arXiv subjects

Sohan Kumar Jha

Publications and source records attributed to Sohan Kumar Jha.

30 records · Page 2Linked to original sources

arXiv2023

Superradiance scattering of scalar, electromagnetic, and gravitational fields and thin accretion disk around non-commutating Kerr black hole

We consider the non-commutative(NC) Kerr black hole where the mass of the central object is smeared over a region of linear size $\sqrt{b}$, $b$ is the strength of the NC character of spacetime. For the spacetime under consideration, we calculate the amplification factor for scalar, electromagnetic, and gravitational fields, and study various properties of a thin accretion disk. The expression for the amplification factor is obtained with the help of the asymptotic matching technique. The amplification factor is then plotted against frequency for various values of the spin $a$ and the NC parameter $b$. We find that though the amplification factor increases with $a$ but decreases with $b$, the cut-off frequency up to which we have amplification increases with $a$ and $b$. We then study the effect of the spin and the NC nature of spacetime on the energy flux, temperature distribution, emission spectrum, energy conversion efficiency, and the radius of the innermost stable circular orbit of a thin accretion disk around the black hole with the help of the steady-state Novikov-Thorne model. Our study reveals that these quantities increase with the spin and the NC parameter. We also find that the disk around the NC Kerr black is hotter and more luminous than that around the Kerr black hole and the NC Schwarzschild black hole. We can conclusively infer from our investigation that the NC nature of spacetime has a significant impact on the superradiance phenomenon as well as on various properties of thin accretion disks.

gr-qc↗

arXiv2023

Superradiant energy extraction from rotating hairy Horndeski black holes

Adopting the manifest of low-frequency and low mass for the scalar perturbation, we perform a semi-classical analysis of the superradiance phenomenon for a rotating hairy Horndeski black hole (BH). For the spacetime under study enriched by the hairy Horndeski parameter $h$, in addition to the mass $M$ and spin $a$, we compute the amplification factor of scalar wave scattering indicating the energy extraction from the BH. We find that due to the addition of the hairy parameter $h$ in the geometry, the superradiance scattering and its frequency range enhance compared to the Kerr BH. This implies that Horndeski's gravity belongs to those alternative theories of gravity that make the amplification factor larger than the Kerr BH so that the energy extraction in its framework is more efficient than general relativity. Calculating the outgoing energy flux measured by an observer at infinity verifies the role of the hairy parameter $h$ in the increase of energy extraction efficiency from the rotating BH. By implementing the BH bomb mechanism, we present an analysis of the superradiant instability of the underlying BH spacetime against massive scalar fields. Our analysis indicates that the hairy Horndeski parameter leaves no imprint on the standard superradiant instability regime.

gr-qc↗

arXiv2022

The inflationary scenario in the $f(R)$ gravity model with a $R^4$ term

We investigate the cosmic inflation scenario of a specific $f(R)$ model that contains more than one higher-order term in $R$. The $f(R)$ considered here has the terms $R^2$, $R^3$, and $R^4$ along with the linear term. A rigorous investigation has been carried out in the presence of these higher-order terms to figure out whether it leads to a physically sensible cosmic inflationary model. We examine in detail, subject to which conditions this $f(R)$ model renders a viable inflationary scenario, and it has been found that the outcomes of our study agree well with the recent PLANCK results.

gr-qc↗

arXiv2022

Superradiance scattering off rotating Simpson-Visser black hole and its shadow in the non-commutative setting

We consider non-commutating Simpson-Visser spacetime and study the superradiance phenomena and the shadow cast by the back hole associated with this modified spacetime. We extensively study the different aspects of the black hole associated with the metric endowed with the corrections linked with non-commutative properties of spacetime. We study the superradiance effect, deviation of shape, size of the ergosphere, and the shadow of black hole in this extended situation and look into their variation taking different values Simpson-Visser parameter $\ell$ and non-commutative parameter $b$. We have made an attempt to constrain the parameter $\ell$ using the data available from the EHT collaboration for $M87^*$ black hole. Our study reveals that black holes are associated with non-commutative Simpson-Visser spacetime may be a suitable candidate for an astrophysical black hole.

gr-qc↗

arXiv2022

Gravitational lensing by the hairy Schwarzschild black hole

In this manuscript, we consider the hairy Schwarzschild black hole that evades the no-hair theorem. The hair is induced by an additional source from surroundings, such as dark matter, that has a constant energy-momentum tensor(EMT). We study the strong gravitational lensing of light in the background of the hairy Schwarzschild black hole. We observe that the lensing coefficient $\overline{a}$ increases with $α$ but decreases with $\ell_0$. The opposite effect is observed for the lensing coefficient $\overline{b}$ and the impact parameter $b_m$. We also notice that the angular position $θ_\infty$ decreases with $α$ but increases with $\ell_0$, whereas the angular separation $s$ increases with $α$ and decreases with $\ell_0$. For all parameters mentioned, we regain their values for the Schwarzschild black hole whenever we put either $α=0$ or $\ell_0=1$. With the help of the Gauss-Bonnet theorem, we briefly describe the weak gravitational lensing in the background of the hairy Schwarzschild black hole.

gr-qc↗

arXiv2022

Lorentz violation and noncommutative effect on superradiance scattering off Kerr-like black hole and on the shadow of it

We consider a Lorentz violating non-commutating Kerr-like spacetime and studied the superradiance effect and the shadow cast by the back hole. We extensively study the different aspects of the black hole associated with a generalized Kerr-like spacetime metric endowed with the corrections licked with Lorentz violation and non-commutativity effect jointly. We investigate the superradiance effect, deviation of shape, and size of the ergosphere, energy emission rate, and black hole shadow in this generalized situation and study their variation taking different admissible values of Lorentz violating parameter $l$ and non-commutative parameter $b$. The admissible range has been determined from the observation of the Event Horizon Telescope (EHT) collaboration concerning $M87_8$ astronomical black hole. We observe that the superradiance phenomena has a crucially depends on the parameter $l$ and $b$ apart from its dependence on $a$ which is linked to the spin of the black hole. We also observe that with the increase in Lorentz violating parameter $l$, the size of the black hole shadow increases, and with the increase in the non-commutative parameter $b$, the size of the black hole decreases. We have made an attempt to constrain parameters $b$ of a non-commutative Kerr-like black hole using the observation available from the EHT collaboration, in the same way, we put constrain on the Lorentz violating parameter $l$. This study shows that black holes associated with non-commutative Kerr-like spacetime may be a suitable candidate for an astrophysical black hole.

gr-qc↗

arXiv2022

Kerr-Sen-like Lorentz violating black holes and Superradiance phenomena

A Kerr-Sen-like black hole solution results from Einstein-bumblebee gravity. It contains a Lorentz violating (LV) parameter that enters when the bumblebee field receives vacuum expectation value through a spontaneously breaking of the symmetry of the classical action. The geometrical structure concerning the singularity of this spacetime is studied with reference to the parameters involved in the Kerr-Sen-like metric. We introduce this Einstein-bumblebee modified gravity to probe the role of spontaneous Lorentz violation on the superradiance scattering phenomena and the instability associated with it. We observe that for the low-frequency range of the scalar wave the superradiance scattering gets enhanced when the Lorentz-violating parameter $\ell$ takes the negative values and it reduces when values of $\ell$ are positive. The study of the black hole bomb issue reveals that for the negative values of $\ell$, the parameter space of the scalar field instability increase prominently, however, for its positive values, it shows a considerable reduction. We also tried to put constraints on the parameters contained in the Kerr-Sen-like black hole by comparing the deformation of the shadow produced by the black hole parameters with the observed deviation from circularity and the angular deviation from the $M87*$ data.

gr-qc↗

arXiv2022

Study of Einstein-bumblebee gravity with Kerr-Sen-like solution in the presence of a dispersive medium

A Kerr-Sen-like black hole solution appears in the Einstein-bumblebee theory of gravity. The solution contains contains a Lorentz violating parameter in an explicit manner. We study the null geodesics in the background of this Kerr-Sen-like black hole surrounded by a dispersive medium like plasma. We investigate the effect of the charge of the black hole, the Lorentz violation parameter, and the plasma parameter on the photon orbits with the evaluation of the effective potential in the presence of both the Lorentz violation parameter and the plasma parameter. We also study the influence of the Lorentz violation parameter and plasma parameter on the emission of energy from the black hole due to thermal radiation. Besides, we compute the angle of deflection of massless particles with weak-field approximation in this generalized situation and examine how it varies with the Lorentz violation parameter in presence of plasma. Constraining the parameters of this Lorentz violating Kerr-Sen-like black hole is also attempted here with the result obtained from the observations of the Event Horizon Telescope (EHT) collaboration.

gr-qc↗

arXiv2020

Bumblebee gravity and particle motion in Snyder noncommutative spacetime structures

A metric with a Lorentz violating parameter is associated with the bumblebee gravity model. We study the motion of a particle in this bumblebee background where the dynamical variables satisfy non-canonical Snyder algebra along with some critical survey on the classical observations in the bumblebee background to see how these would likely differ from Schwarzschild background. It has been found that the perihelion shift acquires a generalized expression with two independent parameters. One of these two is connected with the Lorentz violating factor and the other is involved in the Snyder algebraic formulation. We also observe that the time period of revolution, in general, acquires a Lorentz violating factor in the bumblebee background, however, for the circular orbit, it remains unchanged even in the presence of the Lorentz violating factor in the bumblebee background. The parameters used here can be constrained with the same type of conjecture used earlier.

hep-th↗

arXiv2020

Bumblebee gravity with a Kerr-Sen-like solution and its Shadow

Lorentz-Violating (LV) scenario gets involved through a bumblebee field vector field $B_μ$. A spontaneous symmetry breaking allows the field to acquires a vacuum expectation value that generates LV into the system. A Kerr-Sen-like solution has been found out starting from the generalized form of a radiating stationery axially symmetric black-hole metric. We compute the effective potential offered by the null geodesics in the bumblebee rotating black-hole spacetime. The shadow has been sketched for different variations of the parameters involved in the system. A careful investigation has been carried out to study how the shadow gets affected when Lorentz violation enters into the picture. The emission rate of radiation has also been studied and how it varies with the LV parameter $\ell$ is studied scrupulously.

gr-qc↗

arXiv2020

On the reformulation of Thomas-Fermi model to make it compatible to the Planck-scale

Thomas-Fermi model is considered here to make it cogent to capture the Planck-scale effect with the use of a generalization of uncertainty relation. Here generalization contains both linear and quadratic terms of momentum. We first reformulate the Thomas-Fermi model for the non-relativistic case. It has been shown that it can also be reformulated for taking into account the relativistic effect. Dialectic screening for the non-relativistic cases has been studied and the expression of screening length has been found out explicitly.

physics.gen-ph↗

arXiv2020

Planck-scale effect through a new MDR

In order to the expected Planck-scale correction in the physical systems we have put forwarded a novel modified dispersion relation (MDR). It has a generalized structure. A specific choice of the function used in the construction of this MDR, it has Lorentz invariance. A toy model like relativistic harmonic oscillator has been studied to get the necessary Planck-scale correction. It has been found that each laves of harmonic oscillator acquires Planck-scale correction and the result agrees with negligible deviation with the result obtained for this system for the same purpose using generalized uncertainty relation (GUP). The relativistic Hydrogen atom problem has also been studied with this MDR and it is found that like harmonic oscillator each energy level of the Hydrogen atom too has got Planck-scale correction.

physics.gen-ph↗