arXiv subjects

Sanjay Mandal

Publications and source records attributed to Sanjay Mandal.

At least 19 recordsLinked to original sources

arXiv2026

Testing $\Lambda$CDM with ANN-Reconstructed Expansion History from Cosmic Chronometers

In modern cosmology, the rapid growth of high-precision observational data, along with significant theoretical advances, has intensified the challenge of identifying a robust, model-independent framework to probe the expansion history of the Universe. In this work, we propose a novel artificial neural network (ANN)-based framework for the non-parametric reconstruction of the late-time cosmic expansion. The framework is trained and validated through a three-stage screening pipeline prior to its application to real observational data. As a demonstration of its effectiveness, we reconstruct the Hubble parameter $H(z)$ using the latest cosmic chronometer measurements. Our results show that the reconstructed expansion history aligns with the predictions of the $\Lambda$CDM model within observational uncertainties, thereby supporting the robustness and reliability of the proposed approach.

astro-ph.CO↗

arXiv2026

Energy conditions of bouncing solutions in quadratic curvature gravity coupled with a scalar field

We examine the validity of classical energy conditions in nonsingular bouncing cosmological solutions arising in quadratic curvature gravity minimally coupled to a scalar field. Focusing on the null, weak, strong, and dominant energy conditions, we perform a systematic analysis under two distinct formulations of the energy-momentum tensor. In the first approach, the energy-momentum tensor is assumed to be sourced solely by the scalar field, whereas in the second, an effective energy-momentum tensor is constructed that incorporates the higher-curvature corrections characterizing deviations from general relativity. Our results reveal that, in the scalar-field description, the null, weak, and dominant energy conditions remain satisfied throughout the cosmological evolution, while the strong energy condition is necessarily violated during the bounce phase, enabling the avoidance of the initial singularity. In contrast, when the effective energy-momentum tensor is considered, all four energy conditions are violated near the bounce, highlighting the intrinsically non-Einsteinian nature of the underlying gravitational dynamics. These findings clarify the role of higher-order curvature terms in facilitating nonsingular cosmological bounces, providing important insights into the energy condition violations required in modified theories of gravity.

gr-qc↗

arXiv2024

Reconstruction of the scalar field potential in nonmetricity gravity through Gaussian processes

The accelerated expansion of the universe has been widely confirmed, posing challenges to the standard $\Lambda$CDM model, particularly the cosmological coincidence problem. This has motivated the exploration of modified gravity theories, including non-metricity gravity, which explains cosmic acceleration without dark energy. In this work, we incorporate a quintessence scalar field into the non-metricity framework to model both inflation and late-time acceleration. Employing the Gaussian process method with a square exponential kernel, we reconstruct the scalar field potential, $V(\phi)$, from observational Hubble data sets coming from cosmic chronometers (CC) as well as from the method of radial baryon acoustic oscillations (BAO) in a model-independent approach. This approach allows us to obtain a suitable quintessence scalar field model that aligns with the observational Hubble data under the framework of power-law non-metricity gravity. Additionally, we compare our reconstructed potential with power-law scalar field potentials, revealing that these models show better agreement with the observational data, providing new insights into the dynamics of the universe. In contrast, we find that the early dark energy has minimal effect on the present-time accelerated expansion of the universe.

gr-qc↗

arXiv2024

Modeling and Analyzing Stability of Hybrid Stars within $f(Q)$ Gravity

This study uses the Krori-Barua type metric to represent hybrid stars within the $f(Q)$ theory of gravity. We postulate that the hybrid star also contains strange quark matter in addition to regular baryonic matter. To investigate the physical viability of the hybrid star model, we present the graphical behavior of the energy density, radial pressure, and tangential pressure, equation of state parameters, anisotropy, and stability analysis, respectively, by choosing the compact star EXO 1785-248 with a mass of $1.3_{-0.2}^{+0.2}~M_{\odot}$ and radius $8.849_{-0.4}^{+0.4}$ km with five different values of the coupling constants as $a=2$, $a=4$, $a=6$, $a=8$, and $a=10$. The maximum allowed masses and corresponding radii have been calculated using the $M-R$ curve for three different coupling parameter '$a$' values which match the observational data of three distinct compact stars, namely LMC X-4, SMC X-1, and 4U 1538-52. So, the $f(Q)$ theory of gravity can provide results that are plausible for describing the macroscopical characteristics of hybrid star candidates.

gr-qc↗

arXiv2024

$f(Q,T)$ gravity: From early to late-time cosmic acceleration

In this article, we explore the comprehensive narrative of cosmic evolution within a cosmological framework by utilizing a novel form of gravity known as generalized symmetric teleparallel gravity, denoted as $f(Q,T)$ gravity. Here, $Q$ represents the non-metricity scalar, while $T$ denotes the trace of the energy-momentum tensor. We present and analyze two distinct $f(Q,T)$ cosmological models, each characterized by its unique Lagrangian. Our investigation delves into the cosmological parameters of these models, scrutinizing various energy conditions, examining the inflationary dynamics of the early universe through scalar field formulations, and probing the mysterious nature of dark energy using statefinder diagnostics and $(ω-ω')$ phase space analysis. Ultimately, our findings offer a comprehensive account of cosmic evolution, spanning from the early universe to its late-time evolution.

physics.gen-ph↗

arXiv2024

The Stability of Anisotropic Compact Stars Influenced by Dark Matter under Teleparallel Gravity: An Extended Gravitational Deformation Approach

In our investigation, we pioneer the development of geometrically deformed strange stars within the framework of teleparallel gravity theory through gravitational decoupling via the complete geometric deformation (CGD) technique. The significant finding is the precise solution for deformed strange star (SS) models achieved through the vanishing complexity factor scenario. Further, we introduce the concept of space-time deformation caused by dark matter (DM) content in DM haloes, leading to perturbations in the metric potentials $g_{tt}$ and $g_{rr}$ components. Mathematically, this DM-induced deformation is achieved through the CGD method, where the decoupling parameter $\alpha$ governs the extent of DM influence. To validate our findings, we compare our model predictions with observational constraints, including GW190814 (with a mass range of $2.5-2.67 M_{\odot}$) and neutron stars (NSTRs) such as EXO 1785-248 [mass=$1.3_{-0.2}^{+0.2}~M_{\odot}$], 4U 1608-52 [mass=$1.74_{-0.14}^{+0.14}~M_{\odot}$], and PSR J0952-0607 [mass=$2.35_{-0.17}^{+0.17}~M_{\odot}$]. Our investigation delves into the stability of the model by considering causality conditions, Herrera's Cracking Method, the adiabatic index, and the Harrison-Zeldovich-Novikov criterion. We demonstrate that the developed model mimics a wide range of recently observed pulsars. To emphasize its compatibility, we highlight the predicted mass and radius in tabular form by varying both the parameters $\alpha$ and $\zeta_1$. Notably, our findings are consistent with the observation of gravitational waves from the first binary merger event. Furthermore, we compare our results with those obtained for a slow-rotating configuration. In addition to this, we discuss the moment of inertia using the Bejger-Haensel approach in this formulation.

gr-qc↗

arXiv2024

Gaussian Process Approach for Model-Independent Reconstruction of $f(Q)$ Gravity with Direct Hubble Measurements

The increase of discrepancy in the standard procedure to choose the arbitrary functional form of the Lagrangian $f(Q)$ motivates us to solve this issue in modified theories of gravity. In this regard, we investigate the Gaussian process (GP), which allows us to eliminate this issue in a $f(Q)$ model-independent way. In particular, we use the 57 Hubble measurements coming from cosmic chronometers and the radial Baryon acoustic oscillations (BAO) to reconstruct $H(z)$ and its derivatives $H'(z)$, $H''(z)$, which resulting lead us to reconstruct region of $f(Q)$, without any assumptions. The obtained mean curve along $Λ$CDM constant in the reconstructed region follows a quadratic behavior. This motivates us to propose a new $f(Q)$ parametrization, i.e., $f(Q)= -2Λ+ εQ^2$, with the single parameter $ε$, which signifies the deviations from $Λ$CDM cosmology. Further, we probe the widely studied power-law and exponential $f(Q)$ models against the reconstructed region and can improve the parameter spaces significantly compared with observational analysis. In addition, the direct Hubble measurements, along with the reconstructed $f(Q)$ function, allow the $H_0$ tension to be alleviated.

gr-qc↗

arXiv2023

Theory of gravity with nonminimal matter-nonmetricity coupling and the de-Sitter swampland conjectures

In this study, we investigate swampland conjectures within the setup of matter and non-metricity nonminimal coupling theories of gravity. We examine how the inflationary solution produced by a single scalar field can be resolved with the swampland criteria in string theory regarding the formation of de Sitter solutions. The new important findings are that the inflationary scenario in our study differs from the one in general relativity because of the presence of a nonminimal coupling term, and that difference gives the correction to general relativity. In addition, we observe that the slow-roll conditions and the swampland conjectures are incompatible with each other for a single scalar field within the framework of nonminimally coupled alternative gravity theories. We predict that these results will hold for a wide range of inflationary scenarios in the context of nonminimal coupling gravitational theories.

gr-qc↗

arXiv2023

Cosmological observational constraints on the power law $f(Q)$ type modified gravity theory

In modern cosmology, the curiosity of ultimately understanding the nature of the dark energy controlling the recent acceleration of the Universe motivates us to explore its properties by using some novel approaches. In this work, to explore the properties of dark energy we adopt the modified $f(Q)$ gravity theory, where the non-metricity scalar $Q$, emerging from Weyl geometry, plays the dynamical role. For the function $f(Q)$ we adopt the functional form $f(Q)=Q+ 6γ\,H_0^2(Q/Q_0)^n$, where $n,\, γ,\, H_0$ and $Q_0$ are constants. Then, we test our constructed model against the various observational datasets, such as the Hubble, and the Pantheon+SHOES samples, and their combined sample, through the Markov Chain Monte Carlo (MCMC) statistical analysis. We also employ the parameter estimation technique to constrain the free parameters of the model. In addition, we use the constrained values of the model parameters to explore a few implications of the cosmological model. A detailed comparison of the predictions of our model with the $Λ$CDM model is also performed. In particular, we discuss in detail some cosmographic parameters, like the deceleration, the jerk, and the snap parameters, as well as the behavior of the dark energy and matter energy densities to see the evolution of various energy/matter profiles. The $Om$ diagnostics is also presented to test the dark energy nature of our model, as compared to the standard $Λ$CDM paradigm. Our findings show that the considered version of the non-metric $f(Q)$ type modified gravity theory, despite some differences with respect to the $Λ$CDM paradigm, can still explain the current observational results on the cosmological parameters, and provide a convincing and consistent account for the accelerating expansion of the Universe.

gr-qc↗

arXiv2023

Cosmology in $f(R,L_m)$ gravity

In this letter, we investigate the cosmic expansion scenario of the universe in the framework of $f(R,L_m)$ gravity theory. We consider a non-linear $f(R,L_m)$ model, specifically, $f(R,L_m)=\frac{R}{2}+L_m^n + β$, where $n$ and $β$ are free model parameters. Then we derive the motion equations for flat FLRW universe and obtain the exact solution of corresponding field equations. Then we estimate the best fit ranges of model parameters by using updated $H(z)$ datasets consisting of 57 points and the Pantheon datasets consisting of 1048 points. Further we investigate the physical behavior of density and the deceleration parameter. The evolution of deceleration parameter depicts a transition from deceleration to acceleration phases of the universe. Moreover, we analyze the stability of the solution of our cosmological model under the observational constraint by considering a linear perturbation. Lastlty, we investigate the behavior of Om diagnostic parameter and we observe that our model shows quintessence type behavior. We conclude that our $f(R,L_m)$ cosmological model agrees with the recent observational studies and can efficiently describe the late time cosmic acceleration.

gr-qc↗

arXiv2023

Slow-roll inflation in $f(T,\mathcal{T})$ modified gravity

In this article, we explore the concept of cosmological inflation within the framework of the $f (T,\mathcal{T})$ theory of gravity, where $f$ is a general function of the Torsion scalar $T$ and the trace, $\mathcal{T}$, of the energy-momentum tensor. It is assumed that the conditions of slow-roll inflation are applicable in$f (T,\mathcal{T})$ gravity. To determine different observables related to inflation, such as the tensor-to-scalar ratio $r$, scalar spectral index $n_s$, spectral index $ α_s $, and tensor spectral index $n_t$, the Hubble slow-roll parameters are utilized for a particular model of $f (T,\mathcal{T})$. Lastly, an assessment has been carried out to determine the feasibility of the models by conducting a numerical analysis of the parameters. The findings indicate that it is feasible to achieve compatibility with the observational measurements of slow-roll parameters by utilizing different values of the free parameters.

gr-qc↗

arXiv2023

Correction to Lagrangian for Bouncing Cosmologies in $f(Q)$ Gravity

Symmetric teleparallel gravity offers to reformulate the gravitational formalism without the presence of curvature and torsion with the help of non-metricity tensors. Interestingly, Symmetric teleparallel gravity can be formulated equivalently to teleparallel gravity or general relativity for an appropriate setup. In this study, our aim lies in exploring the bouncing cosmologies as an alternative to the initial singularity of the Universe in the background of modified symmetric teleparallel gravity. To explore this, we adopt the reconstruction technique to present the possible reconstructed Lagrangian for various cosmological bouncing solutions in a flat Friedmann-Lemaître-Robertson-Walker spacetime with a perfect fluid matter distribution. We study the reconstructed gravitational Lagrangians, which are capable of reproducing analytical solutions for \textit{symmetric bounce}, \textit{super-bounce}, \textit{oscillatory bounce}, \textit{matter bounce}, and \textit{exponential bouncing} model settings. Further, we examine the dark energy profiles of the models using reconstructed Lagrangians. In addition, we found that an additional term arises in each reconstructed Lagrangian compared to general relativity (GR). That extra term corrected the background GR to present bouncing cosmology in modified gravity. These newly motivated cosmological models may have an effect on gravitational phenomena at other cosmological scales.

gr-qc↗

arXiv2023

$H_0$ Tension in Torsion-based Modified Gravity

The rising concern in the Hubble constant tension ($H_0$ tension) of the cosmological models motivates the scientific community to search for alternative cosmological scenarios that could resolve the $H_0$ tension. In this regard, we aim to work on a torsion-based modified theory of gravity which is an alternative description to the coherence model. We solve numerically for the Hubble parameter using two exponential Lagrangian functions of torsion $T$ and a trace of energy-momentum tensor $\mathcal{T}$ for the dust case. Further, we constrain the cosmological and model parameters; to do that, we use Hubble, SNe Ia, Baryon Acoustic Oscillations, Cosmic Microwave Background samples, and Markov Chain Monte Carlo (MCMC) simulation through Bayesian statistics. We obtain the values of Hubble constant $H_0$ for our model, and the outputs align with the recent observational measurements of $H_0$. In addition, we check the deviation of our results from model-independent measurements of $H_0$ from Planck2018, S$H_0$ES, and $H_0$LiCOW experiments. In contrast, our finding partially solved the $H_0$ tension but gave a new possible direction to alleviate the $H_0$ tension.

astro-ph.CO↗

arXiv2023

Constraining $f(T,\mathcal{T})$ Gravity with Gravitational Baryogenesis

Gravitational baryogenesis is one of the mechanisms which help us to explore more about our early universe, especially baryon-anti-baryon asymmetry. As we know, modified theories of gravity are very successful in describing the present accelerated scenario of the universe. Therefore, in this letter, we aim to constrain the generalized torsion-based modified theory of gravity, namely, $f(T,\mathcal{T})$ gravity with gravitational baryogenesis, where $T$, $\mathcal{T}$ are the torsion scalar, trace of the energy-momentum tensor, respectively. For this, we examine how the various Lagrangian forms of $f(T,\mathcal{T})$ affect the baryogenesis. We also impose the constraints on the extra degrees of freedom induced by modified theory with the observational values of the baryon-to-entropy ratio. In addition, we further explore how more generalized gravitational baryogenesis can attribute in a physically viable and consistent way to the cosmologies of the modified theory of gravity.

gr-qc↗

arXiv2023

Viscous fluid cosmology in symmetric teleparallel gravity

In this manuscript, we analyze the viscous fluid cosmological model in the framework of recently proposed $f(Q)$ gravity by assuming three different forms of bulk viscosity coefficients, specifically, $(i)ζ=ζ_{0}+ζ_{1}\left( \frac{\dot{a}}{a}\right) +ζ_{2}\left( \frac{\ddot{a}}{\dot{a}}\right) $, $(ii)ζ=ζ_{0}+ζ_{1}\left( \frac{\dot{a}}{a}\right)$, and $(iii)ζ=ζ_{0}$ and a linear $f(Q)$ model, particularly, $f(Q)=αQ$ where $α\neq 0$ is free model parameter. We estimate the bulk viscosity coefficients and the model parameter values using the combined H(z)+Pantheon+BAO data set. We study the asymptotic behavior of our cosmological bulk viscous model by utilizing the phase space method. We find that corresponding to all three cases, our model depicts the evolution of the universe from matter dominated decelerated epoch (a past attractor) to a stable de-sitter accelerated epoch (a future attractor). Further, we study the physical behavior of effective pressure, effective equation of state (EoS), and the statefinder parameters. We find that the pressure component in the presence of bulk viscosity shows negative behavior and the effective EoS parameter predicts the accelerated expansion phase of the universe for all three cases. Moreover, we obtain that the trajectories of our model lie in the quintessence region and it converges to the $Λ$CDM fixed point in the far future. We find that the accelerated deSitter like phase comes purely from the $\barζ_0$ case without any geometrical modification to GR. Moreover, we find that the late-time behavior of all three cases of viscosity coefficients are identical.

gr-qc↗

arXiv2023

Constraining viscous dark energy equation of state in $f(R,L_m)$ gravity

In this article, we attempt to describe cosmic late-time acceleration of the universe in the framework of $f(R,L_m)$ gravity by using an effective equation of state when the account is taken of bulk viscosity. We presume a non-linear $f(R,L_m)$ functional form, specifically, $f(R,L_m)=\frac{R}{2}+L_m^α$, where $α$ is free model parameter. We obtain the exact solution of our bulk viscous matter dominated $f(R,L_m)$ model, and then we utilize the combined $H(z)+Pantheon+Analysis$ data sets to estimate the best fit values of the free parameters of our model. Then we characterize the behaviour of the matter-energy density, effective pressure, and the equation of state (EoS) parameter incorporating the viscous type fluid. The evolution profile of the effective EoS parameter depicts an acceleration phase of the cosmic expansion whereas the pressure with the effect of viscosity exhibits negative behaviour that can lead to the accelerating expansion of the universe. Moreover, the cosmic matter-energy density shows expected positive behaviour. Further, we investigate the behaviour of statefinder parameters for the assumed $f(R,L_m)$ model. We find that the evolutionary trajectory of the given model lies in the quintessence region. In addition, we employ the Om diagnostic test that indicates our model exhibits quintessence behavior. Lastly, we check the energy condition criteria and find that violation of SEC occurs in the past, whereas NEC and DEC satisfies the positivity criteria. We find that our $f(R,L_m)$ cosmological model with the effect of bulk viscosity provides a good fit of the recent observational data and can efficiently describe the cosmic expansion scenario.

gr-qc↗

arXiv2023

Gravastar in the Framework of Symmetric Teleparallel Gravity

In the current research, we present a novel gravastar model based on the Mazur-Mottola (2004) method with an isotropic matter distribution in $f(Q)$ gravity. The gravastar, a hypothesized substitute for a black hole, is built using the Mazur-Mottola mechanism. This approach allows us to define gravastar as having three stages. The first one is an inner region with negative pressure; the next region is a thin shell that is made up of ultrarelativistic stiff fluid, we have studied proper length, energy, entropy, and surface energy density for this region. Apart from that, we have demonstrated the possible stability of our suggested thin shell gravastar model through the graphical study of surface redshift. Exterior Schwarzschild geometry describes the outer region of the gravastar. In the context of $f(Q)$ gravity, we have discovered analytical solutions for the interior of gravastars that are free of any kind of singularity and the event horizon.

gr-qc↗

arXiv2022

Paramerization of deceleration parameter in $f(Q)$ gravity

In this article, we investigate the modified symmetric teleparallel gravity or $f(Q)$ gravity, where $Q$ is the non-metricity, to study the evolutionary history of the universe by considering the functional form of $f(Q)=αQ^n$, where $α$ and $n$ are constants. Here, we consider the parametrization form of the deceleration parameter as $q=q_0+\frac{q_1\,z}{(1+z)^2}$ which provides the desired property for sign flip from a decelerating to an accelerating phase. We get the solution of the Hubble parameter by examining the mentioned parametric form of $q$, and then we impose the solution in Friedmann equations. Employing the Bayesian analysis for the Observational Hubble data (OHD), we estimated the constraints on the associated free parameters $(H_0,q_0,q_1)$ to determine if this model may challenge the $Λ$CDM limitations. Furthermore, the constrained current value of the deceleration parameter $q_0=-0.832^{+0.091}_{-0.091}$ shows that the present universe is accelerating. We also investigate the evolutionary trajectory of energy density, pressure, and EoS parameters to conclude the accelerating behavior of the universe. Finally, we try to demonstrate that the considered parametric form of the deceleration parameter is compatible with $f(Q)$ gravity.

gr-qc↗