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Raja Solanki

Publications and source records attributed to Raja Solanki.

At least 19 recordsLinked to original sources
arXiv2026

$f(T,\mathcal{L}_m)$ Cosmology Embedded in a Viscous Barotropic Fluid

We examine the cosmological dynamics of a viscous fluid within the framework of $f(T,\mathcal{L}_m)$ gravity by using a barotropic equation of state for cosmic fluid. The modified Friedmann equations are used to construct an analytical Hubble model. Then $H(z)$, Pantheon+SH0ES, DESI DR II BAO, and Cosmic Microwave Background datasets are used in a Bayesian Markov Chain Monte Carlo analysis to constrain its free parameters. Physically viable estimations of the Hubble constant and barotropic equation of state parameter are given by the obtained best-fit values. Based on the constrained parameters, we examine the redshift dependence of deceleration parameter, effective equation of state, dark energy equation of state, the effective pressure and viscous pressure quantities. While the deceleration parameter reveals how the universe evolved from an initially decelerating state to its present accelerating expansion, both equation of state parameters persist in the quintessence regime in the late epoch. Moreover, throughout the cosmic evolution, the viscous and effective pressures stay negative. The observational viability of the suggested viscous $f(T,\mathcal{L}_m)$ cosmological model is supported by the estimated Hubble constant, whose values agree well with the results from independent cosmological observations.

physics.gen-ph↗
arXiv2026

Polytropic $f(Q)$ cosmology and its implications for the $H_0$ tension

Understanding the late-time cosmic phenomenon of the universe commonly referred to as the dark energy problem, which is one of the prominent tension in the field of theoretical as well as observational cosmology. In this work, we attempt to analyze the nature of the missing fluid of the universe. In order to do so, we employ a poly tropic equation of state consisting of free parameters rather assuming directly a particular form of the fluid. In addition, for the background geometry we consider a $f(Q)$ cosmology exhibiting power-law assumption, which is recently proposed and found to be attractive in the study of late-time cosmology. We find exact cosmological solution along with a rigorous data analysis, utilizing the Bayesian statistics approach and the emcee ensemble sampler, to find the parameter constraints and then we interpret the parameters of physical interests such as deceleration and statefinder parameter. Also, we present status of the $H_0$ tension predicted by our polytropic $f(Q)$ cosmological model.

gr-qc↗
arXiv2025

Accelerating Expansion of the Universe in Modified Symmetric Teleparallel Gravity

In the last century, theoretical and experimental developments have established the General Relativity theory as the most successful theory for describing the gravitational phenomenon. On the other hand, in the last two decades, multiple observational probes have strongly favored the discovery of the acceleration of cosmic expansion. The observational enhancement and development in precision cosmology indicate a requirement to go beyond General Relativity and to search for an alternate description that can resolve the persistent issues. In Chapter 1, we highlight some important elements of observational cosmology. In Chapters 2 and 3, we investigate the f(Q) gravity in the presence of viscosity in the cosmic fluid. In Chapters 4 and 5, we explore the constraints on the various classes of non-linear f(Q) gravity models in both coincident and non-coincident formalism, respectively. In Chapter 6, we present a covariant formulation and energy balance equation for the f(Q,T) gravity, which is an extension of f(Q) gravity. Finally, in Chapter 7, we briefly summarize the outcomes of the present thesis and the future scope.

gr-qc↗
arXiv2024

Cosmological constraints on $f(Q)$ gravity models in the non-coincident formalism

The article investigates cosmological applications of $f(Q)$ theories in a non-coincident formalism. We explore a new $f(Q)$ theory dynamics utilizing a non-vanishing affine connection involving a non-constant function $\gamma(t)=-a^{-1}\dot{H}$, resulting in Friedmann equations that are entirely distinct from those of $f(T)$ theory. In addition, we propose a new parameterization of the Hubble function that can consistently depicts the present deceleration parameter value, transition redshift, and the late time de-Sitter limit. We evaluate the predictions of the assumed Hubble function by imposing constraints on the free parameters utilizing Bayesian statistical analysis to estimate the posterior probability by employing the CC, Pantheon+SH0ES, and the BAO samples. Moreover, we conduct the AIC and BIC statistical evaluations to determine the reliability of MCMC analysis. Further, we consider some well-known corrections to the STEGR case such as an exponentital $f(Q)$ correction, logarithmic $f(Q)$ correction, and a power-law $f(Q)$ correction and then we find the constraints on the parameters of these models via energy conditions. Finally, to test the physical plausibility of the assumed $f(Q)$ models we conduct the thermodynamical stability analysis via the sound speed parameter.

gr-qc↗
arXiv2024

Bulk viscous cosmological model in $f(T,\mathcal{T})$ modified gravity

This article explores the impact of bulk viscosity on understanding the universe's accelerated expansion within the context of modified $f(T,\mathcal{T})$ gravity, which is an extension of the $f(T)$ gravitational theory, allowing a broad coupling between the energy-momentum scalar $\mathcal{T}$ and the torsion scalar $T$. We consider two $f(T,\mathcal{T})$ functions, specifically $f(T,\mathcal{T})=\alpha T + \beta \mathcal{T}$ and $f(T,\mathcal{T})=\alpha \sqrt{-T} + \beta \mathcal{T}$, where $\alpha$ and $\beta$ are arbitrary constants, along with the fluid part incorporating the coefficient of bulk viscosity $\zeta=\zeta_0 > 0$. We calculate the analytical solutions of the corresponding field equations for a flat FLRW environment, and then we constrain the free parameters of the obtained solution using CC, Pantheon+, and the CC+Pantheon+ samples. We perform the Bayesian statistical analysis to estimate the posterior probability utilizing the likelihood function and the MCMC random sampling technique. Further, to assess the effectiveness of our MCMC analysis, we estimate the corresponding AIC and BIC values, and we find that there is strong evidence supporting the assumed viscous modified gravity models for all three data sets. Also, we find that the linear model precisely mimics the $\Lambda$CDM model. We also investigate the evolutionary behavior of some prominent cosmological parameters. We observe that the effective equation of state parameter for both models predict the accelerating behavior of the cosmic expansion phase. In addition, from the statefinder test, we find that the parameters of the considered MOG models favor the quintessence-type behavior.

gr-qc↗
arXiv2024

Extended Bose-Einstein condensate dark matter in $f(Q)$ gravity

In this article, we attempt to explore the dark sector of the universe i.e. dark matter and dark energy, where the dark energy components are related to the modified $f(Q)$ Lagrangian, particularly a power law function $f(Q)= \gamma \left(\frac{Q}{Q_0}\right)^n$, while the dark matter component is described by the Extended Bose-Einstein Condensate (EBEC) equation of state for dark matter, specifically, $p = \alpha \rho + \beta \rho^2$. We find the corresponding Friedmann-like equations and the continuity equation for both dark components along with an interacting term, specifically $\mathcal{Q} = 3b^2H \rho$, which signifies the energy exchange between the dark sector of the universe. Further, we derive the analytical expression of the Hubble function, and then we find the best-fit values of free parameters utilizing the Bayesian analysis to estimate the posterior probability and the Markov Chain Monte Carlo (MCMC) sampling technique corresponding to CC+Pantheon+SH0ES samples. In addition, to examine the robustness of our MCMC analysis, we perform a statistical assessment using the Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC). Further from the evolutionary profile of the deceleration parameter and the energy density, we obtain a transition from the decelerated epoch to the accelerated expansion phase, with the present deceleration parameter value as $q(z=0)=q_0=-0.56^{+0.04}_{-0.03}$ ($68 \%$ confidence limit), that is quite consistent with cosmological observations. In addition, we find the expected positive behavior of the effective energy density. Finally, by examining the sound speed parameter, we find that the assumed theoretical $f(Q)$ model is thermodynamically stable.

gr-qc↗
arXiv2024

Bouncing cosmological models in $f(R,L_m)$ gravity

This article explores matter bounce non-singular cosmology in $f(R,L_m)$ gravity. We consider two non-linear $f(R,L_m)$ functional forms, specifically, $f(R,L_m) = \frac{R}{2} + \lambda R^2 + \alpha L_m$ and $f(R,L_m) = \frac{R}{2} + L_m ^\beta + \gamma$ representing a minimal coupling case. We derive the corresponding Friedmann-like equations for both the assumed models in the FLRW background, and then we present the impact of the model parameters along with the parameter of bouncing scale factor on the equation of state parameter, pressure, and the energy density. In addition, we examine the dynamical behavior of cosmographic parameters such as jerk, lerk, and snap parameters. Further, we find that the violation of the null energy condition along with the strong energy condition depicts the non-singular accelerating behavior, corresponding to both assumed non-linear $f(R,L_m)$ functions. Lastly, we present the behavior of the adiabatic speed of sound to examine the viability of the considered cosmological bouncing scenario.

gr-qc↗
arXiv2024

Dynamical system analysis of Dirac-Born-Infeld scalar field cosmology in coincident $f(Q)$ gravity

In this article, we offer the dynamical system analysis of the DBI (Dirac-Born-Infeld) scalar field in a modified $f(Q)$ gravity context. We have taken a polynomial form of modified gravity and used two different kinds of scalar potential, i.e., polynomial and exponential, and found a closed autonomous dynamical system of equations. We have analyzed the fixed points of such a system and commented on the conditions under which deceleration to late-time acceleration happens in this model. We have noted the similarity of the two models and have also shown that our result is indeed consistent with the previous work done on Einstein's gravity. We have also investigated the phenomenological implications of our models by plotting the EoS ($\omega$), Energy density ($\Omega$), and deceleration parameter ($q$) w.r.t. to e-fold time and comparing with the present value. Finally, we conclude the paper by observing how the dynamical system analysis differs in modified $f(Q)$ gravity, and we also provide some of the future scope of our work.

gr-qc↗
arXiv2024

Phase-space analysis of the viscous fluid cosmological models in the coincident $f(Q)$ gravity

In this article, we consider a newly proposed parameterization of the viscosity coefficient $\zeta$, specifically $\zeta=\bar{\zeta}_0 {\Omega^s_m} H $, where $\bar{\zeta}_0 = \frac{\zeta_0}{{\Omega^s_{m_0}}} $ within the coincident $f(Q)$ gravity formalism. We consider a non-linear function $f(Q)= -Q +\alpha Q^n$, where $\alpha$ and $n$ are arbitrary model parameters, which is a power-law correction to the STEGR scenario. We find an autonomous system by invoking the dimensionless density parameters as the governing phase-space variables. We discuss the physical significance of the model corresponding to the parameter choices $n=-1$ and $n=2$ along with the exponent choices $s=0, 0.5$, and $1.05$. We find that model I shows the stable de-Sitter type or stable phantom type (depending on the choice of exponent $s$) behavior with no transition epoch, whereas model II shows the evolutionary phase from the radiation epoch to the accelerated de-Sitter epoch via passing through the matter-dominated epoch. Hence, we conclude that model I provides a good description of the late-time cosmology but fails to describe the transition epoch, whereas model II modifies the description in the context of the early universe and provides a good description of the matter and radiation era along with the transition phase.

gr-qc↗
arXiv2023

Dynamical system analysis of scalar field cosmology in coincident $f(Q)$ gravity

In this article, we investigate scalar field cosmology in the coincident $f(Q)$ gravity formalism. We calculate the motion equations of $f(Q)$ gravity under the flat Friedmann-Lema\^{i}tre-Robertson-Walker background in the presence of a scalar field. We consider a non-linear $f(Q)$ model, particularly $f(Q)=-Q+\alpha Q^n$, which is nothing but a polynomial correction to the STEGR case. Further, we assumed two well-known specific forms of the potential function, specifically the exponential from $V(\phi)= V_0 e^{-\beta \phi}$ and the power-law form $V(\phi)= V_0\phi^{-k}$. We employ some phase-space variables and transform the cosmological field equations into an autonomous system. We calculate the critical points of the corresponding autonomous systems and examine their stability behaviors. We discuss the physical significance corresponding to the exponential case for parameter values $n=2$ and $n=-1$ with $\beta=1$, and $n=-1$ with $\beta=\sqrt{3}$. Moreover, we discuss the same corresponding to the power-law case for the parameter value $n=-2$ and $k=0.16$. We also analyze the behavior of corresponding cosmological parameters such as scalar field and dark energy density, deceleration, and the effective equation of state parameter. Corresponding to the exponential case, we find that the results obtained for the parameter constraints in Case III is better among all three cases, and that represents the evolution of the universe from a decelerated stiff era to an accelerated de-Sitter era via matter-dominated epoch. Further, in the power-law case, we find that all trajectories exhibit identical behavior, representing the evolution of the universe from a decelerated stiff era to an accelerated de-Sitter era. Lastly, we conclude that the exponential case shows better evolution as compared to the power-law case.

gr-qc↗
arXiv2023

Wormhole solutions in $f(R,L_m)$ gravity

In this work, we intend to explore wormhole geometries in the framework of $f(R,L_m)$ gravity. We derive the field equations for the generic $f(R,L_m)$ function by assuming the static and spherically symmetric Morris-Thorne wormhole metric. Then we consider two non-linear $f(R,L_m)$ model, specifically, $f(R,L_m)=\frac{R}{2}+L_m^\alpha$ and $f(R,L_m)=\frac{R}{2}+(1+\lambda R)L_m$, where $\alpha$ and $\lambda$ are free model parameters. We obtain the wormhole solutions by assuming three cases, namely, a linear barotropic EoS, anisotropic EoS, and isotropic EoS corresponding to model I. We observe that for both barotropic and anisotropic cases, the corresponding wormhole solutions obey the flaring-out condition under asymptotic background, while for the isotropic case, the shape function does not follow the flatness condition. Also, we find that the null energy condition exhibits negative behavior in the vicinity of the throat. Further, we consider two different shape functions to investigate the behavior of model II. We find some constraints on the model parameter for which the violation of the null energy condition exhibits. Finally, we employ the volume integral quantifier to calculate the amount of exotic matter required near the wormhole throat for both models. We conclude that the modification of standard GR can efficiently minimize the use of exotic matter and provide stable traversable wormhole solutions.

gr-qc↗
arXiv2023

Cosmic acceleration with bulk viscosity in an anisotropic $f(R,L_m)$ background

In this article, we investigate the observed cosmic acceleration in the framework of a cosmological $f(R,L_m)$ model dominated by bulk viscous matter in an anisotropic background. We consider the LRS Bianchi type I metric and derive the Friedmann equations that drive the gravitational interactions in $f(R,L_m)$ gravity. Further, we assume the functional form $f(R,L_m)=\frac{R}{2}+L_m^\alpha $, where $\alpha$ is a free model parameter, and then find the exact solutions of fields equations corresponding to our viscous matter dominated model. We incorporate the updated H(z) data and the Pantheon data to acquire the best-fit values of parameters of our model by utilizing the $\chi^2$ minimization technique along with the Markov Chain Monte Carlo (MCMC) random sampling method. Further, we present the behavior of physical parameters that describe the universe's evolution phase, such as density, effective pressure and EoS parameters, skewness parameter, and the statefinder diagnostic parameters. We found that the energy density indicates expected positive behavior, whereas the negative behavior of bulk viscous pressure contributes to the universe's expansion. The effective EoS parameter favors the accelerating phase of the universe's expansion. Moreover, the skewness parameter shows the anisotropic nature of spacetime during the entire evolution phase of the universe. Finally, from statefinder diagnostic test, we found that our cosmological $f(R,L_m)$ model lies in the quintessence region, and it behaves like a de-Sitter universe in the far future. We analyze different energy conditions in order to test the consistency of the obtained solution. We found that all energy conditions except SEC show positive behavior, while the violation of SEC favors the recently observed acceleration with the transition from decelerated to an accelerated epoch of the universe's expansion in the recent past.

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^\alpha $, where $\alpha$ 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

$f(Q,T)$ gravity, its covariant formulation, energy conservation and phase-space analysis

In the present article we analyze the matter-geometry coupled $f(Q,T)$ theory of gravity. We offer the fully covariant formulation of the theory, with which we construct the correct energy balance equation and employ it to conduct a dynamical system analysis in a spatially flat Friedmann-Lema\^{i}tre-Robertson-Walker spacetime. We consider three different functional forms of the $f(Q,T)$ function, specifically, $f(Q,T)=\alpha Q+ \beta T$, $f(Q,T)=\alpha Q+ \beta T^2$, and $f(Q,T)=Q+ \alpha Q^2+ \beta T$ . We attempt to investigate the physical capabilities of these models to describe various cosmological epochs. We calculate Friedmann-like equations in each case and introduce some phase space variables to simplify the equations in more concise forms. We observe that the linear model $f(Q,T)=\alpha Q+ \beta T$ with $\beta=0$ is completely equivalent to the GR case without cosmological constant $\Lambda$. Further, we find that the model $f(Q,T)=\alpha Q+ \beta T^2$ with $\beta \neq 0$ successfully depicts the observed transition from decelerated phase to an accelerated phase of the universe. Lastly, we find that the model $f(Q,T)= Q+ \alpha Q^2+ \beta T$ with $\alpha \neq 0$ represents an accelerated de-Sitter epoch for the constraints $\beta < -1$ or $ \beta \geq 0$.

gr-qc↗
arXiv2022

Scalar field evolution at background and perturbation levels for a broad class of potentials

In this paper, we investigate a non-interacting scalar field cosmology with an arbitrary potential using the $f$-deviser method that relies on the differentiability properties of the potential. Using this alternative mathematical approach, we present a unified dynamical system analysis at a scalar field's background and perturbation levels with arbitrary potentials. For illustration, we consider a monomial and double exponential potential. These two classes of potentials comprise the asymptotic behaviour of several classes of scalar field potentials, and, therefore, they provide the skeleton for the typical behaviour of arbitrary potentials. Moreover, we analyse the linear cosmological perturbations in the matterless case by considering three scalar perturbations: the evolution of the Bardeen potentials, the comoving curvature perturbation, the so-called Sasaki-Mukhanov variable, or the scalar field perturbation in uniform curvature gauge. Finally, an exhaustive dynamical system analysis for each scalar perturbation is presented, including the evolution of Bardeen potentials in the presence of matter.

gr-qc↗
arXiv2022

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)\zeta =\zeta_{0}+\zeta_{1}\left( \frac{\dot{a}}{a}\right) +\zeta_{2}\left( \frac{{\ddot{a}}}{\dot{a}}\right) $, $(ii)\zeta =\zeta_{0}+\zeta_{1}\left( \frac{\dot{a}}{a}\right)$, and $(iii)\zeta =\zeta_{0}$ and a linear $f(Q)$ model, particularly, $f(Q)=\alpha Q$ where $\alpha \neq 0$ 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 $\Lambda$CDM fixed point in the far future. We find that the accelerated deSitter like phase comes purely from the $\bar{\zeta}_0$ 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↗
arXiv2022

Statefinder Analysis of Symmetric Teleparallel Cosmology

Statefinder diagnostic is a convenient method that can differentiate between the various dark energy models. In this article, we analyze the statefinder parameters in symmetric teleparallel cosmology. The $f(Q)$ gravity theory is an alternative theory to GR, where gravitational interactions attribute to the non-metricity scalar $Q$. In the present work, we consider two $f(Q)$ models which contains a linear and a non-linear form of non-metricity scalar, specifically, $f(Q)=\alpha Q + \frac{\beta}{Q}$ and $f(Q)=\alpha Q + \beta Q^2$, where $\alpha$ and $\beta$ are free parameters and then statefinder parameters $(r,s)$ are evaluated. We plot the trajectory of our models in the $r-s$ plane. In addition, we analyze the physical behavior of different cosmological parameters such as density, deceleration, and the EoS parameters. Further, we use Om diagnostic to differentiate the behavior of both $f(Q)$ models. We found that both $f(Q)$ models predicts that the present universe is accelerating due to the dark energy component evolving due to non-metricity. Moreover, model I represents phantom type behavior while model II follows quintessence scenario.

gr-qc↗
arXiv2022

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, $f(R,L_m)=\frac{R}{2}+L_m^n + \beta$, where $n$ and $\beta$ 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↗