arXiv subjects

Kalyan Bhuyan

Publications and source records attributed to Kalyan Bhuyan.

27 records · Page 2Linked to original sources

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.

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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.

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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.

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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.

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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.

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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.

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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.

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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

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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.

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