arXiv subjects

Anil Kumar Yadav

Publications and source records attributed to Anil Kumar Yadav.

At least 19 recordsLinked to original sources

arXiv2026

Dark Energy Phenomenology in a $f(R,Σ,T)$ Gravity Framework: $O_m(z)$ Parameterization Approach

This study investigates the cosmological implications of $f(R,Σ,T)$ gravity by reconstructing the Hubble parameter from a logarithmic parameterization of the $O_m(z)$ diagnostic. Our approach offers a model-independent way to probe the nature of dark energy and differentiate it from a cosmological constant. We derive the field equations for $f(R,Σ,T) = R + Σ+ 2πηT$ within the homogeneous, isotropic, and spatially flat Friedmann-Robertson-Walker (FRW) metric. A comprehensive analysis of key physical parameters, including the equation of state (EoS) parameter $ω(z)$, the $(ω-ω')$-plane, the squared sound speed $\vartheta^2$, and various energy conditions (Null, Dominant, Strong), is presented. Our findings reveal a dynamic EoS parameter that consistently remains within the quintessence regime ($-1 < ω< -1/3$), approaching $ω= -1$ in the far future, thereby avoiding phantom behavior and maintaining the weak energy condition. The model successfully reproduces the cosmic transition from deceleration to acceleration, as indicated by the deceleration parameter $q(z)$ crossing zero. While the model aligns well with observational data for cosmic expansion, the analysis of $\vartheta^2$ indicates classical instability, a point requiring further theoretical refinement. Overall, this work demonstrates the viability of $f(R,Σ,T)$ gravity as a framework capable of describing the universe's accelerated expansion, consistent with current cosmological observations, while offering a dynamic alternative to the $Λ$CDM model.

gr-qc↗

arXiv2026

Cosmological Implications of the Gong-Zhang Parameterization in Rastall Gravity: A Deep Learning and Observational Study

In this study, we have explored the cosmological dynamics of an isotropic, homogeneous universe in Rastall gravity. For this purpose, we use the parameterization of the EoS parameter in the form $ω(z) = \frac{ω_{0}}{(z+1)} $ to derive the explicit solution of the field equations in Rastall gravity. We constrained the cosmological parameters for the derived model by the Markov Chain Monte Carlo (MCMC) approach utilizing OHD, BAO, and Pantheon plus compilation of SN Ia datasets. We also constrained the model parameters using deep learning techniques and the CoLFI Python package. This paper introduces an innovative deep-learning approach for parameter inference. The deep learning method significantly surpasses the MCMC technique regarding optimal fit values, parameter uncertainties, and relationships among parameters. This conclusion is drawn from a comparative analysis of the two methodologies. Additionally, we determined the transition redshift $z_t = 0.941$, which signifies the shift in the Universe's model from an early deceleration phase to the present acceleration phase. The diagnosis of the model with diagnostic tools like statefinders, jerk parameter, and $O_m$ diagnostics are presented and analyzed. The validation of the model's energy conditions is also examined.

gr-qc↗

arXiv2025

Effective $\Lambda$CDM model emerging from $f(Q,T)$ under a special EOS limit in symmetric cosmology with Bayesian and ANN observational constraints

In this work, we investigate the cosmological consequences of an effective $\Lambda$CDM model emerging from the more general $f(Q,T)$ gravity theory under the special equation-of-state condition $\rho + p = 0$. Under this limit, the field equations yield the constraint $F(Q,T)H(t)=C$, implying that the function $F=f_Q$ becomes purely dependent on the non-metricity scalar $Q$, and the background evolution mimics that of the standard $\Lambda$CDM model. We derive the resulting functional forms of $f(Q)$, obtain the corresponding effective cosmological constant, and analyze the physical nature of this reduction. To test the model against observations, we perform a background-level parameter estimation for $H_{0}$ and $\Omega_{m}$, and evaluate the derived parameter $S_{8}$ (anchored to the Planck baseline $\sigma_8$) using cosmic chronometers (CC), baryon acoustic oscillations (BAO), and Pantheon+ SN Ia datasets. A dual-pipeline analysis is carried out using conventional Bayesian Markov Chain Monte Carlo (MCMC) sampling alongside a machine-learning based Artificial Neural Network (ANN) emulator deployed as a complementary consistency check. We demonstrate that the ANN approach successfully emulates the continuous parameter space with high computational efficiency, yielding results in close agreement with the standard MCMC likelihood framework. Because the background evolution strictly mimics the standard $\Lambda$CDM paradigm, the model successfully reproduces recent observational trends and recovers standard parameter constraints without dynamically resolving the baseline $H_{0}$ and $S_{8}$ tensions. Our results indicate that effective $\Lambda$CDM scenarios derived from $f(Q,T)$ gravity provide a viable and consistent with recent observational data.

gr-qc↗

arXiv2025

Observational constraints using Bayesian statistics and Deep Learning in $f(Q)$ gravity

This study investigates the evolution of Friedmann-Robertson-Walker (FRW) cosmological models within the $f(Q)$ gravity framework, utilizing a specific $f(Q)$ formulation and a novel Hubble parameter $H(z)$ parameterization to probe the universe's accelerating expansion. A central aspect is the application of advanced machine learning techniques for cosmological parameter estimation, alongside comparisons with traditional Bayesian (MCMC) methods. We employ a hybrid Mixed Neural Network (MNN), which synergistically combines Artificial Neural Networks (ANNs) and Mixture Density Networks (MDNs), to enhance the accuracy and robustness of parameter constraints. This MNN architecture is integrated into the CoLFI (Cosmological Likelihood-Free Inference) framework. CoLFI facilitates likelihood-free inference, a significant methodological advancement that provides an efficient and robust alternative, particularly for complex models with computationally expensive or intractable likelihood functions. Training efficiency for the neural networks is optimized by generating data via hyperellipsoid sampling. The $f(Q)$ model, constrained using these diverse approaches, successfully describes a universe transitioning from an early decelerating phase to the current accelerated expansion, with a computed transition redshift of $z_t = 0.60$. The physical and kinematic properties of the model are discussed, underscoring the efficacy of the MNN-CoLFI methodology and its consistency with MCMC results, while highlighting its advantages for obtaining observational constraints in $f(Q)$ gravity.

gr-qc↗

arXiv2025

The CosmoVerse White Paper: Addressing observational tensions in cosmology with systematics and fundamental physics

The standard model of cosmology has provided a good phenomenological description of a wide range of observations both at astrophysical and cosmological scales for several decades. This concordance model is constructed by a universal cosmological constant and supported by a matter sector described by the standard model of particle physics and a cold dark matter contribution, as well as very early-time inflationary physics, and underpinned by gravitation through general relativity. There have always been open questions about the soundness of the foundations of the standard model. However, recent years have shown that there may also be questions from the observational sector with the emergence of differences between certain cosmological probes. In this White Paper, we identify the key objectives that need to be addressed over the coming decade together with the core science projects that aim to meet these challenges. These discordances primarily rest on the divergence in the measurement of core cosmological parameters with varying levels of statistical confidence. These possible statistical tensions may be partially accounted for by systematics in various measurements or cosmological probes but there is also a growing indication of potential new physics beyond the standard model. After reviewing the principal probes used in the measurement of cosmological parameters, as well as potential systematics, we discuss the most promising array of potential new physics that may be observable in upcoming surveys. We also discuss the growing set of novel data analysis approaches that go beyond traditional methods to test physical models. [Abridged]

astro-ph.CO↗

arXiv2025

Cosmological Analysis of $f(R, Σ, T)$ Gravity with EoS Parameterization

In this paper, we present a comprehensive cosmological analysis within the framework of $f(R, Σ, T)$ gravity, a modified theory that incorporates nonlinear matter-geometry coupling via the inclusion of both the trace of the energy-momentum tensor $T$ and the scalar $Σ= T_{μν}T^{μν}$. We consider a spatially flat Friedmann-Robertson-Walker (FRW) universe and introduce a linear parameterization for the equation of state (EoS) parameter based on the Chevallier-Polarski-Linder (CPL) form, which allows us to explore the dynamical evolution of dark energy without imposing restrictive assumptions. To confront the theoretical model with observations, we utilize the latest Hubble parameter measurements from cosmic chronometers. The model parameters are constrained using Markov Chain Monte Carlo (MCMC) simulations with the \texttt{emcee} package, leading to tight bounds on the parameters. The analysis reveals that the model remains consistent with the $Λ$CDM scenario while allowing mild deviations consistent with observational data. Furthermore, we examine the physical and kinematical features of the model by studying the behavior of the physical parameters. Finally, the calculated age of the universe within this framework is found to be in excellent agreement with Planck 2018 results, highlighting the and viability of $f(R, Σ, T)$ gravity as a candidate for explaining late-time cosmic acceleration.

gr-qc↗

arXiv2024

Constraining anisotropic universe under $f(R,T)$ theory of gravity

We try to find the possibility of a Bianchi V universe in the modified gravitational field theory of $f(R,T)$. We have considered a Lagrangian model in the connection between the trace of the energy-momentum tensor $T$ and the Ricci scalar $R$. In order to solve the field equations a power law for the scaling factor was also considered. To make a comparison of the model parameters with the observational data, we put constraints on the model under the datasets of the Hubble parameter, Baryon Acoustic Oscillations, Pantheon, joint datasets of Hubble parameter + Pantheon, and collective datasets of the Hubble parameter + Baryon Acoustic Oscillations + Pantheon. The outcomes for the Hubble parameter in the present epoch are reasonably acceptable, especially since our estimation of this $H_0$ is remarkably consistent with various recent Planck Collaboration studies that utilize the $Λ$-CDM model.

gr-qc↗

arXiv2024

Constraining hybrid potential scalar field cosmological model in Lyra's geometry with recent observational data

In the current study, we investigate a scalar field cosmological model with Lyra's geometry to explain the present cosmic expansion in a homogeneous and isotropic flat FRW universe. In Einstein's field equations, we presupposed a variable displacement vector as an element of Lyra's geometry. In the context of the conventional theory of gravity, we suggest a suitable parameterization of the scalar field's dark energy density in the hybrid function of redshift $z$, confirming the essential transition behavior of the universe from a decelerating era to the present accelerated scenario. We present constraints on model parameters using the most recent observational data sets from OHD, BAO/CMB, and Pantheon, taking Markov Chain Monte Carlo (MCMC) analysis into account. For the proposed model, the best estimated values of parameters for the combined dataset (OHD, BAO/CMB, and Pantheon) are $ H_0 = 71.15\pm 0.26$ km/s/Mpc, $ Ω_{m0}=0.2625\pm 0.0024$, $ Ω_{\phi0} = 0.676\pm0.038$, $ α=-0.22\pm0.13$, $n = 0.096\pm0.079$, and $k = 0.38\pm0.32$. The model exhibits a flipping nature, and the redshift transition occurs at $z_t = 0.756^{+0.005}_{-0.015}$. The current value of the decelerated parameter for the proposed model is calculated as $q_0 = -0.625^{+0.067}_{-0.085}$ for the combined dataset. Some dynamical properties of the model like energy density ($ρ_ϕ$), scalar field pressure ($p_ϕ$), EoS parameter of scalar field ($ω_ϕ$), and effective EoS parameter ($ω_{eff}$) are analyzed and presented. Further, we have also examined the statefinder diagnosis and jerk parameters of the derived model. The total density parameter for the derived model is found to be unity which is in nice agreement with recent standard findings.

gr-qc↗

arXiv2024

Observation constraints on scalar field cosmological model in Anisotropic universe

In this study, we have explored a scalar field cosmological model in the axially symmetric Bianchi type-I universe. In this study, our aim is to constrain the scalar field dark energy model in an anisotropic background. For this purpose, the explicit solution of the developed field equations for the model is determined and analysed. Constraints on the cosmological model parameters are established utilizing Markov Chain Monte Carlo (MCMC) analysis and using the latest observational data sets of OHD, BAO, and Pantheon. For the combined dataset (OHD, BAO, and Pantheon), the best-fit values of Hubble and density parameters are estimated as $ H_{0} = 71.54\pm 0.28$, $Ω_{m0}=0.2622\pm0.0021$ $Ω_{\phi0} = 0.7331\pm0.0046$, and $Ω_{σ0} = 0.000162\pm0.000063$. The model shows a flipping nature and redshift transition occurs at $z_{t} = 0.6964^{+0.0136}_{-0.0006}$, and the present value of decelerated parameter is computed to be $q_{0} = -0.6964\pm0.028$ for the combined dataset. We have explored characteristics like the universe's age, particle horizon, deceleration parameter, and jerk parameter. The dynamical properties such as energy density $ρ_ϕ$, scalar field pressure $p_ϕ$, and equation of state parameter $ω_ϕ$ are analyzed and presented. We have also described the behavior of the scalar potential $V(ϕ)$ and scalar fields. Furthermore, the authors also described the behavior of energy conditions in scalar-tensor cosmology. The scenario of the present accelerated expansion of the universe is described by the contribution of the scalar field.

gr-qc↗

arXiv2023

Reconstruction of symmteric teleparallel gravity with energy conditions

This research investigates the impact of modified gravity on cosmic scales, focusing on $f(Q)$ cosmology. By applying energy conditions, the study reconstructs various $f(Q)$ models, considering an accelerating Universe, quintessence, and a cosmological constant $Λ$. Using up-to-date observational data, including the Supernova Pantheon sample and cosmic chronometer data, Hubble constants $H_0$ are estimated as $70.37^{+0.84}_{-0.92}$ km/sec/Mpc (from $H(z)$ data) and $70.02^{+0.44}_{-0.25}$ km/sec/Mpc (from pantheon compilation of SN Ia data). The matter energy density parameter ($Ω_{0m}$) is calculated as $0.26^{0.015}_{-0.010}$(OHD) and $0.27^{0.025}_{-0.014}$(SN Ia). Furthermore, as a function of redshift $z$, explicit expressions of $f(Q)$ and the EOS parameter $ω$ are produced, and their graphical analysis describes the late time acceleration of the Universe without the usage of dark energy.

gr-qc↗

arXiv2023

Observational Constraints on generalized dark matter properties in the presence of neutrinos with the final Planck release

In this paper, we investigate an extension of the standard $Λ$CDM model by allowing: a temporal evolution in the equation of state (EoS) of DM via Chevallier-Polarski-Linder parametrization, and the constant non-null sound speed. We also consider the properties of neutrinos, such as the effective neutrino mass and the effective number of neutrino species as free parameters. We derive the constraints on this scenario by using the data from the Planck-2018 cosmic microwave background (CMB), baryonic acoustic oscillation (BAO) measurements, Pantheon+ compilation of Type Ia supernovae (SNe Ia), and some large scale structure (LSS) information from the cosmic shear surveys: Kilo Degree Survey (KiDS)-1000 and Dark Energy Survey (DES). We find constraints on the EoS and sound speed of DM very close to the null value in all the analyses, and thus no significant evidence is found beyond the standard CDM paradigm. In all the analyses, we find the significantly tight upper bounds on the sum of neutrino masses, and significantly lower mean values of $S_8$, which are in agreement with the LSS measurements. Thus, the well-known $S_8$ tension is reconciled in the considered model.

astro-ph.CO↗

arXiv2023

Models of f(Q) gravity with electromagnetic field

There are so many ideas that potentially explain the dark energy phenomenon, current research is focusing on a more in-depth analysis of the potential effects of modified gravity on both local and cosmic scales. In this paper we have investigated some cosmic reconstructions in $f (Q)$ cosmology where $Q$ is the non-metricity corresponding to the evolution background in the Friedmann-Lamatre-Robertson-Walker $(FLRW)$ universe. This allows us to determine how any $FLRW$ cosmology can emerge from a particular $f (Q)$ theory. We employ the reconstruction technique to generate explicit formulations of the $f (Q)$ Lagrangian for several types of matter sources like perfect fluid, dust like fluid, stiff fluid and the binary mixture of two fluids. Furthermore, we computed the field equations and equation of state (EoS) parameter $ω$ for two different reconstructed $f(Q)$ models with the variation of the involved constants, which gives the scenario of accelerating universe, quintessence region and cosmological constant. We also observed that the time dependence of $ω$ admits cosmic acceleration. These new $f(Q)$ gravity inspired models may have an impact on gravitational phenomena at other cosmological scales.

gr-qc↗

arXiv2023

Interacting Dark Sectors in Anisotropic Universe: Observational Constraints and $H_{0}$ Tension

The present study reveals observational constraints on the coupling between dark components of anisotropic Bianchi type I universe. We assume interaction between dark matter and dark energy and split the continuity equation with inclusion of interaction term $Γ$. Two scenarios have been considered (i) when coupling between dark components is constant and (ii) when it is a function of redshift ($z$). Metropolis-Hasting algorithm has been used to perform Monte Carlo Markov Chain (MCMC) analysis by using observational Hubble data obtained from cosmic chronometric (CC) technique, cosmic microwave background (CMB) baryon acoustic oscillation (BAO), Pantheon compilation of Supernovae type Ia (SNIa), their joint combination and a Gaussian prior on the Hubble parameter $H_{0}$. It is obtained that the combination of all databases plus $H_{0}$ prior marginalized over a present dark energy density gives stringent constraints on the current value of coupling as $-0.001<δ<0.041$ in constant coupling model and $-0.042<δ<0.053$ in varying coupling model at 68\% confident level. In general, for both models, we found $ω^{X}\approx -1$ and $δ(δ_{0})\approx 0$ which indicate that still recent data favor uncoupled $Λ$CDM model. Our estimations show that in constant coupling model $(H_{0}=73.9^{+1.5}_{-0.95}, δ=0.023^{+0.017}_{-0.024})$ which naturally leads to consistent value of the Hubble constant. This result is interesting because the previous works show that such a high value of Hubble constant requires the significant value of coupling parameter $δ$. It has been also observed that in the constant coupling model, we do not find any disagreement between the estimated $H_{0}$ and those reported by Hubble space telescope (HST) and large scale structure (LSS) experiments.

astro-ph.CO↗

arXiv2021

Gravitational baryogenesis of cosmological constant dominated universe

In this paper, we have studied the gravitational baryogenesis of isotropic and homogeneous universe in the frame-work of general relativity. We investigate an exact and new solution of Einstein's field equations for FRW metric. Our solution represents a transitioning model of the universe which was expanding in decelerated mode and it transit in accelerated mode after dominance of cosmological constant $Λ$. We observe that gravitational baryogenesis occurs in the derived universe and derived baryon entropy ratio is in good agreement with its observational value.

physics.gen-ph↗

arXiv2021

The simplest parametrization of equation of state parameter in the scalar field Universe

In this paper, we have investigated a scalar field cosmological model of accelerating Universe with the simplest parametrization of equation of state parameter of the scalar field. We used $H(z)$ data, pantheon compilation of SN Ia data and BAO data to constrained the model parameters using $χ^{2}$ minimization technique. We obtain the present values of Hubble constant $H_{0}$ as $66.2^{+1.42}_{-1.34}$, $70.7^{+0.32}_{-0.31}$ and $67.74^{+1.24}_{-1.04}$ for $H(z)$, $H(z)$ + Pantheon and $H(z)$ + BAO respectively. Also, we have estimated the present age of the Universe in derived model $t_{0} = 14.38^{+0.63}_{-0.64}$ for joint $H(z)$ and pantheon compilation of SN Ia data which has only $0.88~σ$ tension with its empirical value obtained in Plank collaboration \cite{Ade/2016}. Moreover, the present values of the deceleration parameter $q_{0}$ come out to be $-0.55^{+0.031}_{-0.038}$, $-0.61^{+0.030}_{-0.021}$ and $-0.627^{+0.022}_{-0.025}$ by bounding the Universe in derived model with $H(z)$, $H(z)$ + Pantheon compilation of SN Ia and $H(z)$ + BAO data sets respectively. We also have performed the state-finder diagnostics to discover the nature of dark energy.

astro-ph.CO↗

arXiv2021

Null Geodesics and QNMs in the field of Regular Black Holes

We analyze the null geodesics of regular black holes. A detailed analysis of geodesic structure both null geodesics and time-like geodesics have been investigated for the said black hole. As an application of null geodecics, we calculate the radius of photon sphere and gravitational bending of light. We also study the shadow of the black hole spacetime. Moreover, we determine the relation between radius of photon sphere~$(r_{ps})$ and the shadow observed by a distance observer. Furthermore, We discus the effect of various parameters on the radius of shadow $R_s$. Also we compute the angle of deflection for the photons as a physical application of null-circular geodesics. We find the relation between null geodesics and quasinormal modes frequency in the eikonal approximation by computing the Lyapunov exponent. It is also shown that~(in the eikonal limit) the quasinormal modes~(QNMs) of black holes are governed by the parameter of null-circular geodesics. The real part of QNMs frequency determines the angular frequency whereas the imaginary part determines the instability time scale of the circular orbit. Next we study the massless scalar perturbations and analyze the effective potential graphically. Massive scalar perturbations also discussed. As an application of time-like geodesics we compute the innermost stable circular orbit~(ISCO) and marginally bound circular orbit~(MBCO) of the regular BHs which are closely related to the black hole accretion disk theory. In the appendix, we calculate the relation between angular frequency and Lyapunov exponent for null-circular geodesics.

gr-qc↗

arXiv2021

Accelerating Universe with binary mixture of bulk viscous fluid and dark energy

In this paper, we have proposed a model of accelerating Universe with binary mixture of bulk viscous fluid and dark energy. and probed the model parameters: present values of Hubble's constant $H_{0}$, Equation of state paper of dark energy $ω_{de}$ and density parameter of dark energy $(Ω_{de})_{0}$ with recent OHD as well as joint Pantheon compilation of SN Ia data and OHD. Using cosmic chronometric technique, we obtain $H_{0} = 69.80 \pm 1.64~km~s^{-1}Mpc^{-1}$ and $70.0258 \pm 1.72~km~s^{-1}Mpc^{-1}$ by restricting our derived model with recent OHD and joint Pantheon compilation SN Ia data and OHD respectively. The age of the Universe in derived model is estimated as $t_{0} = 13.82 \pm 0.33\; Gyrs$. Also, we observe that derived model represents a model of transitioning Universe with transition redshift $z_{t} = 0.7286$. We have constrained the present value of jerk parameter as $j_{0} = 0.969 \pm 0.0075$ with joint OHD and Pantheon data. From this analysis, we observed that the model of the Universe, presented in this paper shows a marginal departure from $Λ$CDM model.

gr-qc↗

arXiv2021

Reexamining RHDE Models in FRW Universe with two IR cut-off with redshift parametrization

In this paper, we have investigated that the gravitational field equations are not compatible with conservation equation in Dixit et al. \textcolor{blue}{[Dixit et al. Euro. Phys. J. Plus \textbf{135}, 831 (2020)]}. Therefore, the expression for equation of state parameter along with the dynamics of $ω_{T}$ - $ω_{T}^{\prime}$ plane does not reflect the actual behaviors of RHDE models in $f(R,T)$ gravity and thus the method and technique given in Dixit et al. \textcolor{blue}{[Dixit et al. Euro. Phys. J. Plus \textbf{135}, 831 (2020)]} represents a fractured way for analyzing RHDE models in $f(R,T)$. We also investigate that the derived Universe is in decelerating phase of expansion for $0 \leq β\leq 1.5$ which is contrary to the result obtained in above targeted paper.

gr-qc↗