity predict that strongly interacting matter can reveal new physical phenomena under extreme conditions. Taking into account these interaction effects, we investigate the role of anisotropic pressure in quark stars composed of interacting quark matter. Adopting two physically well-motivated anisotropy profiles, we numerically solve the stellar structure equations in order to explore the consequences of anisotropic pressure on various macroscopic properties such as radius, gravitational mass, surface redshift, moment of inertia, tidal Love number and oscillation spectrum. Remarkably, for both anisotropy models, negative anisotropies increase the radial stability of interacting quark stars, while the opposite occurs for positive anisotropies. However, for the Bowers-Liang profile, the central density corresponding to the maximum-mass point does not coincide with the central density where the squared oscillation frequency vanishes, indicating that the existence of stable anisotropic interacting quark stars is possible beyond the maximum mass for negative anisotropies. Additionally, we compare our theoretical predictions with several observational mass-radius measurements and tidal deformability constraints, which suggest that both strong interaction effects and anisotropy effects play a crucial role in describing compact stars observed in the Universe.

gr-qc↗
arXiv2023

Non-Commutative Effects on Wormholes in Rastall-Rainbow Gravity

In this present article, we explore the physical properties and characteristics of static, spherically symmetric wormholes in the background of Rastall-Rainbow gravity. The Rastall-Rainbow gravity theory has recently been proposed as a combination of two theories, namely, the Rastall theory and the Rainbow description. We implemented noncommutativity by adopting two different distributions of energy density (Gaussian and Lorentzian) in the Morris and Thorne metric. We solve the field equations analytically and discuss all the properties of wormholes depending on the two model parameters. Notably, for specific parameter ranges, one can alleviate the violation of the WEC at the throat and its neighbourhood.

gr-qc↗
arXiv2023

A flat FLRW dark energy model in f(Q,C)-gravity theory with observational constraints

In the recently suggested modified non-metricity gravity theory with boundary terms in a flat FLRWspacetime universe, dark energy scenarios of cosmological models are examined in this study. An arbitrary function, $f(Q, C)=Q+αC^{2}$, has been taken into consideration, where Q is the non-metricity scalar, C is the boundary term denoted by $C = R-Q$, and $α$ is the model parameter, for the action that is quadratic in C. The Hubble function $H(z) = H0[c_{1} (1+z)^{n}+c_{2}]^{1/2}$, where H0 is the current value of the Hubble constant and n c, and $c_{2}$ are arbitrary parameters with $c_{1}+c_{2}= 1$, has been used to examine the dark energy characteristics of the model. We discovered a transit phase expanding universe model that is both decelerated in the past and accelerated in the present, and we discovered that the dark energy equation of state (EoS) $(ω^{de})$ behaves as $(-1\leq ω^{de}<2)$. The $O_{m}$ diagnostic analysis reveals the quintessence behavior in the present and the cosmological constant scenario in the late-time universe. Finally, we calculated the universe's current age, which was found to be quite similar to recent data.

gr-qc↗
arXiv2023

Traversable wormholes in Rastall-Rainbow Gravity

In this paper, we investigate the existence of asymptotically flat wormhole geometries within the framework of Rastall-Rainbow modified gravity, a synthesis of two distinct theoretical models: Rastall theory and the Rainbow description. Our study uncovers that, when considering specific combinations of free parameters and equations of state, the emergence of static and spherically symmetric wormholes is not feasible within a zero-tidal-force context. By considering the subset of viable solutions, we conduct a rigorous assessment of their stability through adiabatic sound velocity analysis and scrutinize their compliance with the Weak Energy Condition (WEC). In summary, our inquiry provides insights into how the interplay between Rastall parameters and Rainbow functions may alleviate violations of energy conditions in these modified gravity scenarios.

gr-qc↗
arXiv2023

An FLRW accelerating universe model in Weyl type $f(Q)$ gravity and Observational Constraints

We propose to develop a cosmological model of the universe based on Weyl type $ f(Q) $ gravity which shows the transition from decelerating in the past to acceleration at present by considering a particular functional form of $ f(Q) $ gravity as $ f(Q) = ({H_0}^2) (α_1 + α_2 \hskip0.05in log ({H_0^{-2}} Q)) $. We have solved Weyl type $ f(Q) $ gravity field equations numerically and have obtained numerical solutions to the Hubble and deceleration parameters, distance modulus, and apparent magnitudes of stellar objects like SNIa Supernovae. We have also obtained numerical solutions for the Weyl vector $ w $, non-metricity scalar $ Q $, and the Lagrangian multiplier $ λ$ appearing in the action of $ f(Q) $ gravity. We have compared our theoretical solutions with the error bar plots of the Observed Hubble data set of $ 77 $ points, $ 580 $ distance modulus SNIa data set, and $ 1048 $ supernova Pantheon data sets of apparent magnitudes. It is found that our results fit well with the observed data set points. \bf{The model envisages a unique feature that although the universe is filled with perfect fluid as dust whose pressure is zero, still the weyl vector dominance f(Q) creates acceleration in it. }

gr-qc↗
arXiv2023

An f(R,T) Gravity Based FLRW Model and Observational Constraints

We attempt to construct a Friedmann-Lemaitre-Robertson-Walker(FLRW) cosmological model in $f(R, T)$ gravity which exhibits a phase transition from deceleration to acceleration at present. We take $f(R,T) = R + 2 λT$, $λ$ being an arbitrary constant. In our model, the $λ$ parameter develops a negative pressure in the universe whose Equation of state is parameterized. The present values of model parameters such as density, Hubble, deceleration, Equation of state, and $λ$ are estimated statistically by using the Chi-Square test. For this, we have used three different types of observational data sets: the $46$ Hubble parameter data set, the SNeIa $715$ data sets of distance modulus, and the 66 Pantheon data set (the latest compilation of SNeIa 40 bined plus 26 high red shift apparent magnitude $m_b$ data set in the red shift ranges from $0.014 \leq z \leq 2.26 $). We have calculated the transitional red shift and time. The estimated results for the present values of various model parameters are found as per expectations and surveys. Interestingly, we get the present value of the density $ρ_0$, $\simeq 1.5 ρ_c $. The critical density is estimated as $ρ_c\simeq 1.88 ~ h_0^2~10^{-29}~gm/cm^3 $ in the literature. The higher value of the present density is attributed to the presence of some additional energies in the universe apart from baryon energy. We have examined the behavior of the pressure in our model. It is negative and produces acceleration in the universe. Its present value is obtained as $p_0 \simeq - 0.7 ρ_0$.

gr-qc↗
arXiv2023

Wormholes and energy conditions in $f(R,T)$ gravity

We explore the existence of wormholes in the context of $f(R,T)$ gravity. The $f(R,T)$ theory is a curvature-matter coupled modified gravity that depends on an arbitrary function of the Ricci scalar $R$ and the trace of the stress-energy tensor $T$. In this work, we adopt two different choices for the matter Lagrangian density ($\mathcal{L}_m= \mathcal{P}$ and $\mathcal{L}_m= p_r$) and investigate the impact of each one on wormhole structure. By adequately specifying the redshift function and the shape function, we found a variety of exact wormhole solutions in the theory. Our finding indicates that, for both classes of wormholes the energy density is always positive throughout the spacetime, while the radial pressure is negative. This means exotic matter is necessary for the existence of wormholes in $f(R,T)$ gravity.

gr-qc↗
arXiv2023

The Reconstruction of Constant Jerk Parameter with $f(R,T)$ Gravity in Bianchi-I spacetime

We have developed a Bianchi I cosmological model of the universe in $f(R,T)$ gravity theory which fit good with the present day scenario of accelerating universe. The model displays transition from deceleration in the past to the acceleration at the present. As in the $Λ$CDM model, we have defined the three energy parameters $Ω_m$, $Ω_μ$ and $Ω_σ$ such that $Ω_m$ + $Ω_μ$ + $Ω_σ$ = 1. The parameter $Ω_m$ is the matter energy density (baryons + dark matter), $Ω_μ$ is the energy density associated with the Ricci scalar $R$ and the trace $T$ of the energy momentum tensor and $Ω_σ$ is the energy density associated with the anisotropy of the universe. We shall call $Ω_μ$ dominant over the other two due to its higher value. We find that the $Ω_μ$ and the other two in the ratio 3:1. 46 Hubble OHD data set is used to estimate present values of Hubble $H_0$, deceleration $q_0$ and jerk $j$ parameters. 1$σ$, 2$σ$ and 3$σ$ contour region plots for the estimated values of parameters are presented. 580 SNIa supernova distance modulus data set and 66 pantheon SNIa data which include high red shift data in the range $0\leq z\leq 2.36$ have been used to draw error bar plots and likelihood probability curves for distance modulus and apparent magnitude of SNIa supernova's. We have calculated the pressures and densities associated with the two matter densities, viz., $p_μ$, $ρ_μ$, $p_m$ and $ρ_m$, respectively. The present age of the universe as per our model is also evaluated and it is found at par with the present observed values.

gr-qc↗
arXiv2023

Reconstruction of an Observationally Constrained $f(R, T)$ gravity model

In this paper, an attempt is made to construct a Friedmann-Lemaitre-Robertson-Walker model in $f(R,T)$ gravity with a perfect fluid that yields acceleration at late times. We take $f(R,T)$ as $R$ + $8πμT$. As in the $Λ$CDM model, we take the matter to consist of two components, viz., $Ω_m$ and $Ω_μ$ such that $Ω_m$ + $Ω_μ$=1. The parameter $Ω_m$ is the matter density (baryons + dark matter), and $Ω_μ$ is the density associated with the Ricci scalar $R$ and the trace $T$ of the energy momentum tensor, which we shall call dominant matter. We find that at present $Ω_μ$ is dominant over $Ω_m$, and that the two are in the ratio 3:1 to 3:2 according to the three data sets: (i) 77 Hubble OHD data set (ii) 580 SNIa supernova distance modulus data set and (iii) 66 pantheon SNIa data which include high red shift data in the range $0\leq z\leq 2.36$. We have also calculated the pressures and densities associated with the two matter densities, viz., $p_μ$, $ρ_μ$, $p_m$ and $ρ_m$, respectively. It is also found that at present, $ρ_μ$ is greater than $ρ_m$. The negative dominant matter pressure $p_μ$ creates acceleration in the universe. Our deceleration and snap parameters show a change from negative to positive, whereas the jerk parameter is always positive. This means that the universe is at present accelerating and in the past it was decelerating. State finder diagnostics indicate that our model is at present a dark energy quintessence model. The various other physical and geometric properties of the model are also discussed.

gr-qc↗
arXiv2023

The Reconstruction of Constant Jerk Parameter with $f(R,T)$ Gravity

In this work, we have developed an FLRW type model of a universe which displays transition from deceleration in the past to the acceleration at the present. For this, we have considered field equations of $f(R,T)$ gravity and have taken $f(R,T) = R + 2 λT$, $λ$ being an arbitrary constant. We have estimated the $λ$ parameter in such a way that the transition red shift is found similar in the deceleration parameter, pressure and the equation of state parameter $ω$. The present value of Hubble parameter is estimated on the basis of the three types of observational data set: latest compilation of $46$ Hubble data set, SNe Ia $580$ data sets of distance modulus and $66$ Pantheon data set of apparent magnitude which comprised of 40 SN Ia binned and 26 high redshift data's in the range $0.014 \leq z \leq 2.26 $. These data are compared with theoretical results through the $ χ^2 $ statistical test. Interestingly, the model satisfies all the three weak, strong and dominant energy conditions. The model fits well with observational findings. We have discussed some of the physical aspects of the model, in particular the age of the universe.

gr-qc↗
arXiv2023

Barrow entropic Quintessence and Dilation dark energy Models with Generalized HDE cut-off

In the present work, we have analyzed the behaviors of extension of generalized Barrow holographic dark energy(`BHDE'). A ``generalized BHDE model based on the particle and the future horizon using infrared cut-off" was proposed by Nojiri et al. (2022). In this work, we have reviewed the generalized BHDE extension under the assumption of a generalized HDE cut-off. Using a scale factor of the form $a = k t^m$, the dynamics of the cosmos have been discussed through graphic demonstration. By applying the ``open-source emcee Python package", the values of the free parameters $k$ and $m$ are estimated on 57 OHD points by the Markov Chain Monte Carlo (MCMC) technique. We have examined the behavior of the equation of state (EoS) parameter, $( p_{de})$, and dark energy density $(ρ_{de})$. We have also discussed the equivalence of holographic dark energy (DE) with the Barrow entropic DE and its extension. Also, we have explained quintessence and dilation dark energy models in the context of Barrow entropic DE.

gr-qc↗
arXiv2023

Transit String Dark Energy Models in $f(Q)$ Gravity

In this paper, we have investigated an anisotropic cosmological model in $f(Q)$ gravity with string fluid in LRS Bianchi type-I universe. We have considered the arbitrary function $f(Q)=Q+α\sqrt{~Q}+2Λ$ where $α$ is model free parameter and $Λ$ is the cosmological constant. We have established a relationship between matter energy density parameter $Ω_{m}$ and dark energy density parameter $Ω_Λ$ through Hubble function using using constant equation of state parameter $ω$. We have made observational constraint on the model using $χ^2$-test with observed Hubble datasets $H(z)$ and SNe Ia datasets, and obtained the best fit values of cosmological parameters. We have used these best fit values in the result and discussion. We have discussed our result with cosmographic coefficients and found a transit phase dark energy model. Also, we analyzed the Om diagnostic function for anisotropic universe and found that our model is quintessence dark energy model.

gr-qc↗
arXiv2022

Observational constraints for an axially symmetric transitioning model with bulk viscosity parameterization

In this paper, we have analyzed the significance of bulk viscosity in an axially symmetric Bianchi type-I model to study the accelerated expansion of the universe. We have considered four bulk viscosity parameterizations for the matter-dominated cosmological model. The function of the two significant Hubble $H(z)$ and deceleration parameters are discussed in detail. The energy parameters of the universe are computed using the most recent observational Hubble data (57 data points) in the redshift range $0.07 \leq z \leq 2.36)$. In this model, we obtained all feasible solutions with the viscous component and analyzed the universe's expansion history. Finally, we analyzed the statefinder diagnostic and found some interesting results. The outcomes of our developed model now properly align with observational results.

gr-qc↗
arXiv2022

Quintessence Behaviour of an Anisotropic Bulk Viscous Cosmological Model in Modified $f(Q)$-Gravity

In this article, we have discussed the results of our investigation. We consider an anisotropic viscous cosmological model of (LRS) Bianchi type I spacetime universe filled with a viscous fluid under $f(Q)$ gravity. We have studied the modified $f(Q)$ gravity with quadratic form $f(Q)=αQ^{2}+β$, where $Q$ is called as a non-metricity scalar and $α$, $β$ are positive constants. We obtain the modified Einstein field equation by considering the viscosity coefficient $ξ(t)=ξ_{0}H$ and obtained the scale factor $a(t)=2~sinh\left (\frac{m+ 2} {6}\sqrt{\frac{ξ_{0}}{α(2m+1)}}t\right)$.We applied the observational constraint on the apparent magnitude $m(z)$ using the $χ^{2}$ test formula with observational data set like JLA or Union 2.1 compilation and obtained the best approximate values of the model parameters ${ m,α, H_{0} ξ_{0}}$. We have analyzed our model and found a transit phase quintessence anisotropic accelerating universe. We also examined the bulk viscosity equation of state (EoS) parameter $ω_{v}$ and obtained its current value as $ω_{v}<-1/3$, which shows the dark energy dominant model, cosmological constant, phantom, and super-phantom dark energy models, and tends to the $Λ$CDM value ($ω_{v}=-1$) in the late time. We also estimate the current age of the universe as $t_{0}\approx13.6$ Grys and analyze the Statefinder parameters with $(s,r)\to(0,1)$ as $t \to \infty$.

gr-qc↗
arXiv2022

The Mass-Radius relation for Quark Stars in Energy-Momentum Squared Gravity

We study the structure of quark stars (QSs) adopting homogeneously confined matter inside the star with a 3-flavor neutral charge and a fixed strange quark mass $m_s$. We explore the internal structure, and the physical properties of specific classes of QSs in the recently proposed energy-momentum squared gravity (EMSG). Also, we obtain the mass-radius $(M-R)$ and mass-central energy density $(M-ρ_c)$ relations for QS using the QCD motivated EoS. The maximum mass for QSs in EMSG is investigated depending on the presence and absence of the free parameter $α$. Furthermore, the stability of stars is determined by the condition $\frac{d M}{d ρ_c}>0$. We observe that consideration of the EMSG has specific contributions to the structure of QSs.

gr-qc↗