arXiv subjects

Praveen Kumar Dhankar

Publications and source records attributed to Praveen Kumar Dhankar.

At least 19 recordsLinked to original sources

arXiv2026

Classical and Loop Quantum Cosmology of Interacting Dark Energy: A Dynamical System Analysis with Superfluid Dark Matter and Dust Matter

We study the cosmological dynamics of interacting dark energy and dark matter in Classical Einstein Gravity and Loop Quantum Cosmology. Two dark matter scenarios are considered: superfluid dark matter described by a generalized cubic equation of state and the standard pressureless fluid. The dark energy component is modeled using both a generalized nonlinear equation of state and a constant equation of state. We examine two phenomenological interaction terms, $Q=α\dotρ_m$ and $Q=β\dotρ_d$, which govern the energy transfer between the dark sectors. In classical gravity, the pressureless matter model exhibits stable late-time attractors, whereas the superfluid dark matter model admits only saddle and non-hyperbolic critical points. Extending the analysis to Loop Quantum Cosmology, quantum geometric corrections replace the Big Bang singularity with a nonsingular quantum bounce, and significantly modify the phase-space dynamics. As a result, the stable attractors of the classical pressureless matter model disappear, and all interacting models possess only saddle and non-hyperbolic critical points. These findings highlight the significant influence of both dark matter properties and quantum gravitational effects on the asymptotic evolution of interacting dark-sectors.

gr-qc↗

arXiv2026

Probing Nonlinear Logarithmic Kalb-Ramond Black Holes: Particle Dynamics, Epicyclic Oscillations and Thermodynamic Signatures

In contrast to conventional linear coupling frameworks, the proposed work investigated the nonlinear effects that become more significant in the strong field regime of a black hole. We have investigated a new class of Kalb Ramond black holes generated by a nonlinear logarithmic coupling of the field, referred to as a Logarithmic Kalb Ramond black hole. The logarithmic coupling introduces strong-field modifications to the spacetime geometry, leading to significant departures from the Schwarzschild and Reissner Nordstrom BHs. We analyze the motion of test particles using the effective potential formalism and derive the conserved energy and angular momentum for circular equatorial geodesics. The stability of circular orbits and the location of the innermost stable circular orbit are examined, revealing a strong dependence on the model parameters Q, l, and $β$. The epicyclic frequencies (radial, vertical, and azimuthal) together with the associated periastron precession demonstrate that nonlinear logarithmic corrections can substantially modify quasi periodic oscillation observables. We further investigate the Hawking temperature and energy emission rate, which show that the nonlinear coupling also impacts distinct imprints on the thermodynamic behavior and evaporation characteristics of the black hole. Our results also identify the BH parameters as key regulators of the orbital dynamics, oscillatory properties, and thermal evolution, providing a unified framework for probing nonlinear KR gravity through strong-field astrophysical phenomena.

gr-qc↗

arXiv2026

Observational Constraints and Cosmic Growth Index of Realistic $f(G)$ Gravity Frameworks using MCMC Analysis

We present a comprehensive observational analysis of modified Gauss--Bonnet, or $f(G)$, gravity by investigating both the background cosmological expansion and sub-horizon linear matter perturbations. We consider two viable functional forms: an arctangent parameterization (Model~I) and a generalized polynomial power-law model (Model~II). Using a Markov Chain Monte Carlo (MCMC) ensemble sampler, we constrain their parameter spaces through background and structure-growth observations. Our analysis employs cumulative combinations of Cosmic Chronometers (CC), the Pantheon+ Type Ia Supernovae compilation (PP), Redshift-Space Distortions (RSD), and the Year-1 Baryon Acoustic Oscillation measurements from the Dark Energy Spectroscopic Instrument (DESI BAO). The inclusion of DESI BAO data produces a noticeable downward shift in the preferred values of the Hubble constant, $H_0$, and present matter density parameter, $Ω_{m0}$. For Model~II, the full analysis yields $H_0=64.02^{+4.13}*{-3.08},\mathrm{km,s^{-1},Mpc^{-1}}$ and $Ω*{m0}=0.249^{+0.047}_{-0.038}$. We further assess the statistical performance of the models relative to $Λ$CDM using $Δ\mathrm{AIC}_c$, $Δ\mathrm{BIC}$, and $Δ\mathrm{DIC}$. For Model~II, the full dataset gives $Δ\mathrm{AIC}_c=4.233$ and $Δ\mathrm{DIC}=5.971$, favoring the $Λ$CDM baseline. At the perturbation level, both models exhibit stable growth histories compatible with large-scale structure observations. The models predict transitions in the late-time expansion dynamics at $z\approx0.5005$ and $z\approx0.6086$ for Models~I and II, respectively, highlighting differences in their cosmological evolution.

physics.gen-ph↗

arXiv2026

Testing $f(Q)$ Gravity with DESI DR2 and Strong-Lensing Time Delays

Symmetric teleparallel gravity provides an alternative description of gravitation in which non-metricity replaces curvature and torsion. Its extension through $f(Q)$ gravity offers a different geometric description of the late-time expansion of the Universe and its accelerated phase. In this work, we investigate two $f(Q)$ models, a normalized power-law model and a square-root exponential model, and test their ability to describe the late-time expansion history. We constrain the model parameters through Markov chain Monte Carlo analyses using Cosmic Chronometer measurements, DESI DR2 baryon acoustic oscillations, strong-lensing time-delay observations, and three Type Ia supernova compilations, Pantheon$^+$, Union 3.0, and DES Y5. We compare both models with the flat $Λ$CDM model using the minimum $χ^2$, Akaike information criterion, and Bayesian information criterion. The square-root exponential model provides a better statistical fit than $Λ$CDM for the combinations of Cosmic Chronometer, DESI DR2, and strong-lensing time-delay data with Pantheon$^+$ and Union 3.0, with improvements in both the goodness of fit and information criteria. The normalized power-law model remains statistically competitive with $Λ$CDM for the supernova-inclusive combinations, although the information criteria do not favor its additional parameter. We also determine the transition redshift from cosmic deceleration to acceleration for both models, obtaining consistent values across the different dataset combinations. The transition redshifts agree with observational estimates of the cosmic acceleration epoch. Overall, our results support $f(Q)$ gravity as a viable alternative to $Λ$CDM for explaining the late-time accelerated expansion of the Universe without requiring a cosmological constant.

gr-qc↗

arXiv2026

Observational Constraints on $f(Q,T)$ Gravity in the Presence of DBI-Essence Scalar Field

We investigate late-time cosmology in extended symmetric teleparallel gravity coupled to a Dirac-Born-Infeld (DBI) scalar field within $f(Q,T)$ gravity, where $Q$ is the non-metricity scalar and $T$ is the trace of the matter energy-momentum tensor. Working on a spatially flat Friedmann-Lemaître-Robertson-Walker background and treating the cosmic medium as an effective perfect fluid, we derive the background field equations for $f(Q,T)+\mathrm{DBI}$ gravity and obtain analytic solutions for the linear choice $f(Q,T)=αQ+βT$. We then constrain the model parameters with a Markov Chain Monte Carlo analysis using Hubble-rate data, DESI BAO (DR2) measurements, and the Pantheon+SHOES Type~Ia supernova sample. The joint posteriors (Tables II and III) are broadly consistent with current late-time constraints and allow a direct comparison with $Λ$CDM, quantifying the departures driven by the $βT$ coupling and the DBI sector. Although the model does not reproduce every observational feature exactly, it provides a statistically viable alternative avenue to the standard paradigm and a useful framework for exploring potential remedies to existing tensions, including the $H_0$ discrepancy, without claiming a definitive resolution.

gr-qc↗

arXiv2026

Cosmic Hysteresis in Reconstructed $f(T)$ Bounce Models A Torsion-Based Thermodynamic Perspective

We investigate the emergence of cosmic hysteresis in cyclic and bouncing cosmologies within the framework of reconstructed $f(T)$ gravity. In contrast to curvature-based modifications of General Relativity, teleparallel gravity attributes gravitation to spacetime torsion encoded in the torsion scalar $T$. By reconstructing viable $f(T)$ functions corresponding to analytically prescribed nonsingular bouncing scale factors and coupling the geometry to a minimally interacting canonical scalar field, we demonstrate that asymmetric scalar field dynamics between expansion and contraction phases give rise to a non-vanishing thermodynamic work integral $\oint p_ϕ\, dV$ over complete cycles. This hysteresis manifests as closed loops in the $(w_ϕ,a)$ plane, signifying thermodynamic memory and irreversibility. We derive the modified Friedmann equations, establish exact bounce and turnaround conditions, and discuss the implications of torsion-induced hysteresis for the cosmological arrow of time. Our results confirm that cosmic hysteresis is a generic feature of cyclic universes in modified gravity, extending beyond curvature-based theories.

gr-qc↗

arXiv2026

A Bayesian Statistical Study of Bianchi Type-I Universe in $f(R,T^ψ)$ Modified Gravity

We have examined the cosmological actions of LRS (Locally Rationally Symmetric) Bianchi type-I universe model in $f(R,T^ψ)$ gravity. For this, we have estimated the Hubble parameter, the effective equation of state parameter ($ω^{eff}$), and the potential of the scalar field as a function of time using equation $H = W(ψ)$. The graphical representation of the potential function $V(ψ)$ with respect to cosmic time t is described. This study explores the dynamical properties of a Bianchi Type-I universe by utilizing Bayesian statistical techniques to constrain the model parameters and evaluate the viability of anisotropic cosmology under extended matter-geometry couplings. Also, we have applied the Markov Chain Monte Carlo (MCMC) mechanism on the derived $H(z)$ model by using observational Hubble data (OHD), the Baryon Acoustic Oscillation (BAO) dataset, and the Pantheon dataset. From the confidence-level contours and best-fit parameter values obtained, along with the corresponding reduced $χ^{2}$, it is evident that the model aligns strongly with observational data, demonstrating statistical stability and consistency in describing late-time cosmic acceleration. Likewise, the error analyses presented in this research, including a comparison between the $Λ$CDM cosmology and the reconstructed $H(z)$ model, confirm the model's compatibility with current observations by yielding a reliable and accurate account of the universe's expansion history.

gr-qc↗

arXiv2025

Statistical Constraints on Anisotropic Bianchi-III Cosmology in $f(R,T)$-Gravity Using MCMC Methods

Anisotropic Bianchi type-III cosmology is examined within the framework of f(R,T) gravity, where R denotes the Ricci scalar and T the trace of the energy-momentum tensor. In this work, we investigate the statistical constraints on anisotropic Bianchi type-III cosmology within the framework of f(R,T) gravity. The specific choice $f(R,T)=R+2f(T)$ is considered and exact solutions are derived for the background dynamics of the model. The physical parameters, such as the Hubble parameter H(z), spatial volume V(z), energy density $ρ(z)$, and pressure p(z), are derived and their evolutionary behaviors are analyzed. To examine the observational viability of the model, we employ Markov Chain Monte Carlo (MCMC) methods and perform a comprehensive statistical analysis using the latest observational datasets, including the Hubble parameter measurements, Baryon Acoustic Oscillations (BAO), and the Pantheon compilation of type Ia supernovae. The combined data analysis provides constraints on the free parameters of the model and allows a comparison with the standard $Λ$CDM cosmology. Our results show that the anisotropic Bianchi-III universe in f(R,T) gravity can successfully accommodate current observational data, offering new insights into the role of matter-geometry coupling in the late-time cosmic acceleration.

gr-qc↗

arXiv2025

Viscous Fluid Models of Cosmic Acceleration in FRW Spacetime Using MCMC Constraints

This study combines theoretical advancements with observational limitations to investigate the cosmological implications of a bulk viscous modified Chaplygin gas (MCG) in a Friedmann--Robertson--Walker (FRW) in (3+1) dimensional spacetime framework. We provide analytical solutions for both viscous and non-viscous cases, pointing out variations in the energy density evolution, the Hubble parameter dynamics, and the deceleration parameter transitions. Bulk viscosity suppresses oscillations in structure creation, a well-known drawback of Chaplygin gas models in larger dimensions, as shown by a thorough perturbation analysis. Using the bulk viscosity coefficient and Hubble expansion parameter, which are incorporated by the total pressure and the appropriate pressure and by using energy momentum conservation law determined time time-dependent density. With the help of three conditions ($ξ= 0$, $ξ\neq0$, and we neglect both bulk viscosity and presence of Chaplygin gas, i.e $A=0$ and $ξ=0$) created three different models as the Hubble parameter is a function of redshift $z$. By applying the MCMC method to these models, we have gone through observational analysis by using the Hubble and BAO datasets.

gr-qc↗

arXiv2025

Large scale structure constraints and matter power spectrum in $f (Q,\mathcal{L}_{m})$ gravity

In the present work, we take into account the dynamical system analysis to investigate the matter power spectrum within the framework of the $f(Q,\mathcal{L}_{m})$ gravitational theory. After obtaining autonomous dynamical system variables for two different particular pedagogical choices of $f(Q,\mathcal{L}_{m})$ models (A and B), we derive the full system of perturbation equations using the $1+3$ covariant formalism to study the matter fluctuations. We present and solve the energy density perturbation equations to obtain the energy density contrast, which decays with redshift for both models for a particular choice of model parameters. After obtaining the numerical results of the density contrast, we computed the matter spectra for each model and conducted a comparative analysis with the $Λ$CDM. Furthermore, by employing the Markov Chain Monte Carlo (MCMC) analysis,the model parameters were constrained using a combination of different observational data sets to improve the robustness and accuracy of the parameter estimation. Our results indicate that only model A can be compatible with the considered observational data sets.

gr-qc↗

arXiv2025

Testing the Generalized Second Law in $(2+1)$-Dimensional Cosmology: Holographic Entropy Bounds and Observational Constraints

We investigate the validity of the Generalized Second Law (GSL) of thermodynamics in a $(2+1)$-dimensional holographic cosmological model with a negative cosmological constant. Adopting a horizon thermodynamics framework, we examine two prominent entropy bounds, the Fischler--Susskind (FS) bound and the Hubble Entropy (HE) bound, in both expanding and contracting universes, including the effects of quantum entropy corrections. Our theoretical analysis shows that the FS bound is intrinsically incompatible with the GSL in contracting $(2+1)$-dimensional universes, regardless of spatial curvature or exotic matter content, and that this incompatibility persists even when quantum corrections are considered. In contrast, the HE bound is consistent with the GSL in expanding universes under classical conditions and can also be reconciled in certain contracting scenarios when quantum effects are included. To complement the theoretical study, we perform a Markov Chain Monte Carlo (MCMC) analysis using recent Baryon Acoustic Oscillations (BAO), Cosmic Chronometer (CC), and Hubble parameter datasets to constrain the model parameters. The best-fit results reveal good cross-dataset consistency, with the cosmological constant parameter $ψ$ remaining stable across all probes. These findings identify the HE bound as a more robust candidate for holographic constraints in lower-dimensional cosmology, while demonstrating the limitations of the FS bound. Our results not only clarify the status of the GSL in $(2+1)$-dimensional settings but also provide a framework for testing entropy bounds with future high-precision cosmological data.

gr-qc↗

arXiv2025

Constraints on multi-fluid cosmology in modified Gauss-Bonnet gravity models with different observational data sets

In the present work, we incorporate redshift-space distortion measurement to investigate the growth of large scale structure within the framework of multi-fluid cosmology in the context of modified Gauss-Bonnet gravity. Using three different modified Gauss-Bonnet gravity models, we compare the predictions of modified Gauss-Bonnet gravity expansion history-through the Friedmann equation with Hubble and BAO data sets and constrain models parameters. Within the context of multi-fluid cosmology in modified Gauss-Bonnet gravity, we obtain the structure growth equation. This equation is then combined with Sigma_8 to get f_Sigma_8 predictions-which is compared with redshift-space distortion data to constrain models parameters to obtain best-fit values including Sigma_8. This involves performing a Markov Chain Monte Carlo (MCMC) analysis for these specific forms of modified Gauss-Bonnet models.

gr-qc↗

arXiv2025

Cosmic Hysteresis in Reconstructed $f(R)$ Bounce Models: A Thermodynamic Study

We study the emergence of cosmic hysteresis in cyclic bouncing universes within the framework of analytically reconstructed $f(R)$ gravity. Using exact bouncing scale factor solutions of exponential and power-law forms, we reconstruct the corresponding $f(R)$ models and investigate the thermodynamic behavior of a minimally coupled scalar field in these geometries. The pressure evolution during expansion and contraction phases is shown to be asymmetric, leading to a non-vanishing thermodynamic work integral over each cycle, defined by $\oint p_ϕ\, dV$. We identify closed hysteresis loops in the equation-of-state space and quantify the net energy transfer per cycle. Our results reveal that such reconstructed $f(R)$ models generically support irreversible evolution, demonstrating a natural emergence of the thermodynamic arrow of time. These findings provide new insight into the dissipative features of modified gravity and the long-term dynamics of cyclic cosmological scenarios.

gr-qc↗

arXiv2025

Observational constraints on holography in $(2 + 1)$-dimensional cosmology with a generalized equation of state

In this study we explore the cosmic holographic principle, as proposed by Fischler and Susskind~\cite{Fischler}, within the framework of $(2 + 1)$-dimensional cosmological models. A generalized equation of state is employed, given by $p = (ζ- 1)(ρ+ ρ_0)$, where $ζ$ and $ρ_0$ are treated as two free parameters. The analysis confirms the validity of the holographic principle in all flat and open universes. However, for a $(2 + 1)$-dimensional closed universe, we apply the method proposed by Kaloper and Linde~\cite{Kaloper}, and observe that the holographic principle is generally not satisfied. Furthermore, we examine the stability of the proposed model using the Markov chain Monte Carlo (MCMC) method, and estimate the best-fit values for the model parameters based on observational Hubble data sets.

physics.gen-ph↗

arXiv2025

Testing Gauss-Bonnet Gravity with DESI BAO Data

In the present paper, we observationally constrain f (G) gravity at the background level using Type Ia supernovae from the Pantheon Plus (PP) sample, cosmic chronometer (CC) data, and the recent Baryon Acoustic Oscillation (BAO) measurements released by DESI. For the analysis, we consider two combinations of datasets: (i) PP + CC, and (ii) PP + CC + DESI BAO. In both cases, we determine the best-fit parameters by numerically solving the modified Friedmann equations for two distinct f (G) models, namely the power-law and exponential forms. This is achieved through Markov Chain Monte Carlo (MCMC) simulations. To assess the statistical significance of the f (G) models, we employ both the Akaike Information Criterion (AIC) and the Bayesian Information Criterion (BIC). Our results show that both f (G) models are statistically favored over the standard ΛCDM model. Notably, the exponential model exhibits an additional future transition at redshift closer to -0.1, indicating a possible return to a decelerating phase. This distinctive behavior sets it apart from both the power-law model and the ΛCDM scenario, which predict continued acceleration into the future.

gr-qc↗

arXiv2025

Matter power spectrum in a power-law $f(G)$ gravity

Cosmological models based on $f(G)$ gravity are efficient in fitting different observational datasets at both background and perturbation levels. This motivates the current study to take into account dynamical system analysis to investigate the matter power spectrum within the framework of modified Gauss-Bonnet gravity. After defining the dimensionless dynamical system variables for a power-law $f(G)$ model, We derive the full system of equations governing the energy density perturbations for both matter and Gauss-Bonnet fluids using the $1+3$ covariant formalism. After solving the energy density perturbation equations, we compute the matter power spectrum. The importance of studying first order perturbations for the defined $f(G)$ model and the relevance of different initial conditions in computing the matter power spectrum are also stressed. It is reported that matter power spectrum for $f(G)$ gravity, for a particular functional form of $f(G)$ model considered is not scale invariant as the case for General Relativity.

gr-qc↗

arXiv2025

Joint Analysis of Constraints on f(R) Parametrization from Recent Cosmological Observations

In this study, we present constraints on the parameters of three well-known $f(R)$ gravity models, viz. (i) Hu-Sawicki, (ii) Starobinsky, and (iii) ArcTanh by using a joint analysis of recent cosmological observations. We perform analytical approximations for the Hubble parameter, $H(z)$, and cosmological distances in terms of the Hubble constant $(H_0)$, matter density $(Ω_{m0})$, and a deviation parameter $b$ for each model. {Our analysis combines early and late-universe cosmological data from five cosmological observations:} (a) Hubble parameter measurements (Cosmic Chronometers), (b) Type Ia Supernovae (Union 3.0), (c) Baryon Acoustic Oscillations (DESI-2025), (d) Gamma-Ray Bursts (GRBs) and (e) Cosmic Microwave Background (CMB). We first optimize the models using each dataset independently, and subsequently, we perform a comprehensive joint analysis combining all four datasets. Our results show that the Hu-Sawicki and ArcTanh models do not deviate significantly from the $Λ$CDM model at 95% confidence level for individual datasets and remain consistent at 99% confidence level in the joint analysis. In contrast, the Starobinsky model shows a strong deviation and appears as a viable alternative to $Λ$CDM. We also constrain the transition redshift parameter ($z_t$), and check that the obtained value agrees with the values inferred from both early-time measurement (Planck) and late-time data from Type Ia Supernovae. These results support the potential support of $f(R)$ gravity to explain the late-time cosmic acceleration effectively. Finally, a statistical model comparison using $χ^2_{\text{min}}$, AIC, and BIC indicates that all three $f(R)$ models are favored over $Λ$CDM, with the Starobinsky model receiving very strong support.

astro-ph.CO↗

arXiv2025

Observational analysis of bulk viscous modified Chaplygin gas in (2+1)-dimensional universe using MCMC

This paper investigates regarding cosmological implications of a bulk viscous modified Chaplygin gas (MCG) in (2+1)-dimensional Friedmann-Robertson-Walker spacetime, incorporating both theoretical analysis and observational constraints. We derive analytical solutions for both viscous and non-viscous cases, revealing distinct behavior in energy density evolution, Hubble parameter dynamics, and deceleration parameter transitions. A comprehensive perturbation analysis illustrates how bulk viscosity dampens the structure growth oscillations, addressing a key challenge faced by Chaplygin gas models in higher dimensions. Using Markov chain Monte Carlo (MCMC) techniques with Hubble parameter and Pantheon supernova datasets, we impose constraints on our model parameters, obtaining $H_0 = 67.90$ km s$^{-1}$ Mpc$^{-1}$, showing remarkable consistency with Planck $Λ$CDM estimations despite the dimensional reduction. Our findings suggest that lower-dimensional viscous cosmology captures essential features of cosmic evolution while providing valuable theoretical insights into the interplay between dissipative effects and exotic equations of state.

gr-qc↗