arXiv subjects

S. Surendra Singh

Publications and source records attributed to S. Surendra Singh.

36 records · Page 2Linked to original sources

arXiv2025

Reconstructing cosmic expansion in $f(R, G)$ gravity using a log-periodic deceleration model

We study the late-time cosmology in $f(R, G)=R+αR^{2}+βe^{γG}$, using a logarithmic parametrization of the deceleration parameter $q(z)=q_{0}+q_{1}sin[log(1+z)]$. The Hubble parameter $H(z)$ is reconstructed and model parameters are constrained via MCMC analysis using CC ($31$), BAO ($26$) and Pantheon+SHOES ($1701$) datasets. Our results yield a Hubble constant in the range $H_0 = 71.7$--$72.8$ km/s/Mpc, consistent with late-time observations. The present deceleration parameter is found to be $q_{0}=-0.484$ to $-0.517$, while the evolution parameter $q_{1}\approx 1$, indicating increasing acceleration. The transition redshift shifts from $z_{tr}=0.879$ (CC) to $0.744$ (CC+BAO+Pantheon+SHOES), supporting a dynamic acceleration phase. The model reproduces early radiation behavior with $ω(z>>1) \approx 0.33$ and predicts present-day values $ω_{0} \approx -0.49$. Energy conditions NEC and DEC are satisfied, while SEC is violated at late times. The statefinder parameters $\{r_0, s_0\} = (0.866, 0.046)$ lie near the $Λ$CDM point. Estimated age of the Universe ranges from $13.01$ to $13.59$ Gyr. Thermodynamic analysis confirms consistency with the generalized second law. Overall, the model offers a viable and observationally consistent description of cosmic acceleration.

gr-qc↗

arXiv2025

Phase space analysis of cosmological models on $f(R,G,T)$ gravity

In the present article, we developed a dynamical system in the context of modified $f(R,G,T)$ gravity, where $R$, $G$ and $T$ are Ricci scalar, Gauss-Bonnet term and energy-momentum tensor respectively. Development of the dynamical system is done by first defining 9 dimensionless variables and formulate a ordinary differential equations by taking derivative of the variables with respect to logarithmic time $N=\log a(t)$, where $a(t)$ is the scale factor. This formulation is applied to the model $f(R,G,T)=αR^{l}+βG^{m} + γT^{n} $, and its solutions and their corresponding stabilities are analysed in details. From the plot of density parameters vs $N=-log(1+z)$, we conclude that our Universe is currently dominated by dark energy, which is compatible with current observation. Taking $l=2$, cosmic parameters such as deceleration parameter $(q)$, equation of state $(ω)$ and state finder parameters are also discussed by fixing one dimensionless variable, showing our Universe's expansion is accelerating with the present value of $q=-0.5$. Present value of $ω=-0.66$ suggests that our Universe is in Quintessence phase. Lastly, the validity of the model with respect to the $Λ$CDM model is checked by using 77 Hubble, 15 BAO and 1024 pantheon datasets, which implies that our model is aligned with the $Λ$CDM model's bahaviour.

gr-qc↗

arXiv2025

Cylindrical gravastars with Kuchowicz metric potential

Mazur and Mottola's gravastar model represents one of the few serious alternatives to the traditional understanding of the black hole. The gravastar is typically regarded as a theoretical alternative for the black hole. This article investiagtes the creation of gravastar(gravitational vacuum star) within the realm of cylindrically symmetric space-time utilizing the Kuchowicz metric potential. A stable gravastar comprises of three distinct regions, starting with an interior region marked by positive energy density and negative pressure $(p=-ρ)$ which is followed by an intermediate thin shell, where the interior negative pressure induces a outward repulsive force at each point on the shell. Ultra-relativistic stiff fluid makes up the thin shell governed by the equation of state(EoS) $(p=ρ)$, which meets the Zel'dovich criteria. And then comes the region exterior to it which is total vacuum. In this scenario, the central singularity is eliminated and the event horizon is effectively substituted by the thin bounding shell. Employing the Kuchowicz metric potential we have derived the remaining metric functions for the interior region and the shell regions yielding a non-singular solution for both the regions. Additionally, we have investigated various characteristics of this shell region including its proper shell length, the energy content and entropy. This theoretical model successfully resolves the singularity issue inherent to the black holes. Therefore, this gravastar model presents a viable alternative to the traditional black holes, reconciled within the context of Einstein's theory of General Relativity.

physics.gen-ph↗

arXiv2024

Constraining model parameters in f(Q,C) gravity: Observational analysis and geometric diagnostics

We investigate the cosmological implications of $f(Q,C)$ gravity with $f(Q,C)=αQ+βC$, where $Q$ is the non-metricity scalar and $C$ encapsulates cosmological expansion terms. Three parameterizations of the EoS for dark energy, $ω=ω_{0}+ω_{1}z$, $ω=ω_{0}+\frac{ω_{1}z(1+z)}{1+z^{2}}$ and $ω=ω_{0}+\frac{ω_{1}z^{2}}{1+z^{2}}$ are tested using the Hubble, Hubble plus BAO, and Hubble plus BAO plus Pantheon datasets to constrain model parameters. The resulting Hubble and deceleration parameters reveal a transition from deceleration to acceleration, supporting current cosmic acceleration observations. Analysis of the energy density and pressure confirms positive energy density and a negative pressure for dark energy, potentially driving the late-time acceleration. We examine energy conditions, showing compliance with NEC, WEC and DEC, while SEC remains negative, supporting an accelerated expansion. Statefinder diagnostics suggest that two of the EoS parameterizations lead to Quintessence-like behavior with a time-varying dark energy component, while the third closely approaches $Λ$CDM showing slight deviations consistent with recent observations. Sound speed analysis demonstrates the physical stability of all parameterizations.

gr-qc↗

arXiv2024

A novel approach to baryogenesis in $f(Q,L_{m})$ gravity and its cosmological implications

We present an examination of the $f(Q,L_{m})$ gravity model, in which the functional form $f(Q,L_{m})=αQ^{n}+βL_{m}$ is postulated and discuss its potential impact on cosmological dynamics and the phenomenon of gravitational baryogenesis. Combining observational insights from Hubble, BAO and phantom datasets, we conduct a comprehensive analysis to constrain the model's parameters and determine the baryon-to-entropy ratio $\frac{η_{B}}{s}$, providing valuable insights into the model's performance and cosmological implications. In the context of baryogenesis and generalized gravitational baryogenesis, we show that setting $n=\frac{1}{2}$ results in a zero baryon-to-entropy ratio, which is physically implausible. Through a detailed examination of the dependence of $\frac{η_{B}}{s}$ on $n$ and $β$, we demonstrate that our model predicts a baryon-to-entropy ratio that is both positive and consistent with the observational upper limit of $9.42\times10^{-11}$ for $1.32965<n<1.39252$ and appropriate of $β$ and $n$ with $α\simeq-1.95084\times10^{86}$. The excellent agreement between our model's predictions and the phantom dataset demonstrates the model's capacity to accurately describe the physics of baryogenesis and its ability to reproduce the observed features of the cosmological data, showcasing its potential as a reliable tool for understanding the evolution of the Universe.

gr-qc↗

arXiv2024

Barrow Holographic Dark Energy in $f(Q,L_{m})$ gravity: A dynamical system perspective

In this work, we investigate the cosmological implications of the Barrow Holographic Dark Energy (BADE) model within the framework of $f(Q,L_{m})$ gravity, specifically considering the model $f(Q,L_{m})=αQ+βL_{m}$. Using a dynamical system approach for both non-interacting and interacting scenarios, we identify critical points corresponding to different phases of the Universe's evolution, including matter domination, radiation domination and dark energy-driven accelerated expansion. Our analysis reveals two stable critical points in the non-interacting case and three stable critical points in the interacting case, each indicating a transition to a stable phase dominated by BADE. The phase plots clearly demonstrate the evolution of the Universe's dynamics toward these stable points. At these stable points, the deceleration parameter is negative, consistent with accelerated expansion and the equation of state parameter suggests that BADE behaves as a dark energy component. These findings highlight the BADE model's strength as a viable explanation for the Universe's late-time acceleration inside $f(Q, L_{m})$ gravity, and they provide novel perspectives on the cosmic development of dark energy-matter interactions.

gr-qc↗

arXiv2024

Gravastar model in the structure of $f(R,L_{m}, T)$ modified theory of gravity

The Gravastar ( or the Gravitational Vacuum Star ) is a very serious alternatives proposed to the principle of the Black Hole, the model of which was originally developed by Mazur and Mottola. A Gravastar is an astronomically hypothetically condensed object which is a gravitationally dark vacuum star or a gravitational vacuum condensate star, which is singularity free, spherically symmetric and also super compact. The current study concerns about the model of the Gravastar in the modified $f(R,L_{m}, T)$ gravity considering the form $f(R,L_{m},T) = R + αTL_{m} $. From Mazur-Mottola \cite{Mazur}-\cite{Mottola}, we get to know that a Gravastar model has three distinct regions having various Equations of State (EoS). We have inquired into the interior portion with the space-time considering $ ρ= - p $, for the dark sector of the interior region, here the negative matter-energy density exerting a repulsive force on the immediate thin shell with the EoS $ ρ= p $ where it is considered as an ultra-relativistic fluid. We have studied the properties such as energy density, proper length, total energy and entropy. Next comes the vacuum exterior region of the Gravastar which is being described by the Schwarzschild-de-Sitter solution. And also from Darmois-Israel formalism, we have probed the junction connecting the inner and the outer surfaces of the Gravastar.

gr-qc↗

arXiv2024

Behaviours of rip cosmological models in $f(Q,C)$ gravity

In this study, the Universe's rip cosmology theories have been provided for the $f(Q,C)$ gravity theory, where $Q$ and $C$ stand for the non-metricity scalar and boundary term. We assumed $f(Q,C)=αQ^{n}+βC$ and analyzed the nature of the physical parameters for the Little Rip, Big Rip and Pseudo Rip models. In the LR and PR models, the EoS parameter exhibits phantom characteristics but remains closely aligned with the $Λ$CDM line. After investigating the energy conditions, we recognised that our model violates the strong energy constraint. Avoiding singularity situations has been noted in all of these accelerated models. The characteristics of the jerk and snap parameters have been investigated. Our model provides an effective description of the Universe's evolutionary history and fits well with contemporary cosmic data.

gr-qc↗

arXiv2023

Stability Analysis of Cosmological models in $f(T,ϕ)$ Gravity

We investigated the stability condition in $f(T,ϕ)$ gravity theory for considering two models by using dynamical system. We assume the forms of $G(T)$ are $(i)$ $G(T)$ = $αT+\fracβ{T}$, $(ii)$ $G(T)$ = $ζT$ ln$(ψT)$, where $α$, $β$, $ζ$ and $ψ$ be the free parameters. We evaluated the equilibrium points for these models and examine the stability behavior. We found five stable critical points for Model I and three stable critical points for Model II. The phase plots for these systems are examined and discussed the physical interpretation. We illustrate all the cosmological parameters such as $Ω_{m}$, $Ω_ϕ$, $q$ and $ω_{Tot}$ at each fixed points and compare the parameters with observational values. Further, we assume hybrid scale factor and the equation of redshift and time is $t(z)=\fracδσW\bigg[\fracσδ\bigg(\frac{1}{a_{1}(1+z)}\bigg)^{\frac{1}δ}\bigg]$. We transform all the parameters in redshift by using this equation and examine the behavior of these parameters. Our models represent the accelerating stage of the Universe. The energy conditions are examined in terms of redshift and SEC is not satisfied for the model. We also find the statefinder parameters $\{r,s\}$ in terms of z and discuss the nature of $r-s$ and $r-q$ plane. For both pairs $\{r,s\}$ and $\{r,q\}$ our models represent the $Λ$CDM model. Hence, we determine that our $f(T,ϕ)$ models are stable and it satisfies all the observational values.

gr-qc↗

arXiv2023

Stability analysis of cosmological models coupled minimally with scalar field in $f(Q)$ gravity

In this work, in the framework of dynamical system analysis, we focus on the study of the accelerated expansion of the Universe of $f(Q)$ gravity theory where $Q$ be the non-metricity that describes the gravitational interaction. We consider the linear form of $f(Q)$ gravity i.e. $f(Q)=-α_{1}Q-α_{2}$ where $α_{1}$ and $α_{2}$ are constants. We consider an interaction between dark matter (DM) and dark energy (DE) in $f(Q)$ gravity. To reduce the modified Friedmann equations to an autonomous system of first-order ordinary differential equations, we introduce some dimensionless new variables. The nature of the critical points are discussed by finding the eigenvalues of the Jacobian matrix. We get six critical points for interacting DE model. We also analyze the density parameter, equation of state (EoS) parameter and deceleration parameter and draw their plots and we conclude that for some suitable range of the parameters $λ$ and $α$, the value of the deceleration parameter is $q=-1$ which shows that the expansion of Universe is accelerating and the value of EoS parameter is $ω_ϕ=-1$ which shows that the model is $Λ$CDM model. Finally, we discussed the classical as well as quantum stability of the model.

gr-qc↗

arXiv2022

Qualitative Stability Analysis of Cosmological Parameters in $f(T,B)$ Gravity

We analyze the cosmological solutions of $f(T,B)$ gravity using dynamical system analysis where $T$ is the torsion scalar and $B$ be the boundary term scalar. In our work, we assume two specific cosmological models. For first model, we consider $ f(T,B)=f_{0}(B^{k}+T^{m})$, where $k$ and $m$ are constants. For second model, we consider $f(T,B)=f_{0}T B$. We generate an autonomous system of differential equations for each models by introducing new dimensionless variables. To solve this system of equations, we use dynamical system analysis. We also investigate the critical points and their natures, stability conditions and their behaviors of Universe expansion. For both models, we get four critical points. The phase plots of this system are analyzed in detail and study their geometrical interpretations also. In both model, we evaluated density parameters such as $Ω_{r}$, $Ω_{m}$, $Ω_Λ$ and $ω_{eff}$ and deceleration parameter $(q)$ and find their suitable range of the parameter $λ$ for stability. For first model, we get $ω_{eff}=-0.833,-0.166$ and for second model, we get $ω_{eff}=-\frac{1}{3}$. This shows that both the models are in quintessence phase. Further, we compare the values of EoS parameter and deceleration parameter with the observational values.

gr-qc↗

arXiv2022

Renyi Holographic dark energy and its behaviour in f(G) gravity

In this work, the Renyi holographic dark energy (RHDE)and its behaviour has been explored with the anisotropic and spatially homogeneous Bianchi type-I Universe in the framework of $f(G)$ gravity. We use IR cutoff as the Hubble and Granda-Oliveros (GO) horizons. To find a consistent solutions of the field equations of the models, it is assumed that the deceleration parameter is defined in terms of function of Hubble parameter $H$. With reference to current cosmological data, the behaviors of the cosmological parameters relating to the dark energy model are evaluated and their physical significance is examined. It is observed that for both the models, the equation of state parameter approaches to $-1$ at late times. However, the RHDE model with the Hubble horizon exhibits stability from the squared sound speed, but the RHDE model with the GO horizon exhibits instability. In both the models, deceleration parameter and statefinder diagnostic confirm the accelerated expansion of the Universe and also correspond to the $Λ$CDM model at late times.

hep-th↗

arXiv2022

Anisotropic Universe in f(Q) gravity with Hybrid expansion

Despite having a reasonably successful account of accelerated cosmology, understanding the early evolution of Universe has always been difficult for mankind. Our promising strategy is based on a novel class of symmetric teleparallel theories of gravity called $f(Q)$, in which the gravitational interaction is caused by the non-metricity scalar $Q$, which may help to solve some problems. We consider the locally rotationally symmetric (LRS) Bianchi type-I spacetime cosmological models and derive the motion of equations to study the early evolution of the cosmos. By assuming the Hybrid Expansion Law (HEL) for the average scale factor, we are able to determine the solutions to the field equations of Bianchi type-I spacetime. We discuss the energy density profile, the equation of state, and the skewness parameter and conclude that our models preserve anisotropic spatial geometry during the early stages of the Universe with the possibility of an anisotropic fluid present. However, as time goes on, even in the presence of an anisotropic fluid, the Universe may move towards isotropy due to inflation while the anisotropy of the fluid dims away at the same time. It is seen from the squared speed of sound that Universe shows phantom nature at the beginning then approaches to dark energy at present epoch. We analyze both geometrical and physical behaviors of the derived model.

gr-qc↗

arXiv2022

FRW cosmology with varying cubic deceleration parameter

In this work a new law of varying deceleration parameter of third degree have been proposed. The solutions of the modified field equations have been derived under the newly proposed law of the deceleration parameter. Model exhibits the Big-bang singularity at cosmic time ($t=0$) and shows Big Rip at ($t=n$) then it re-enter the phase of initial singularity at $t=2n$ and ends its cyclic behavior at $t=3n$. The evolution of the physical and dynamical parameters of the Universe have been studied and the graphical representation has also been shown. Further $Om(z)$ diagnostic parameter and the energy conditions have also been studied together with their graphical representations.

gr-qc↗

arXiv2022

Dynamical systems of cosmological models for different possibilities of $G$ and $ρ_Λ$

The present paper deals with the dynamics of spatially flat Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmological model with a time varying cosmological constant $Λ$ where $Λ$ evolves with the cosmic time (t) through the Hubble parameter (H). We consider that the model dynamics has a reflection symmetry $H \rightarrow -H $ with $Λ(H)$ expressed in the form of Taylor series with respect to H. Dynamical systems for three different cases based on the possibilities of gravitational constant G and the vacuum energy density $ρ_Λ$ have been analysed. In Case I, both G and $ρ_Λ$ are taken to be constant. We analyse stability of the system by using the notion of spectral radius, behavior of perturbation along each of the axis with respect to cosmic time and Poincare sphere. In Case II, we have dynamical system analysis for G=constant and $ρ_Λ \neq $ constant where we study stability by using the concept of spectral radius and perturbation function. In Case III, we take $G \neq$ constant and $ρ_Λ \neq$ constant where we introduce a new set of variables to set up the corresponding dynamical system. We find out the fixed points of the system and analyse the stability from different directions: by analysing behaviour of the perturbation along each of the axis, Center Manifold Theory and stability at infinity using Poincare sphere respectively. Phase plots and perturbation plots have been presented. We deeply study the cosmological scenario with respect to the fixed points obtained and analyse the late time behavior of the Universe. Our model agrees with the fact that the Universe is in the epoch of accelerated expansion. The EOS parameter $ω_{eff}$, total energy density $Ω_{tt}$ are also evaluated at the fixed points for each of the three cases and these values are in agreement with the observational values in [1].

gr-qc↗

arXiv2021

Interaction of Anisotropic Dark Energy with Generalized Hybrid Expansion Law

Interaction of dark energy in the anisotropic Locally Rotationally Symmetric (LRS) Bianchi type-I metric is investigated in the context of modified f(R,T) theory of gravity, where R is the Ricci scalar and T is the trace of stress energy momentum tensor. We choose the particular form of the functional f(R,T)=f_1 (R,T)+f_2 (R,T) then we find the exact solutions of the field equations by applying inhomogeneous equation of state, p= ω\r{ho}-Λ(t) and a generalized form of hybrid expansion law. Transition of deceleration to acceleration is observed in this model. It is also observed that the Universe shows accelerated expansion at late epoch. The derived model overlaps with ΛCDM at late time which is in agreement with present observation. Energy conditions of the derived model are also investigated. From the plot, we observe the age of Universe t_0=13.821 Gyr for the observed H_0=70.07Kms^(-1) Mpc^(-1). The physical and geometrical behaviours of these models are also discussed.

gr-qc↗

arXiv2021

Dynamical system perspective of cosmological models minimally coupled with scalar field

The stability criteria for spatially flat homogeneous and isotropic cosmological dynamical system is investigated with the interaction of a scalar field endowed with a perfect fluid.In this paper, we depict the dynamical system perspective to study, qualitatively, the scalar field cosmology under two special cases, with and without potential. For analysis with potential we use simple exponential potential form, $V_{o}e^{-λϕ}$. We generate, by introducing new dimensionless variables, an autonomous system of ordinary differential equations $(ASODE)$ for each case and obtain respective fixed points. We also analyse the type of fixed points, nature and stability of the fixed points and how their nature and behavior reflect towards the cosmic scenarios. Throughout the whole work, the investigation of this model has shown us the deep connection between these theories and cosmic acceleration phenomena. The phase plots of the system at different conditions and different values of $γ$ have been analyzed in detail and their interpretations have been worked out.The perturbation plots of the dynamical system have also been studied and analyzed which emphasize our analytical findings.

gr-qc↗

arXiv2019

Evolution of Kaluza-Klein Like Wet Dark Fluid in $f(R,T)$ Theory of Gravitation

Here we study the essence of $f(R,T)$ gravitation theory in five dimensional Universe and see the role of dark energy in the form of wet dark fluid in such a Universe. It is found that the dark energy is not exaggerated in contributing to the accelerating expansion of the Universe though the expansion is inherent as a result of the theory itself and due to the geometric contribution of matter. It is interesting to see that in some model it is found that there was some era before the beginning of the present era, and some of the model Universe came out to be either oscillatory or cyclic. Some of the models are seen to go to $ΛCDM$ models in late future as in Einstein gravitation theory, starting the evolution with a big bang. Most of the models undergo early inflation as well as late time accelerating expansion thus defining as good models for real astrophysical situations, with dark energy playing fundamental role in these Universe.

physics.gen-ph↗