arXiv subjects

S. Surendra Singh

Publications and source records attributed to S. Surendra Singh.

At least 19 recordsLinked to original sources

arXiv2026

Evolutionary Behavior of Fractional Holographic Dark Energy within $f(T)$ Teleparallel Gravity

We investigate the cosmological dynamics of FHDE within $f(T)$ gravity by employing the dynamical system approach in a spatially flat FRW background. By introducing appropriate dimensionless variables, the field equations are reformulated as a closed system, which allows a systematic phase-space analysis. The resulting system admits four critical points, including two saddle points corresponding to radiation and matter-dominated epochs, and two stable points associated with a DE-dominated phase and a de Sitter solution. The radiation- and matter-dominated critical points are found to possess a saddle character in phase space, ensuring their transient nature and enabling the cosmological evolution to naturally progress toward a stable late-time accelerated attractor. The stable critical points describe accelerated expansion with effective equations of state compatible with DE and de Sitter regimes. Overall, the analysis indicates that $f(T)$ gravity is capable of reproducing the standard cosmological sequence within a consistent dynamical framework.

gr-qc↗

arXiv2026

Bose-Einstein condensate stars in massive gravity

This study explores the construction, validity and the properties of Boson or Bose-Einstein condensate (BEC) stars under the framework of de Rham-Gabadadze-Tolley (dRGT) like massive gravity, employing the Kuchowicz metric potential to model their internal structure. This gravitational framework accounts for a massive graviton while ensuring the absence of ghost instabilities during propagation. The BEC stellar configuration in this study was obtained by determining the solutions characterized by static and spherical symmetric metric. This study provides a detailed account of the stellar structure, highlighting the roles played by massive gravity and the Kuchowicz metric through a combination of analytical and numerical solutions. Our work specifically utilizes the Colpi-Wasserman-shapiro (CWS) and Gross-Pitaevskii (GP) equations of state (EoS) to model the internal thermodynamic behavior of the BEC. We have evaluated the physical viability of the BEC stellar framework by analyzing the energy conditions, and the EoS parameter along with the gradients of the energy-momentum tensor. The stability criteria such as the study of surface redshift, adiabatic index and squared sound velocity were utilized to confirm that our proposed model is both stable and physically consistent. Hence, this study offers a definitive structural analysis of the BEC stars, providing precise results in this massive gravity environment.

gr-qc↗

arXiv2025

Baryogenesis constraints and parameter bounds in $f(T,T_{G})$ modified gravity

We investigate the generation of the observed baryon asymmetry of the Universe within the framework of $f(T,T_{G})$ gravity, where $T$ is the torsion scalar and $T_{G}$ denotes its teleparallel Gauss--Bonnet counterpart. Two illustrative models, $f(T,T_{G})=αT+β\sqrt{T_{G}}$ and $f(T,T_{G})=-T+δ\, T_{G}\ln(T_{G})$, are examined in a power-law background $a(t)=a_{0} t^{m}$. For both models, we derive analytic expressions for the baryon-to-entropy ratio $η_{B}/s$ using the standard and generalized baryogenesis formalisms, adopting high-energy decoupling conditions with $g_{b}=1$, $g_{s}=106$, $T_{D}=2\times10^{16}\,\mathrm{GeV}$, and $M_{\star}=2\times10^{12}\,\mathrm{GeV}$. Consistency of the cosmological dynamics requires $m>1$, and the observed value $η_{B}/s \simeq 9.42\times10^{-11}$ is obtained for constrained intervals of the parameters $α$, $β$, $δ$, and $m$. Numerical results confirm that both models reproduce the measured baryon asymmetry without invoking extra fields or exotic matter sources. These findings indicate that teleparallel gravity with a Gauss--Bonnet torsion term provides a natural and viable mechanism for baryogenesis, offering a compelling alternative to curvature-based descriptions of the early Universe.

physics.gen-ph↗

arXiv2025

Quintessence Star Solutions with Conformal Symmetry in a Durgapal Spacetime

The accelerated expansion of the Universe can be suitably attributed to the existence of the dark energy (DE). On the backdrop of this concept, this paper introduces a novel, anisotropic compact star model whose stability and structure are governed by the presence of quintessence field, defined by the parameter $ w_{Q} (-1<w_{Q}<-\frac{1}{3}) $ and which admits conformal symmetry. The construction of the model relied on the Durgapal-Fuloria (DP) metric formulation. The model successfully meets all the necessary physical constraints viz., TOV equation, energy conditions, compactness factor, surface redshift and casuality condition. The results are analyzed through analytical methods as well as through the graphical visualization for the various physical attributes.

gr-qc↗

arXiv2025

Late-time cosmic dynamics in $f(R,L_{m})$ gravity with recent observations

In this work, we investigate the late-time cosmic dynamics in the framework of non-linear $f(R, L_m)$ gravity, adopting the functional form $f(R,L_m)=\frac{R}{2}+L_m^2$. To explore the dark energy behavior, we assume an oscillatory parametric equation of state, $ω(z) = ω_0 + b \sin[\log(1+z)]$, which allows smooth deviations from the cosmological constant. Using a joint MCMC analysis with the latest Hubble 31 chronometer data, DESI DR2 BAO measurements, and Type Ia supernova samples (Pantheon+, DES-SN5Y and Union 3), we obtain well-constrained parameters around $H_0 \simeq 67.2~\text{km s}^{-1}\text{Mpc}^{-1}$ and $ω_0\approx-0.5$, consistent with Planck 2018 and other current observations. The model exhibits a clear transition from deceleration to acceleration with $z_{\rm tr} \sim 0.7$--$0.8$, satisfies the NEC and DEC while violating the SEC and yields present EoS values close to $-1$, reproducing $Λ$CDM behavior at late times. The derived Universe ages ($t_0 \approx 13.3~\text{Gyr}$) agree well with CMB and stellar constraints, confirming that the proposed oscillatory $f(R, L_m)$ model provides an observationally consistent and dynamically viable alternative to $Λ$CDM cosmology.

gr-qc↗

arXiv2025

Cosmological implications of LRS Bianchi type-I cosmological model in $f(T)$ gravity

We perform the dynamical system analysis of the Locally Rotationally Symmetric (LRS) Bianchi type-I cosmological model in f(T) gravity in the presence of energy interaction . A cosmologically viable form of $f(T)$ is chosen (where $T$ is the torsion scalar in teleparallelism) in the background of homogenous and anisotropic. For our model, we take $f(T) = T+ζT^{2}$ where $ζ$ is a constant. The evolution equations are reduced to the autonomous system of differential equations by suitable transformation of variables. The behaviour of the equilibrium points is examined by calculating the eigenvalues corresponding to these equilibrium points. We get four equilibrium points for our cosmological model out of which one equilibrium point is stable, two are saddle points and one is an unstable equilibrium point. Corresponding to equilibrium point $T_{2}$, our model is consistent with the quintessence dark energy cosmological model. Along the equilibrium points $T_{1},T_{3}$ and $T_{4}$, our model is consistent to phantom dark energy model. After that, we utilize the autonomous equations to analyse the cosmographic parameters along with the state-finder parameter. We demonstrate the phase plot analysis for our model.

gr-qc↗

arXiv2025

Strange quark stars in mimetic gravitational theory

This paper presents a novel anisotropic quark star model in the backdrop of mimetic gravitational theory. Our study focuses on the strange quark stars using the MIT bag equation of state (EoS) admitting non-singular Buchdahl metric function. The proposed model matches the interior spacetime of the strange quark star with the exterior Schwarzschild spacetime. Our analysis of the energy conditions, radial and tangential EoSs along with the energy momentum tensor gradients and TOV equilibrium condition analysis support the model's physical validity. Further study of adiabatic index, surface redshift function and speed of sound analysis have demonstrated the stability of the strange quark star in mimetic gravity. Thus, we can say that the model stability and consistency have been validated for various parameter values of the Buchdahl function throughout our study in the context of mimetic gravity.

gr-qc↗

arXiv2025

Gravastars with Kuchowicz Metric Potential in $f(R, Σ, T)$ Gravity

This manuscript explores the gravastar model in the $f(R,Σ,T)$ gravity framework, with the help of Kuchowicz metric funcition, offering an alternative to black holes. A gravastar has three regions: interior, intermediate shell, and exterior. The interior region has pressure equal to negative density, generating a repulsive force across the thin shell. The intermediate shell contains ultra-relativistic plasma fluids, with pressure proportional to density, balancing the interior's repulsive force. The exterior region is a vacuum, described by a generalized Schwarzschild solution. Our specifications yield precise, singularity-free gravaster solutions with physically valid features in the $f(R,Σ,T)$ gravity framework, exploring strong gravity and anti-gravity aspects. The gravitational Lagrangian is based on an arbitrary function of torsion scalar $Σ$ and trace of the energy-momentum tensor $T$. Our $f(R,Σ,T)$ gravity analysis explores gravastars inner workings, revealing insights into gravity, strong gravity, and antigravity forces due to torsion effects. We examine shell properties like length, energy, entropy, and discussed junction conditions. Key findings include constant interior density and pressure, denser shell fluid at the outer boundary, and increasing shell length. These results illuminate gravastar behavior and fundamental gravitational principles.

gr-qc↗

arXiv2025

Interaction of polytropic dark energy in cosmological model: Constraints from observational data

We investigate an interacting polytropic dark energy (PDE) model characterised by the equation of state $p_{d} = αρ_{d}^{\,1+\frac{1}β}$, where the interaction between dark energy and pressureless matter is modelled via a linear coupling term $Q = 3ηHρ_{d}$. The background dynamics are formulated by deriving the Hubble parameter in the interacting scenario, and the model parameters are constrained through a Markov Chain Monte Carlo (MCMC) analysis using three joint observational data sets: Hubble77+BAO26, Hubble77+Pantheon$^+$, and Hubble77+BAO26+DESI DR2. The resulting best-fit values of $(H_0, Ω_{d0}, η) $ are $(69.22^{+1.27}_{-1.24},\,0.73^{+0.02}_{-0.02},\,-0.22^{+0.10}_{-0.12})$, $(69.23^{+1.27}_{-1.22},\,0.73^{+0.02}_{-0.02},\,-0.34^{+0.15}_{-0.17})$, and $(67.77^{+1.26}_{-1.24},\,0.73^{+0.02}_{-0.02},\,\\-0.02^{+0.10}_{-0.11})$ respectively for the respective data combinations. Our results indicate a positive energy density and negative pressure over the full redshift range, with the evolution of the equation-of-state parameter and state finder parameters placing the model firmly within the Quintessence regime. The study of the deceleration parameter also reveals a shift from a decelerating to an accelerating cosmic expansion. The estimated present age of the Universe is $14\,\mathrm{Gyr}$, consistent with recent observational data. Furthermore, the sign of $Q$ implies a current energy transfer from dark energy to matter. These findings support the interacting PDE framework as a viable candidate for explaining late-time cosmic acceleration and related large-scale dynamics.

astro-ph.CO↗

arXiv2025

CPL-parametrized cosmic expansion in Galileon gravity: Constraints from recent data

We explore the cosmic expansion history within the framework of Galileon gravity by employing a redshift-based expression for the Hubble rate, $H(z)$, derived from the CPL parametrization $ω_{DE}(z)=ω_{0}+ω_{a}\frac{z}{1+z}$. This parametrization allows for a time-dependent expansion history consistent with the non-linear Galileon field equations. To constrain the model parameters, we perform a MCMC analysis using $46$ Hubble parameter measurements, DESI DR2 BAO data and $1701$ Pantheon+ datasets. The best fit values obtained are $H_0 = 67.7043^{+1.4354}_{-1.4102}$ km/s/Mpc, $Ω_{m0} = 0.2668^{+0.0212}_{-0.0217}$, $ω_0 = -0.8827^{+0.1076}_{-0.0967}$ and $ω_a = 0.0011^{+0.0660}_{-0.0622}$. Model comparison using information criteria yields $ΔAIC=1.46$ and $ΔBIC = 11.5$ indicating that the Galileon model is a strong contender to the $Λ$CDM model. The deceleration parameter shows a transition at $z_{tr} = 0.7873$, with $q_0 = -0.598$. Energy density and pressure remain physically viable with $ρ_{de}(z)>0$ and $p_{de}(z)<0$ with the present day equation of state $ω(z)$ value of $-0.2915$, which suggests mild dynamical dark energy. NEC and DECare satisfied, while SEC is violated. The model yields $r_{0}=0.657$, $s_{0}=0.1173$ and Om diagnostic shows a peak value of $-0.45$ at $z = 0.377$, converging to $-1$ at late times.These results demonstrate that Galileon gravity remains a viable and flexible alternative to $Λ$CDM in describing late-time cosmic acceleration.

astro-ph.CO↗

arXiv2025

Compact stars with gravitational wave echoes in $f(R,L_{m},T)$ gravitational theory

This work explores the gravitational wave echoes (GWEs) from the compact stellar configurations in the backdrop of $f(R,L_{m},T)$ gravity within static and spherically symmetric framework. Our study has utilized the MIT Bag model and color-favour-locked (CFL) phase equations of state (EoS) for matter description. Mass-radius profiles were determined by solving the hydrostatic equilibrium equations. Model parameter variations were used to assess the configuration stability here. TOV solutions helped to evaluate compactness. Our results indicate that MIT bag model and CFL EoS in $f(R,L_{m},T)$ modified gravitational theory are capable of producing GWEs. The calculated wave frequencies lie within the range of $ 7.5-11 $ kHz range. We have also demonstrated that how different gravitational theory parametrization within $f(R,L_{m},T)$ theory affect our star structure and echo frequency characteristics. Surface redshift and adiabatic index analysis confirm the stability of our stellar model here.

gr-qc↗

arXiv2025

Constraining $f(Q,\mathcal{L}_{m})$ gravity with redshift-dependent pressure: Insights from observational probes

We explore the late time cosmological dynamics of the Universe within the framework of $f(Q,\mathcal{L}_{m})$ gravity by considering the specific form $f(Q, \mathcal{L}_m)=-Q+2\mathcal{L}_m+γ$. To describe the cosmic pressure evolution, a redshift dependent parametrization of $p(z)=α+\frac{βz}{1+z}$ is introduced. MCMC analysis is performed using a combined datasets from Hubble ($46$ points), BAO ($15$ points including DESI DR2) and Pantheon$+$ ($1701$ SNe Ia), the model parameters are constrained as $H_{0}=67.9476^{+0.7534}_{-0.7523}$ (km/s/Mpc), $α=-0.0002^{+0.0211}_{-0.0208}$, $β=-0.0001^{+0.0410}_{-0.0404}$ and $γ=0.0002^{+0.0599}_{-0.0602}$. The model predicts a transition from deceleration to acceleration at $z_{tr} \approx 0.493$ with present values $q_{0}=-0.255$ and $ω_{0}=-0.9001$. The evolution of energy density and pressure aligns with observational expectations. An analysis of energy conditions shows that NEC and DEC are satisfied, while SEC is violated, consistent with late time acceleration. Moreover, the slow roll parameters $ε_{1}$ and $ε_{2}$ confirm a smooth inflationary regime. These results demonstrate the capability of the model to unify early Universe inflation with the current phase of cosmic acceleration.

gr-qc↗

arXiv2025

A new parametric observational study of $f(Q,B)$ gravity with modified chaplygin gas

In this work, we explore the cosmological dynamics of a modified gravity framework based on the function $f(Q,B)=δQ^{2}+βB$, where $Q$ denotes the nonmetricity scalar and $B$ is the boundary term that relates $Q$ to the Ricci scalar. The matter sector is modeled using the Modified Chaplygin Gas (MCG) with the equation of state $p=Aρ-\frac{B}{ρ^α}$, allowing the model to interpolate between early-time matter behavior and late-time cosmic acceleration. By deriving an analytical expression for the Hubble parameter $H(z)$, we perform a parameter estimation using Markov Chain Monte Carlo (MCMC) techniques in conjunction with the latest cosmological observations: $46$ Hubble parameter measurements, $15$ BAO data points, DESI DR2 BAO data and the Pantheon+ Type Ia supernovae compilation. The best-fit values are obtained as $H_0 = 72.22^{+3.64}_{-4.46}$, $A_s = 0.696^{+0.082}_{-0.129}$, $α= 0.0029^{+0.022}_{-0.021}$, and $A = 0.0038^{+0.071}_{-0.047}$. The deceleration parameter transitions at redshift $z_{tr} \approx 0.946$, while the present-day value is $q_0 = -0.789$. The model yields an age of the Universe $t_0 \approx 13.53$ Gyr and a present EoS parameter $ω_0 \approx -0.691$, which reflects the late-time acceleration consistent with observational bounds. These results demonstrate that the MCG scenario within $f(Q,B)$ gravity provides a viable and observationally consistent framework for explaining the late-time accelerated expansion of the Universe.

gr-qc↗

arXiv2025

Observational signatures of scalar field dynamics in modified $f(Q, L_m)$ gravity

We investigate the cosmological implications of a tanh-parametrized scalar field model in the framework of modified $f(Q, L_{m})$ gravity by adopting the form $f(Q, L_{m})=βQ+δL_{m}$ along with a scalar field energy density $ρ_ϕ= ρ_{c0} \tanh(A + Bz)$. Using MCMC methods and combining $31$ cosmic chronometer data points, $15$ BAO, DESI DR2 BAO and $1701$ Pantheon+ samples, we constrain the model parameters and obtain $H_{0}=74.284^{+4.155}_{-4.275}$, $Ω_{m0}=0.326^{+0.093}_{-0.072}$ and $B=-0.001^{+0.030}_{-0.030}$. The model predicts a transition redshift $z_{tr}=0.5914$ and a present deceleration parameter $q_0=-0.5167$, consistent with a Universe transitioning from deceleration to acceleration. We further analyze the evolution of the EoS parameter, density components and statefinder diagnostics in which all parameters show asymptotic convergence to a de Sitter phase. Additionally, we study black hole mass accretion, showing its dependence on the scalar field dynamics. This work highlights the compatibility of tanh-scalar field forms with $f(Q, L_m)$ gravity in describing cosmic acceleration and gravitational phenomena.

gr-qc↗

arXiv2025

Observational viability of fractional holographic dark energy in LRS Bianchi type-I cosmological model

We scrutinized the Locally Rotational Symmetry (LRS) Bianchi type-I cosmological model in the presence of fractional holographic dark energy. We calculate the value of the Hubble parameter $H(z)$ using the field equations. After that, we fit the model by employing the MCMC techniques to observational data, which includes Hubble, Hubble+BAO, Hubble+Pantheon+Shoes, and Hubble+BAO+Pantheon+Shoes datasets. With the help of these datasets, we calculate the model parameters, viz. $H_{0}(z), α,$ and $Ω_{m_{0}}$. The value of $H_{0}(z),α$ and $Ω_{m_{0}}$ lies in the range $67.688^{+1.246}_{-1.197}- 67.80^{+1.23}_{-1.23}, 0.86^{+0.14}_{-0.15}- 0.885^{+0141}_{-0.149}$ and $0.264^{+0.018}_{-0.018}- 0.27^{+0.02}_{-0.02}$ respectively. Our results indicate that the evolution of the density parameters corresponding to dark energy (DE) and dark matter (DM), particularly for $α=1.01$, along with the transition at $z_{t} = 0.55$ of the deceleration parameter $q$ from positive values to $-1$, reflects a phase of accelerated expansion that closely resembles the $Λ$CDM model. The behavior of the equation of state (EoS) parameter further demonstrates that the Universe's evolution aligns well with the framework of the fractional holographic dark energy (FHDE) model, suggesting its compatibility with scenarios of late-time cosmology. Moreover, the analysis of the statefinder diagnostics ${(r,s)}$ and the present value of ${(r,s)} = (0.74,0.07)$ reveal characteristics that converge towards the $Λ$CDM fixed point. A comparison between the observational constraints on our model parameters and those of the $Λ$CDM model exhibits a strong degree of agreement, thereby reinforcing the physical plausibility and consistency of the proposed cosmological model.

gr-qc↗

arXiv2025

Durgapal-Fuloria Bose-Einstein condensate stars within $ f(R,T) $ gravity theory

This manuscript studies the Bose-Einstein condensate (BEC) stars in the light of $ f(R,T) $ gravity here with Durgapal-Fuloria (DP) metric ansatz. The function under this study features as $ f(R,T) = R + 2ηT $, where $ η$ represents the coupling constant. With the help of it, we have formulated a stellar model describing the isotropic matter here within. Our analysis covers energy conditions, equation of state (EoS) parameter and gradients of the energy-momentum tensor components for a valid BEC stellar framework within $ f(R,T) $ gravitational theory with satisfactory results. The model's stability has been validated via multiple stability criteria viz., the velocity of sound, study of adiabetic index and surface redshift where all are found to be lying within the acceptable range for our stellar model. Thus in all the cases we have found our model to be stable and realistic. From the graphical representations the impact of the coupling constant and the parameter of the DP metric potential are clearly visible. Thus we can state that with all the above-mentioned features we have introduced new stellar solutions for BEC stars with enhanced precise results in this modified gravity.

gr-qc↗

arXiv2025

Impact of Buchdahl metric potential on thin-shell gravastar framework in de Rham-Gabadadze-Tolley like massive gravity

This paper presents a study on gravitational vacuum stars (gravastars) with an isotropic matter distribution in de Rham-Gabadadze-Tolley (dRGT) massive gravity incorporating Buchdahl metric function. Here we have conducted an analysis on thin-shell singularity-free gravastar configuration. The study demonstrates the viability of gravastars as alternatives to black holes (BHs) in this massive gravity. Our research yields singularity free analytical solutions for gravastar interior and without event horizon. Our discussion focuses on the properties of the thin shell of ultra-relativistic stiff fluid viz., length, energy, entropy and the massive gravity's impact on these physical properties. The junction conditions have been carefully examined and study of surface redshift analysis implies regularity of the model. Our investigation also includes energy condition analysis supporting the thin shell formation. Thus our solutions eliminate singularities and information paradoxes and the implications of these solutions are seemingly noteworthy and exhibit physical desirable properties.

gr-qc↗

arXiv2025

Cosmological insights from an exponential $Om(z)$ function in $f(T,T_{G})$ gravity framework

We examine a modified teleparallel gravity model defined by $f(T,T_{G})=T+γ\sqrt{T_{G}}+δ\sqrt{T}$ by introducing an exponential $Om(z)$ diagnostic of the form $Om(z)=αe^{\frac{z}{1+z}}+β$. This novel form captures smooth redshift evolution and allows for a flexible, model-independent probe of dark energy dynamics. We derive a Hubble function from this expression and use MCMC analysis with $31$ CC, $26$ BAO and $1701$ Pantheon+ data points to constrain the model parameters. The best-fit results yield $H_{0} \in [68.46, 77.38]$km/s/Mpc for $α\in [-0.232, -0.068]$ and $β\in [0.218, 0.560]$ which is consistent with local $H_{0}$ values. Our model predicts a transition redshift $z_{tr} \approx (0.48-0.54)$, present $q_{0}\approx -0.34$, and $ω_{0}\approx-0.33$. It satisfies NEC and DEC, closely tracks $Λ$CDM in the statefinder plane and estimates a cosmic age of $(13.28-13.87)$ Gyr which confirms its strength in explaining late-time acceleration. Our findings demonstrate that the exponential $Om(z)$ parameterization provides a robust and insightful approach to trace dark energy evolution within modified gravity frameworks.

gr-qc↗