arXiv subjects

Mrinnoy M. Gohain

Publications and source records attributed to Mrinnoy M. Gohain.

17 recordsLinked to original sources

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↗

arXiv2025

Dynamics of Geodesics in Non-linear Electrodynamics Corrected Black Hole and Shadows of its Rotating Analogue

We study the nature of particle geodesics around a non-linear electrodynamic black hole (NLED-BH) inspired by the confinement of a heavy quark-antiquark system, which reduces to Maxwell's linear electrodynamics theory at the strong field regime. The corrected BH solution is a special generalisation of the Schwarzschild BH at the linear regime. Such a type of corrected system is parameterised by a charge parameter along with a non-linear electrodynamic term $ζ$. To be specific, we studied the geodesic behaviour of massless null particles through the geodesic equations using the backward ray-tracing method. We also investigated how NLED effects in charged BH spacetimes affect timelike particle orbits, specifically properties like precession frequency and orbital velocity around the NLED BH. Furthermore, to extend the analysis to the rotating case, we used the modified Newman-Janis algorithm to generate the rotating analogue of the NLED BH. We then analysed the ergosphere formation and shadow cast by the rotating analogue of the NLED BH.

gr-qc↗

arXiv2025

Shadows of a Non-Commutative Black Hole under the Influence of a Magnetized Plasma

We investigate the properties of the shadow cast by a black hole (BH) in a non-commutative spacetime surrounded by a plasma medium. BHs in a plasma environment can exhibit complex interactions that profoundly affect their shadow properties. In specific terms, we consider a special type of plasma medium motivated from the interactions among axions, photons and plasma excitations, or commonly termed as axion-plasmon in the presence of a magnetic field. We have studied the effect of these plasma parameters and the non-commutative parameter on the shadow properties by focusing particularly on the photon sphere radius, angular shadow and the shadow observed from the observer's perspective. For this we have considered two different forms of plasma, viz homogeneous and inhomogeneous plasma. In this work we have presented the parameter dependence of these BH shadow properties. Finally, we have also studied the energy emission rate of the BH and how it depends on the model and plasma parameters.

hep-th↗

arXiv2025

Frolov Black Hole Surrounded by Quintessence -- I: Thermodynamics, Geodesics and Shadows

The Frolov black hole (BH) is a charged extension of the Hayward BH, having regularity at the central point $r = 0$ and an asymptotically Schwarzschild form for large values of $r$. Such a BH is parameterized by a length scale parameter, $ α_0 $. In this paper, we analyze the thermodynamic properties, null and timelike geodesics, and shadows of a Frolov BH immersed in a quintessence field. Our results indicate that the smaller BH is locally thermodynamically stable yet globally unstable at all horizon radii. Neither the quintessence parameter nor the other model parameters like the charge $q$ and length scale parameter $α_0$ change this global instability. We extend the study of the null and timelike geodesics to the vicinity of the BH by analyzing how the geodesic motion depends on the model parameters. Finally, we analyze the shadow of the BH system and find that the shadow radii are sensitively dependent on model parameters. In contrast, the influence of the quintessence parameter itself on the size of the shadow is found to be rather weak.

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 $f(Q) = αQ^n$, 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 $ω_{\rm eff}(z)$, 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 $ω_{\rm eff}(0)$ 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 $χ^2$, 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 $Λ$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 $χ^2_{\min}$, AIC, and BIC scores, outperforming the standard $ Λ$CDM model. Further, we investigate the cosmological evolution using the deceleration parameter $ q(z) $, effective equation of state $ ω_{\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 < ω_{\mathrm{eff}}(z) < -1/3 $, indicating quintessence like behavior. Om diagnostics consistently show a negative slope, further confirming deviation from $ Λ$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

Signatures of NED on Quasi periodic Oscillations of a Magnetically Charged Black Hole

In this work, we explore the influence of nonlinear electrodynamics (NED) on the quasi-periodic oscillations (QPOs) of a magnetic charged black hole by analyzing the motion of test particles and their epicyclic frequencies. Starting from the effective potential, angular momentum, and energy of circular orbits, we examine how the NED parameter b alters the orbital dynamics. We find that as b increases, the system transitions smoothly from the RN regime towards the Schwarzschild profile, with observable changes in the innermost stable circular orbit (ISCO) and Keplerian frequencies. We further investigate the variation in the radii of QPOs with respect to the NED parameter b by employing the RP, WD, and ER models. We also perform Markov Chain Monte Carlo (MCMC) analysis using observational QPO data from a diverse set of black hole sources spanning stellar-mass, intermediate-mass, and supermassive regimes. The MCMC results yield consistent constraints on the parameter b across all mass regimes, indicating that NED effects leave a distinguishable signature on the QPO structure of a charged black hole.

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 $β$, $η$ (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 $β$ enlarge both the photon sphere and the subsequent shadow, whereas increase in the Horndeski coupling $η$ 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↗

arXiv2025

Modeling a Non-Singular Universe with Late-Time Acceleration through a Novel Inhomogeneous Barotropic Equation of State

In this study, we investigated the effects of incorporating barotropic fluids on cosmological solutions within the general relativity (GR) framework. We proposed a modified version of the barotropic fluid with the EoS, $p=ζ_0 ρ+ζ_1 ρ\left(t-t_0\right){}^{-2 n}$, where $ζ_0$, $ζ_1$, $t_0$ and $n$ are some constants. Our goal is to explore if this type of EoS might help explain the universe's development, concentrating on the scenario where the universe bounces instead of singularities. Interestingly the generic solutions derived from our model are sufficiently adaptable to illustrate the bounce scenario, cosmic inflation and late-time dark-energy behaviour. The parameters $ζ_0$, $ζ_1$, $t_0$, and $n$ define the universe's phase in this non-singular solution. We investigated several elements of cosmic development, including as the energy density, deceleration parameter, and energy conditions, in order to validate our model. Stability analysis showed that the perturbations approach to zero as the time evolves, indicating the model is stable under scalar perturbation. Additionally, we looked at the statefinder diagnostics and Hubble flow dynamics to get more understanding of the model's dark energy and inflationary behaviour, respectively. Additionally, we conducted a study of the models' relevance to the observational datasets from BAO, DESI and Pantheon+SH0ES.

gr-qc↗

arXiv2025

Frolov Black Hole Surrounded by Quintessence - II: Quasinormal Modes, Greybody Factors, Deflection Angle and Chaos Bound

In this work, we have examined the scalar quasinormal modes, greybody factors, weak gravitational lensing, and the chaos bound of a Frolov black hole surrounded by a quintessence field. We studied the quasinormal frequencies and deduced that an increase in the value of the quintessence parameter $c$ dissipates the oscillatory behavior and thus slow down the decay of perturbations. The greybody factors are highly dependent on the multipole moment $l$ and BH parameters, with a larger $q$ hindering the transmission and smaller $α_0$ making the barrier steeper. The deflection angle of weak lensing under thin lens approximation, monotonically decreases as the value of impact parameter $b$ increases, and is visibly affected by the parameter $c$, and that $α_0$ and $q$ has very minimal effect. Lastly, we investigated the chaos bound behaviour of photon trajectories in the near-horizon neighbourhood, which indicates that it is satisfied for bigger circular photon orbits but broken for smaller ones, with high-angular-momentum photons being chaotic. This violation of the bound points towards the role of quintessence in modifying the thermodynamic features of the BH, with possible adjustments to the standard Bekenstein-Hawking entropy.

gr-qc↗

arXiv2024

Wormhole Formation with Gravitational Particle Creation Mechanism

We explore the possibility of formation of a traversable wormhole in General Relativity supported by particle creation mechanism. The repulsive back-reaction pressure generated through this mechanism can be thought of as a source of sustaining a traversable wormhole. In the first part of this paper, we model a wormhole geometry by assuming an inverse powerlaw variation of particle creation pressure within the wormhole geometry, and the shape function for the wormhole is obtained, assuming a finite redshift function. By stabilizing the wormhole structure, based on the causality of sound-speed we obtained the existence range of the parameter $β$ associated with particle creation mechanism. The general shape function obtained is found to adhere to the feasibility conditions of a viable wormhole geometry. Then we studied the 2D and 3D embedding derived for the obtained shape function. In the second part of the paper, we followed the reverse approach of the aforementioned treatment, where we derived the particle creation pressures by assuming three commonly utilized toy shape-functions. The energy conditions are then investigated for all these cases. In essence, particle creation phenomena inside the wormhole may indeed provide for the possibility of sustenance of stable wormhole structure.

gr-qc↗

arXiv2024

Thermodynamics and Null Geodesics of a Schwarzschild Black Hole Surrounded by a Dehnen Type Dark Matter Halo

In this work, we derive a novel black hole solution surrounded by a Dehnen-(1,4,0) type dark matter halo by embedding a Schwarzschild black hole within the halo, constituting a composite dark matter-black hole system. The thermodynamics of the resulting effective black hole spacetime is then studied with particular attention to the influence of the dark matter parameters on various thermodynamic properties. We examine the specific heat and free energy to assess the thermodynamic stability of the model. Furthermore, the null geodesics and the effective potential of light rays are studied to investigate how the dark matter parameters affect these geodesics and the radii of circular orbits. The stability of circular null geodesics is analyzed using dynamical systems and Lyapunov exponents, which represents the dynamical behaviour of the circular photon orbits. Finally, we studied if the circular geodesics exhibit chaotic behaviour using the chaos-bound condition.

gr-qc↗

arXiv2024

Cosmological Bounce Scenario with a Novel Parametrization of Bulk Viscosity

In this work, we have studied how incorporating viscous fluids leads to exact bounce cosmological solutions in general relativity (GR) framework. Specifically, we propose a novel parameterization of bulk viscosity coefficient of the form $ζ= ζ_0 (t-t_0)^{-2n} H$, where $ζ_0$, $n$ being some positive constants and $t_0$ is the bounce epoch. We investigate how this form of bulk viscosity may assist in explaining the early universe's behaviour, with a particular focus on non-singular bounce scenario by studying the various energy conditions and other related cosmological observables and how the model parameters affect the evolution of the Universe. We demonstrate that the NEC and SEC violation occurs at the bounce point while DEC is satisfied. Finally, we carried out a stability check based on linear order perturbation to the Hubble parameter. We found that the perturbation vanishes asymptotically at later times, which indicates a stable behaviour of the bounce scenario

gr-qc↗

arXiv2024

Emergent Cosmology in 4D Einstein Gauss Bonnet Theory of Gravity

In this paper, in an FLRW background and a perfect fluid equation of state, we explore the possibility of the realization of an emergent scenario in a 4D regularized extension of Einstein-Gauss-Bonnet gravity, with the field equations particularly expressed in terms of scalar-tensor degrees of freedom. By assuming non-zero spatial curvature ($k = \pm 1$), the stability of the Einstein static universe (ESU) and its subsequent exit into the standard inflationary system is tested through different approaches. In terms of dynamical systems, a spatially closed universe rather than an open universe shows appealing behaviour to exhibit a graceful transition from the Einstein static universe to standard cosmological history. We found that under linear homogeneous perturbations, for some constraints imposed on the model parameters, the Einstein static universe is stable under those perturbations. Moreover, it is noted that for a successful graceful transition, the equation of state $ω$ must satisfy the conditions $-1 < ω<0$ and $ω< -1$ for closed and open universes, respectively. Also, under density perturbations, the Einstein static universe is unstable if the fluid satisfies the strong energy condition but is stable if it violates it, for both closed and open universes. Furthermore, the Einstein static universe is seen to be stable under vector perturbations and tensor perturbations, regardless of whether the fluid obeys or violates the SEC.

gr-qc↗