arXiv subjects

Dheeraj Singh Rana

Publications and source records attributed to Dheeraj Singh Rana.

4 recordsLinked to original sources

arXiv2026

Study of Cosmic Acceleration of the Universe in the Presence of Bulk Viscous Matter

Over the past century, both theoretical advancements and experimental observations have established General Relativity (GR) as the most successful framework for describing gravitational phenomena. However, observations from multiple cosmological probes over the last two decades have provided compelling evidence for the accelerated expansion of the Universe. The rapid progress in observational astronomy and precision cosmology has highlighted several challenges that motivate the search for extensions or alternatives to General Relativity. In this thesis, we investigate alternative formulations of gravity based on non-Riemannian geometry, with particular emphasis on (f(Q)) gravity in the presence of bulk viscosity. We first explore the existence of exact cosmological solutions for viscous fluid models within the framework of (f(Q)) gravity and constrain the free parameters of these solutions using observational datasets. The resulting constrained models are then employed to study the evolutionary history of cosmic expansion. Our analysis demonstrates that the proposed viscous (f(Q)) gravity models can successfully account for the observed late-time acceleration of the Universe. Furthermore, we examine several classes of nonlinear viscous (f(Q)) gravity models through both dynamical systems and observational analyses. Particular attention is given to the cosmological viability of these models and their ability to reproduce the different evolutionary epochs of the Universe, ranging from matter-dominated eras to accelerated expansion phases.

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arXiv2024

Cosmological constraints in symmetric teleparallel gravity with bulk viscosity

In this study, we explore the accelerated expansion of the universe within the framework of modified $f(Q)$ gravity. The investigation focus on the role of bulk viscosity in understanding the universe's accelerated expansion. Specifically, a bulk viscous matter-dominated cosmological model is considered, with the bulk viscosity coefficient expressed as $ζ= ζ_0 ρH^{-1} + ζ_1 H $. We consider the power law $f(Q)$ function $f(Q)=αQ^n $, where $α$ and $n$ are arbitrary constants and derive the analytical solutions for the field equations corresponding to a flat FLRW metric. Subsequently, we used the combined Cosmic Chronometers (CC)+Pantheon+SH0ES sample to estimate the free parameters of the obtained analytic solution. We conduct Bayesian statistical analysis to estimate the posterior probability by employing the likelihood function and the MCMC random sampling technique, along with the AIC and BIC statistical assessment criteria. In addition, we explore the evolutionary behavior of significant cosmological parameters. The effective equation of state (EOS) parameter predicts the accelerating behavior of the cosmic expansion phase. Further, by the statefinder and $Om(z)$ diagnostic test, we found that our viscous model favors quintessence-type behavior and can successfully describe the late-time scenario.

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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 $ζ$, specifically $ζ=\barζ_0 {Ω^s_m} H $, where $\barζ_0 = \frac{ζ_0}{Ω^s_{m_0}} $ within the coincident $f(Q)$ gravity formalism. We consider a non-linear function $f(Q)= -Q +αQ^n$, where $α$ 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.

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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.

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