arXiv subjects

Kalyan Bhuyan

Publications and source records attributed to Kalyan Bhuyan.

At least 19 recordsLinked to original sources

arXiv2026

Viability of Big Bang Nucleosynthesis in $f(R,L_m)$ Gravity

We examine the viability of Big Bang Nucleosynthesis (BBN) constraints in $f(R,L_m)$ gravity, where the gravitational Lagrangian is an arbitrary function of the the Ricci scalar $R$ and matter Lagrangian density $L_m$. We derive stringent bounds on the underlying model parameters of four different $f(R,L_m)$ gravity models by examining both the primordial abundances of helium-4 ($^4\text{He}$) and the fractional variations in the neutron--proton freeze out temperature. Our results reveal that $f(R,L_m)$ gravity remains viable under the BBN constraints. With the freeze-out condition offering the most dominat constraint on the parameter space, whereas the bounds from $^4\text{He}$ abundance act as a separate abundance level consistency checks on the modified expansion rate. In summary, the presence of these feasible parameter regions demonstrates that $f(R,L_m)$ gravity models can successfully accommodate primordial element abundances without disrupting standard early Universe dynamics.

astro-ph.CO↗

arXiv2026

Quantum-Corrected Thermodynamics and Phase Structure of AdS Euler-Heisenberg Black Hole

We investigate the thermodynamic behaviour of an AdS black hole arising from nonlinear electrodynamics-corrected gravity, incorporating quantum effects through thermal fluctuations. From the Einstein--Euler--Heisenberg framework, we consider the modified black hole solution and analyse its thermodynamic properties in the extended phase space. Logarithmic and inverse-area corrections to the entropy are obtained, leading to modified expressions for enthalpy, internal energy, Helmholtz free energy and Gibbs free energy. The corrected specific heat exhibits multiple divergences and sign changes, signalling genuine second-order phase transitions and revealing a quantum-stabilized microscopic phase followed by universal macroscopic instability. Our results demonstrate that thermal fluctuations qualitatively restructure the thermodynamic phase space and highlight the dominant role of quantum corrections in governing the black hole stability.

gr-qc↗

arXiv2026

Statistical insights on the decorrelation lengths of solar wind parameters at L1 point under varying conditions

Understanding the spatial coherence of solar wind plasma and magnetic field properties is essential for interpreting multi-spacecraft observations and for characterizing the large-scale structure of heliospheric transients. In this study, we quantify the spatial correlation of six key solar wind parameters - interplanetary magnetic field components, bulk flow speed, proton number density, and the alpha-to-proton abundance ratio - using simultaneous measurements from the ACE and Wind spacecraft as a function of their instantaneous separation distance. The analysis is performed separately for intervals of background solar wind, Interplanetary Coronal Mass Ejections (ICMEs), and Stream Interaction Regions (SIRs). The decay of the Pearson correlation coefficient with distance is modeled using an exponential function to infer characteristic de-correlation length scales. We find that the bulk solar wind speed is the most spatially coherent parameter in all regimes, while plasma composition exhibits the weakest coherence. Magnetic field coherence shows strong dependence on solar wind structure: ICMEs display near-unity correlations and the largest magnetic coherence scales, consistent with organized, flux-rope-like configurations, whereas SIRs exhibit reduced coherence - particularly in the north - south magnetic field component - reflecting compressed and turbulent plasma. The background solar wind exhibits intermediate behavior, with large-scale coherence in bulk plasma properties but shorter coherence lengths in magnetic fluctuations. These results provide a quantitative framework for distinguishing solar wind structures based on their spatial coherence properties and have important implications for multi-point solar wind studies and space weather applications.

astro-ph.SR↗

arXiv2026

Astrophysical Signature and Optical Appearance of Weyl--Corrected Einstein--Maxwell Black Holes

In this work, we investigate the physics of charged black holes modified by Weyl corrections, which emerge from the non-minimal coupling between spacetime curvature and electromagnetism. We begin by revisiting the thermodynamics of these systems, deriving the Hawking temperature, Wald entropy, and heat capacity to examine how the Weyl correction parameter reshapes the landscape of thermal stability and phase transitions. Then, we apply a topological method to classify the thermodynamic states and reveal the impact of the Weyl modifications on this classification. To explore astrophysical signatures, we analyze the null trajectories and shadow cast by photons under two-photon polarization modes, deriving observational constraints on the black hole parameters. Finally, we model the accretion disk around these black holes. By calculating the energy flux, spectral luminosity, and differential luminosity, we show how these corrections leave a detectable trace on the emitted radiation.

gr-qc↗

arXiv2026

Cosmological Realization of Baryon Asymmetry in f(R, G_{\mu\nu}T^{\mu\nu}) Gravity

This work investigates the mechanism of gravitational baryogenesis (GB) under the formalism of f(R, G_{{\mu}{\nu}}T^{{\mu}{\nu}}) gravity, where R denotes the Ricci scalar, G_{{\mu}{\nu}} is the Einstein tensor and T^{{\mu}{\nu}} represents the energy--momentum tensor. f(R, G_{{\mu}{\nu}}T^{{\mu}{\nu}}) model is considered to evaluate the baryon-to-entropy ratio (BnER), which is subsequently compared against the observational limits. The results obtained exhibit compatibility with the estimated matter imbalance. Moreover, the analysis is extended to generalized GB case, resulting in outcomes that closely match empirical bounds. The findings reveal that the f(R, G_{{\mu}{\nu}}T^{{\mu}{\nu}}) formulation yields a viable theoretical setting for explaining the detected matter-antimatter disparity of the universe, highlighting its relevance in early cosmic evolution. To further validate the models, a chi-square ({\chi}^2) analysis of the Hubble parameter, H(z), and distance modulus, {\mu}(z), is performed, confirming their consistency with current cosmological observations. A comparative assessment simultaneously with the {\Lambda}CDM paradigm demonstrates a satisfactory level of agreement between the proposed model and cosmological observations from CC and Pantheon+SH0ES datasets.

gr-qc↗

arXiv2026

From Big Bang Nucleosynthesis to Late-Time Acceleration in $f(Q,L_m)$ Gravity

We perform a comprehensive investigation of the early-to-late time cosmic evolution within the framework of $f(Q,L_m)$ gravity, characterized by a non-minimal coupling between non-metricity and matter. The model is further tested against a combined set of observational data, including DESI DR2 BAO, previous BAO measurements, cosmic chronometers (CC), and gravitational-wave (GW) standard sirens, using a Markov Chain Monte Carlo (MCMC) approach. Further by incorporating the Big Bang Nucleosynthesis (BBN) freeze-out constraint, we place stringent limits on the model parameters, ensuring consistency with early-Universe physics. The resulting constraints exhibit strong agreement with observations, with the model successfully describing the transition from decelerated to accelerated expansion. The evolution of the effective equation-of-state parameter, together with statefinder diagnostics and energy conditions, reveals a quintessence-like nature and confirms the physical viability of the model. Overall, the $f(Q,L_m)$ framework emerges as a viable alternative to $\Lambda$CDM, closely reproducing its predictions while allowing controlled deviations in the expansion history.

gr-qc↗

arXiv2026

Exploring R\'enyi Entropic Cosmology Constrained by DESI DR2 BAO and Complementary Late-Time Observations

We investigate the late-time cosmological viability of R\'enyi entropic cosmology (REC) by confronting its modified Friedmann dynamics with DESI DR2 BAO, cosmic chronometers, gravitational-wave standard sirens, redshift-space distortions, and two independent Type Ia supernova compilations, PantheonPlus and Union3. Unlike earlier works, we obtain a stringent constraint on the R\'enyi parameter $\lambda \sim (3.7\text{--}3.8)\times10^{-124}$, with the resulting value even satisfying recent Big Bang Nucleosynthesis and baryogenesis bounds. Across all observational combinations, REC yields $H_0\simeq68.2$--$68.4$ $km\,s^{-1}\,Mpc^{-1}$ and $\Omega_{m0}\simeq0.291$--$0.295$, with a transition from deceleration to acceleration at $z_{\rm tr}\simeq0.66$--$0.68$. The effective equation of state remains quintessence-like, with $w_{\rm eff}(0)\simeq-0.68$, and does not cross the phantom boundary. REC provides a modest improvement in the minimum $\chi^2$ relative to $\Lambda$CDM, while the AIC, DIC, BIC, and Bayesian evidence indicate that the preference for REC is statistically competitive but not decisive. Conclusively, the results show that R\'enyi entropy can produce a controlled modification of the late-time expansion history while remaining observationally close to $\Lambda$CDM.

gr-qc↗

arXiv2026

Viability of Big Bang Nucleosynthesis in Some Generalized Horizon Entropies

In this work, we investigate the viability of some cosmological models derived from generalized horizon entropies, using Big Bang Nucleosynthesis (BBN) constraints. By analyzing the deviations in the expansion rate, we derive bounds on the model parameters from freeze-out temperature, helium, and deuterium abundances. Our results show that the freeze-out condition provides the most stringent constraint, while helium and deuterium bounds remain consistent across all models. Although lithium constraints are not satisfied, this discrepancy is attributed to the well-known cosmological lithium problem. Furthermore, the parameter values required for late-time cosmic acceleration are found to lie well within the BBN bounds, demonstrating consistency between early- and late-Universe behavior. These results establish the viability of the considered models within the framework of BBN.

gr-qc↗

arXiv2026

Dynamics of Late time cosmology in $f(Q,L_{m})$ Gravity with Constraints from DESI DR2 BAO Data

We investigate late-time cosmology in the context of modified $f(Q,L_m)$ gravity, considering a non-linear model$ f(Q,L_m) = \alpha Q + \beta L_m^n + \lambda$ where, $\alpha$, $\beta$, $\lambda$, and $n$ are some free parameters. The modified Friedmann equations are derived for a barotropic cosmic fluid, and an analytical solution for the Hubble parameter $H(z)$ is obtained. Using the latest DESI DR2 BAO data, previous BAO compilations (P-BAO), and cosmic chronometer (CC) datasets, we constrain the model parameters through a Markov Chain Monte Carlo analysis. Our results show that the model successfully describes the observed late-time cosmic acceleration with slightly tighter constraints from the inclusion of DESI dataset. The present-day Hubble constant is determined as $H_0 \simeq 69.5\ \mathrm{km\ s^{-1}\ Mpc^{-1}}$, while the deceleration parameter confirms accelerated expansion with $q_0 \simeq -0.57$. The transition redshift, where the universe switches from deceleration to acceleration, occurs in the range $z_{\rm tr} \sim 0.56 - 0.77$. Similarly, a smooth and physically consistent transition from a matter-dominated decelerated period at high redshifts to an accelerated phase at late times is revealed by the evolution of $\omega_{eff}(z)$. While statefinder diagnostic shows the model favours a Chaplygin gas like nature for DESI and DESI+CC, whereas the model favours as quintessence dominated evolution for P-BAO+CC in the late time regime. Conclusively, all these results along with the study of the energy conditions and stability analysis showcases the given $f(Q,L_m)$ model offers a viable alternative to GR-based cosmology

gr-qc↗

arXiv2025

Fractional Holographic Dark Energy Driven Reconstruction of $f(Q)$ Gravity and its Cosmological Implications

In order to explain the late-time acceleration of the Universe, we present a reconstructed version of the $f(Q)$ gravity theory in this work, which is inspired by the integrating the fractional holographic dark energy with the Hubble horizon as the infrared cutoff. This reconstructed $f(Q)$ gravity model shows a geometrically motivated dark energy component and naturally recovers General Relativity in the appropriate limit. The free parameters of the model are constrained using the latest DESI BAO data, previous BAO compilations (P-BAO), and cosmic chronometer (CC) datasets through a Markov Chain Monte Carlo (MCMC) analysis. The reconstructed Hubble parameter $H(z)$ exhibits excellent consistency with observational data, with high values of $R^2$ and low values of $\chi^2_{\min}$, AIC, and BIC, confirming the model's strong statistical performance relative to $\Lambda$CDM. With current $q(0) \in [-0.40, -0.32]$ and a transition redshift $z_{\text{tr}} \sim 0.56$--$0.72$, the dynamical diagnostics show a smooth transition from a decelerated to an accelerated phase. While the $Om(z)$ diagnostic exhibits a negative slope, indicating that the model is not $\Lambda$CDM, the effective equation-of-state parameter $\omega_{\text{eff}}(z)$ stays within the quintessence regime ($-1 < \omega_{\text{eff}} < -1/3$). The analysis of classical energy conditions shows that the WEC, DEC, and NEC are satisfied throughout the cosmic evolution, with a violation of the SEC at lower-redshift, which is consistent with late-time acceleration. Linear homogeneous perturbation analysis further confirms the model's dynamical stability. Conclusively, the FHDE-inspired reconstructed $f(Q)$ gravity provides a stable, observationally compatible, and geometrically motivated alternative to $\Lambda$CDM, that successfully describes the late-time cosmic acceleration within the symmetric teleparallel framework.

gr-qc↗

arXiv2025

Viability Constraints on Baryogenesis in f (R, Lm, T) Gravity

Our study explores gravitational baryogenesis in the context of f(R, Lm, T) gravity, where R denotes the Ricci scalar, Lm represents the Lagrangian density of the matter field, and T stands for the metric contraction of T_{mu nu}. We focus on a linear model: f(R, Lm, T) = alpha R + beta Lm + gamma T, and examine the parameter constraints for a successful baryon asymmetry generation in four different eras of the cosmos under the assumption of a power-law cosmic expansion. The computed baryon-to-entropy ratio is found to be consistent with the observed order of asymmetry ratio, 9.42 x 10^-11. Furthermore, the study is extended to the generalized framework of gravitational baryogenesis, where the outcome shows strong agreement with the current observational data. Our findings indicate that the f(R, Lm, T) framework provides a compatible theoretical foundation for producing the observed matter imbalance of the cosmos, thereby emphasizing its potential significance in early-universe cosmology.

gr-qc↗

arXiv2025

Investigating Optical and Ring-Down Gravitational Wave Properties of a Rotating Black Hole in a Dehnen Galactic Dark Matter Halo

We present a comprehensive study of the optical and dynamical properties of a rotating black hole immersed in a Dehnen-type $(1,4,0)$ galactic dark matter halo, modeled by a double power-law density profile commonly used to describe realistic galactic cores. By extending our previous Schwarzschild-Dehnen solution using a modified Newman-Janis algorithm, we construct a Kerr-like axisymmetric spacetime that smoothly incorporates both black hole rotation and the influence of the surrounding dark matter halo. We systematically investigate the effects of the halo parameters-the central density and halo radius-on horizon structure, the shape and extent of the ergoregion, and the null geodesics associated with black hole shadows. Our results show that the presence of a dense or extended halo expands the event horizon and ergoregion, and significantly alters the size and distortion of the black hole shadow. Furthermore, by applying the WKB approximation to scalar field perturbations, we compute the quasinormal mode (QNM) spectra and demonstrate that the frequencies and damping times of ringdown signals are highly sensitive to the halo profile. These results open promising avenues for probing the dark matter environment of astrophysical black holes through black hole imaging and gravitational wave observations.

astro-ph.CO↗

arXiv2025

Constraint on Symmetric Teleparallel Gravity with Different Dark energy Parametrizations from DESI DR2 BAO Data

We investigate the cosmological viability of symmetric teleparallel gravity, specifically the $f(Q)$ gravity model with a power-law form $f(Q) = \alpha Q^n$, in combination with two widely used dark energy parameterizations: Chevallier Polarski Linder (CPL) and Barboza Alcaniz (BA). Employing the most recent DESI DR2 Baryon Acoustic Oscillation (BAO) dataset along with previous BAO measurements, we constrain the model parameters through a robust Markov Chain Monte Carlo (MCMC) analysis. We examine the background evolution via key cosmological indicators including the Hubble parameter $H(z)$, deceleration parameter $q(z)$, the effective equation of state $\omega_{\rm eff}(z)$, and the Om diagnostic. Our results indicate that the inclusion of DESI DR2 data significantly tightens constraints on the model parameters and supports a consistent transition from decelerated to accelerated expansion. The present-day values of $q(0)$ and $\omega_{\rm eff}(0)$ lie within the quintessence regime for all datasets. However, for lower redshift values the behaviour varies between phantom-like and quintessence-like phases. Statistical comparison via $\chi^2$, AIC, BIC, and $R^2$ further demonstrate that both CPL + $f(Q)$ and BA + $f(Q)$ provide better or competitive fits to data compared to $\Lambda$CDM, hence offering a compelling geometric alternative for explaining late-time cosmic acceleration.

gr-qc↗

arXiv2025

New Agegraphic Dark Energy Driven Reconstruction of \boldmath{$f(Q)$} Gravity and its Cosmological Implications

In this work, we perform reconstruction of $ f(Q) $ gravity inspired by the New Agegraphic Dark Energy (NADE) model, aiming to account for the Universe's late time acceleration without invoking a cosmological constant. Utilizing a power law scale factor $ a(t) = a_0 t^h $, we derive an analytic form for $ f(Q) $ based on a correspondence with NADE, where the conformal time serves as the infrared cutoff. The resulting model naturally recovers General Relativity in the limit and exhibits a geometrically motivated dark energy component. We constrain the model parameters using recent Baryon Acoustic Oscillation (BAO) data from DESI DR2 BAO and previous BAO observations through the Markov Chain Monte Carlo (MCMC) analysis. The reconstructed Hubble parameter $ H(z) $ demonstrates excellent agreement with observational data, achieving high $ R^2 $ values and low $\chi^2_{\min}$, AIC, and BIC scores, outperforming the standard $ \Lambda $CDM model. Further, we investigate the cosmological evolution using the deceleration parameter $ q(z) $, effective equation of state $ \omega_{\mathrm{eff}}(z) $, and Om diagnostics. The model exhibits a clear transition from deceleration to acceleration with a present value $ q(0) \in \left[-0.5879, -0.3333\right] $ and transition redshift $z_{\mathrm{tr}} \sim 0.5209-0.8126$, while maintaining $ -1 < \omega_{\mathrm{eff}}(z) < -1/3 $, indicating quintessence like behavior. Om diagnostics consistently show a negative slope, further confirming deviation from $ \Lambda $CDM. Energy condition analysis reveals that WEC, DEC, and NEC are satisfied, while SEC is violated only at low redshifts which is consistent with cosmic acceleration. Overall, the reconstructed $ f(Q) $ model provides a viable, observationally consistent, and theoretically motivated alternative to standard dark energy scenarios.

astro-ph.CO↗

arXiv2025

Gravitational Baryogenesis Constraints on Nojiri-Odintsov $f(R)$ Gravity

This paper focuses on exploring the imbalance between matter and antimatter via the gravitational baryogenesis mechanism within the framework of Nojiri-Odintsov $f(R)$ gravity models in a spatially flat FLRW universe, where $R$ is the Ricci curvature scalar. This mechanism is based on $\mathcal{CP}$-violation generated via introducing an interaction between baryonic matter current ($J^\mu$) and the derivative of curvature scalar ($\partial_\mu R$), which finally results in the baryon asymmetry. We examine three distinct $f(R)$ models: (i) $f(R)=\frac{-\alpha}{R^m}+\frac{R}{2k^2}+\beta R^2$, (ii) $f(R)=\alpha R^{\gamma} +\beta R^{\delta}$ and (iii) $f(R)=\frac{\alpha R^{2m}-\beta R^m}{1+\gamma R^m} $. We demonstrate that even during the matter-dominated epoch of the universe, the $f(R)$ gravity models under consideration can yield a non-vanishing matter-antimatter asymmetry. We constrain the model parameters of each model within the scenario of gravitational baryogenesis to produce the observed value of the baryon-to-entropy ratio, providing a continuous set of acceptable physical values for those parameters. Our results show highly consistent agreement to the baryon-to-entropy ratio with the observational bound from the cosmological data.

gr-qc↗

arXiv2025

Dark Matter Surrounded Quartic Square-root Horndeski Black Hole: Thermodynamics, Optical Properties and Quasinormal Oscillations

In this work, we study a special form of Horndeski solutions, viz. quartic "square-root" Horndeski black hole immersed in a perfect-fluid dark-matter halo, by examining its thermodynamics, null geodesic shape, optical shadow, and quasinormal ringdown spectrum. The model is characterized by three parameters, namely $\beta$, $\eta$ (non-minimal Horndeski coupling parameters), and $b$ (perfect fluid dark matter parameter), which collectively determine horizon properties and observational effects. To study thermodynamic stability, we used the specific heat and free energy arguments, with which we demonstrated that small horizon states are locally stable but are never globally preferred. Analytic solutions of null geodesics reveal the radius of the photon sphere and the critical impact parameter, proving that increases in the dark matter parameters and the Horndeski parameter $\beta$ enlarge both the photon sphere and the subsequent shadow, whereas increase in the Horndeski coupling $\eta$ causes a mild diminishing in the shadow radius. Numerical ray tracing verifies these qualitative trends in the apparent shadow. Using the 6$^{th}$ - order WKB approximation method, we also calculate the scalar quasinormal modes and determine that oscillation frequencies and damping rates behave oppositely according to the sign and magnitude of each parameter. By comparing the shadow radius with the recent Event Horizon Telescope constraints on the Sgr A*, we find a narrow window of parameter space that agrees with observed data. In other words, the coupling parameters should be very small. These measurements restrict modified gravity impacts within realistic astrophysical contexts.

gr-qc↗

arXiv2025

Stability of the Einstein Static Universe in Zero-Point Length Cosmology with Topological Defects

Recently, zero-point length cosmology has shown some positive insights into some non-singular aspects of the early Universe. In addition, topological defects are known to play a significant role by its presence as a part of the total energy in the very early Universe. We investigate the stability issue of the Einstein static phase in the emergent scenario of the Universe in a generalized framework of zero-point length cosmology in the presence of topological defects in the very early times. We derive the modified Friedmann equations, where the matter sector includes an extra energy density term arising from $n$-dimensional topological defects. We have studied the possibility of graceful exit of emergent scenario and its stability using dynamical system analysis and against homogeneous scalar perturbation. We also analysed the stability against inhomogeneous density perturbation, vector perturbation and tensor perturbation. Through the stability analysis, it has been shown that the model parameters associated with zero-point length setting and $n$-dimensional topological defects play a visible role in the phase transition process from the ESU to the inflationary regime. Also, interestingly it is found that there exists a mutual interplay between the zero-point length parameter, and the dimension of topological defect on the stability of the ESU on the basis of inhomogeneous density perturbation. Finally, the stability is also tested against vector and tensor perturbation, which shows that the ESU is stable against such perturbations.

gr-qc↗

arXiv2025

Unified Non-Singular Cosmology with Late-Time Acceleration through a Novel Parametrization of Bulk Viscosity Coefficient

This work explores the influence of viscous fluids on cosmological dynamics within the framework of General Relativity. We introduce a novel time-dependent parametrization for the bulk viscosity coefficient, given by $ζ= ζ_0 (t - t_0)^{-2n} ρ^{1/2}$, where $ζ_0$, $t_0$, and $n$ are model parameters. This formulation is designed to investigate whether bulk viscosity of this nature can effectively describe the evolution of the universe, particularly in scenarios that avoid initial singularities through a cosmological bounce. Remarkably, the general solutions emerging from our model exhibit significant flexibility, accommodating not only a bouncing universe but also an early inflationary phase and a late-time acceleration mimicking dark energy. The roles of $ζ_0$, $t_0$, and $n$ are pivotal, as they govern the cosmic evolution and determine the transitions between different phases. To validate the robustness of our model, we analyze key cosmological quantities such as the energy density, deceleration parameter, and the validity of various energy conditions. Furthermore, we employ statefinder diagnostics to probe the dark energy behavior and examine Hubble flow parameters to shed light on the inflationary aspects of the model. Lastly, we confront our theoretical predictions with observational data sets, including the BAO, DESI and Pantheon+SH0ES datasets, demonstrating the model's consistency with empirical cosmological trends.

gr-qc↗