arXiv subjects

Umananda Dev Goswami

Publications and source records attributed to Umananda Dev Goswami.

At least 55 records · Page 3Linked to original sources

arXiv2023

Deflection angle, quasinormal modes and optical properties of a de Sitter black hole in $f(\mathcal{T}, \mathcal{B})$ gravity

The current study aims to examine the impact of the boundary term on the bending angle of light for a static spherically symmetric black hole in the modified gravity described by the $f(\mathcal{T}, \mathcal{B})$ function. To accomplish this objective, we employ the Ishihara \textit{et al.}~method, which enables us to compute the deflection angle of light for a receiver and source situated at finite distances from a lens object in a non-asymptotically flat spacetime. This method considers the receiver's viewpoint, and the resulting deflection angle diverges as the distance from the lens object increases, owing to the non-asymptotically flat spacetime. Nevertheless, the divergence can be regulated by the boundary term parameter $c_0$. For lower values of the parameter $c_0$, the divergence can be minimized within the finite range of the observer and source. Furthermore, we calculate the quasinormal modes of massless scalar perturbations in the black hole's background using the asymptotic iteration method (AIM) and Padé averaged sixth-order Wentzel-Kramers-Brillouin (WKB) approximation method. Our findings indicate that the real quasinormal modes and damping rates are significantly impacted by the model parameter $c_0$. Subsequently, we investigate two optical characteristics of the black hole, namely the shadow and the emission rate. Our results show that with an increase in the boundary term parameter $c_0$, the shadow's size increases, and the evaporation rate decreases.

gr-qc↗

arXiv2023

Weak gravitational lensing and shadow cast by rotating black holes in axionic Chern-Simons theory

We investigate the impact of the axionic coupling parameter on the bending angle of light and the shadow cast by slowly rotating black holes in Chern-Simons modified gravity. We utilize the Ishihara \etal method to derive the deflection angle of light for an observer and source located at finite distances from a lens object in an asymptotically flat spacetime, using the Gauss-Bonnet theorem. The deflection angle exhibits an increasing trend up to a certain point, followed by a decrease as a function of the impact parameter, with the presence of the axion matter field causing the observed increase. Additionally, we calculate the Einstein ring radius as a direct application of the weak deflection angle. We also investigate the effect of the axion matter field on the time delay of light and analyze its impact on the shadow cast by slowly rotating black holes. Our findings reveal a significant effect of the axionic coupling parameter on the black hole's shadow.

gr-qc↗

arXiv2023

Thermodynamics and Shadows of GUP-corrected Black Holes with Topological Defects in Bumblebee Gravity

In this work we investigate a Schwarzschild-type black hole that is corrected by the Generalized Uncertainty Principle (GUP) and possesses topological defects within the framework of Bumblebee gravity. Our focus is on the thermodynamic characteristics of the black hole, such as temperature, entropy and heat capacity, which vary as functions of the horizon radius, and also on shadow as an optical feature. Our investigation reveals significant changes in the thermodynamic behavior of the black hole due to violations of Lorentz symmetry, GUP corrections, and the presence of monopoles. However, the shadow of the black hole is unaffected by violations of Lorentz symmetry. In addition, we provide a limit on the parameters of Lorentz symmetry violation, GUP and topological defects based on a classical test involving the precession of planetary orbits and the advancement of perihelion in the solar system.

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arXiv2023

Anisotropic LRS-BI Universe with $f(Q)$ gravity theory

The possible anisotropic nature in the early phases of the Universe is one of the interesting aspects of study in cosmology. We investigate the evolution of the Universe in terms of few cosmological parameters considering an anisotropic locally rotationally symmetric (LRS) Bianchi type-I spacetime (LRS-BI) under the $f(Q)$ gravity of symmetric teleparallel theory equivalent to the GR (STEGR). In this study we consider two $f(Q)$ gravity models, viz., $f(Q) = -\,(Q+2Λ)$, a simple model with the cosmological constant $Λ$ and the power law model, $f(Q) = -\, αQ^n$ with two constant parameters $α$ and $n$. Considering a proportionality relation between the directional Hubble parameters with a proportionality constant parameter $λ$ we find a significant contribution of the anisotropic factor in the evolution of the early Universe for both models. The power law model shows the more dominating effect of anisotropy in comparison to the simple model depending on model parameters, especially on the parameter $n$. The power law model also shows some possible effects of anisotropy on the Universe's evolution in the near future. In addition, both models confirm that the anisotropy does not obtain any appreciable signature in the current stage of the Universe. From our analysis we also set rough constraints on our model parameters as $0.5 \le λ\le 1.25$, $0.75 \le α\le 1.5$ and $0.95 \le n \le 1.05$.

gr-qc↗

arXiv2023

Tideless Traversable Wormholes surrounded by cloud of strings in $f(R)$ gravity

We study the tideless traversable wormholes in the $f(R)$ gravity metric formalism. First we consider three shape functions of wormholes and study their viabilities and structures. The connection between the $f(R)$ gravity model and wormhole shape function has been studied and the dependency of the $f(R)$ gravity model with the shape function is shown. We also obtain a wormhole solution in the $f(R)$ gravity Starobinsky model surrounded by a cloud of strings. In this case, the wormhole shape function depends on both the Starobinsky model parameter and the cloud of strings parameter. The structure and height of the wormhole is highly affected by the cloud of strings parameter, while it is less sensitive to the Starobinsky model parameter. The energy conditions have been studied and we found the ranges of the null energy condition violation for all wormhole structures. The quasinormal modes from these wormhole structures for the scalar and Dirac perturbations are studied using higher order WKB approximation methods. The quasinormal modes for the toy shape functions depend highly on the model parameters. In case of the Starobinsky model's wormhole the quasinormal frequencies and the damping rate increase with an increase in the Starobinsky model parameter in scalar perturbation. Whereas in Dirac perturbation, with an increase in the Starobinsky model parameter the quasinormal frequencies decrease and the damping rate increases. The cloud of strings parameter also impacts prominently and differently the quasinormal modes from the wormhole in the Starobinsky model.

gr-qc↗

arXiv2022

Quasinormal modes and thermodynamic properties of GUP-corrected Schwarzschild black hole surrounded by quintessence

We study the Quasinormal Modes (QNMs) of the Schwarzschild black hole surrounded by a quintessence field after implementing the quantum corrections to its solution as required by the Generalised Uncertainty Principle (GUP). We analyse the dependence of the QNMs on the deformation parameters of GUP as well as on the quintessence parameter. In most cases the QNMs show the appreciable dependency on these parameters. For a better idea of the accuracy of calculations of QNMs, we compare the results of the QNMs obtained via Mashhoon method with the 6th order WKB method. A good agreement between these two methods of QNM calculations is seen depending on the different factors. Further, we study the thermodynamic properties of the GUP-corrected Schwarzschild black hole and check for any dependence with the deformation parameters and the quintessence parameter. In particular, we compute the Hawking temperature, heat capacity and entropy for the black hole and analyse the results graphically to show the dependency of the thermodynamic properties on the said parameters. We have seen that the thermodynamic properties of black holes also depend noticeably on the model parameters in most cases. Black hole remnants have been studied and it is shown that the possible existence of remnant radius as well as remnant temperature depends on the deformations introduced. However, it is observed that the GUP-corrected black hole constructed here can not become a remnant.

gr-qc↗

arXiv2022

Gravitational wave echoes from strange stars for various equations of state

The tentative Gravitational Wave Echo (GWE) at a frequency of about $72\,Hz$ has been recently claimed at $4.2σ$ significance level in the GW170817 event. GWEs can be used as a tool to study the characteristics of ultra-compact stellar objects. Considering the final ultra-compact, post-merger object as a strange star, the GWE frequency can be calculated. However, GWEs are observed for only those compact stellar structures whose compactness lies in between 0.33 and 0.44. Alternatively, GWE can be obtained for those compact stars which feature a photon sphere and compactness not crossing the Buchdahl's limit radius $R_{B}=9/4M$. A photon sphere is a surface located at $R=3M$, $R$ being the radius and $M$ is the total mass of the ultra-compact object. Recently using the simplest MIT Bag model Equation of State (EoS) it has been reported that strange stars can produce GWEs with frequencies of tens of kilohertz. In view of this, for a comparative study, we have calculated the respective echo frequencies associated with strange stars by considering three models of strange star EoSs, viz., MIT bag model, linear and polytropic EoSs \cite{JB}. We found that, not being too stiff the polytropic EoS can not emit GWE, whereas the MIT Bag model and the linear EoSs can emit GWEs at a frequency range of about tens of kilohertz. Also, GWE frequency increases with the increase in values of bag constant $B$ and decreases with the increasing values of linear constant $b$. So a model-dependent nature of GWE frequencies is observed.

gr-qc↗

arXiv2022

Gravitational wave echoes from compact stars in $f(\mathcal{R},T)$ gravity

We have calculated the static and spherically symmetric solutions for compact stars in the $f(\mathcal{R},T)$ gravity metric formalism. To describe the matter of compact stars, we have used the MIT Bag model equation of state (EoS) and the color-flavor-locked (CFL) EoS. Solving the hydrostatic equilibrium equations i.e., the modified TOV equations in $f(\mathcal{R},T)$ gravity, we have obtained different stellar models. The mass-radius profiles for such stars are eventually discussed. The stability of these configurations are then analysed using different parameters. From the obtained solutions of TOV equations for mass and radius, we have checked the compactness of such objects. It is found that similar to the unrealistic EoS, like the stiffer form of the MIT Bag model, under some considerations the realistic interacting quark matter CFL EoS can give stellar structures which are compact enough to possess a photon sphere outside the stellar boundary and hence can echo GWs. The obtained echo frequencies are found to lie in the range of 39-55 kHz. Also we have shown that for different parametrizations of the gravity theory, the structure of stars and also the echo frequencies differ significantly. Moreover, we have constrained the pairing constant value $β$ from the perspective of emission of echo frequencies. For the stiffer MIT Bag model $β\geq-2.474$ and for the CFL phase with massless quark condition $β\geq-0.873$, whereas for the massive case $β\geq-0.813$.

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arXiv2022

Galactic rotation dynamics in a new $f(\mathcal{R})$ gravity model

We propose to test the viability of the recently introduced $f(\mathcal{R})$ gravity model in the galactic scales. For this purpose we consider test particles moving in stable circular orbits around the galactic center. We study the Palatini approach of $f(\mathcal{R})$ gravity via Weyl transformation, which is the frame transformation from the Jordan frame to the Einstein frame. We derive the expression of rotational velocities of test particles in the new $f(\mathcal{R})$ gravity model. For the observational data of samples of high surface brightness and low surface brightness galaxies, we show that the predicted rotation curves are well fitted with observations, thus implying that this model can explain flat rotation curves of galaxies. We also study an ultra diffuse galaxy, AGC $242019$ which has been claimed in literature to be a dark matter dominated galaxy similar to low surface brightness galaxies with a slowly rising rotation curve. The rotation curve of this galaxy also fits well with the model prediction in our study. Furthermore, we studied the Tully-Fisher relation for the entire sample of galaxies and found that the model prediction shows the consistency with the data.

gr-qc↗

arXiv2022

Quasinormal Modes of Non-Linearly Charged Black Holes surrounded by a Cloud of Strings in Rastall Gravity

We obtain the black hole solution for the Ayón - Beato - García (ABG) type black hole surrounded by a cloud of strings in Rastall gravity and calculate the scalar quasinormal modes of it for a massless scalar field. To have a better visualization of the results, we also introduce a new non-linear electrodynamic source and obtain a black hole solution surrounded by a cloud of strings in the Rastall gravity. We see that the quasinormal modes are affected by the type of non-linear electrodynamic sources in case of higher magnitudes of charge $q$. With increase in the magnitude of charge $q$, gravitational waves decay rapidly for the new black hole solution, while for the ABG black hole, the decay rate increases initially and finally starts to decrease near to $q=1$. We also study the impact of the cloud of strings and other model parameters, including the Rastall parameter on the quasinormal modes for both of the black holes. The gravitational waves decay slowly with increase in the cloud of strings parameter for both of the black holes. Dependency of quasinormal modes on the Rastall parameter is different from a surrounding dark energy field and the decay of gravitational waves may be slow or rapid depending on the value of this parameter.

gr-qc↗

arXiv2022

Strange stars in $f(\mathcal{R})$ gravity Palatini formalism and gravitational wave echoes from them

The compact stars are promising candidates associated with the generation of gravitational waves (GWs). In this work, we study a special type of compact stars known as strange stars in the $f(\mathcal{R})$ gravity Palatini formalism. Here we consider three promising $f(\mathcal{R})$ gravity models viz., Starobinsky, Hu-Sawicki and Gogoi-Goswami models in the domain of MIT Bag model and linear equations of state (EoSs). We compute the stellar structures numerically and constrained the $f(\mathcal{R})$ model parameters with a set of probable strange star candidates. The study shows that the consideration of stiffer MIT Bag model and linear EoSs within a favourable set of $f(\mathcal{R})$ gravity model parameters may result in strange stars with sufficient compactness to produce echoes of GWs. Thus, we have computed the GWs echo frequencies and characteristic echo times for such stars. It is found that in compliance with the experimentally obtained possible strange star candidates, the obtained GW echo frequencies for all the models are in the range of $65 - 85$ kHz.

gr-qc↗

arXiv2022

Bianchi Type I model of universe with customized scale factors

According to standard cosmology, the universe is homogeneous and isotropic at large scales. However, some anisotropies can be observed at the local scale in the universe through various ways. Here we have studied the Bianchi type I model with customizing the scale factors to understand the anisotropic nature of the universe. We have considered two cases with slight modifications of scale factors in different directions in the generalized Bianchi Type I metric equation, and compared the results with the $Λ$CDM model and also with available cosmological observational data. Through this study, we also want to predict the possible degree of anisotropy present in the early universe and its evolution to current time by calculating the value of density parameter for anisotropy ($Ω_σ$) for both low and high redshift (z) along with the possible relative anisotropy that exist among different directions. It is found that there was a significant amount of anisotropy in the early universe and the anisotropic nature of the universe vanishes at the near past and the present epochs. Thus at near past and present stages of the universe there is no effective distinction between this anisotropic model and the standard $Λ$CDM model.

gr-qc↗

arXiv2022

Quasinormal Modes and Hawking Radiation Sparsity of GUP corrected Black Holes in Bumblebee Gravity with Topological Defects

We have obtained the Generalized Uncertainty Principle (GUP) corrected de Sitter and anti-de Sitter black hole solutions in bumblebee gravity with a topological defect. We have calculated the scalar, electromagnetic and gravitational quasinormal modes for the both vanishing and non-vanishing effective cosmological constant using Padé averaged sixth order WKB approximation method. Apart from this, the time evolutions for all three perturbations are studied, and quasinormal modes are calculated using the time domain profile. We found that the first order and second order GUP parameters $α$ and $β$, respectively have opposite impacts on the quasinormal modes. The study also finds that the presence of a global monopole can decrease the quasinormal frequencies and the decay rate significantly. On the other hand, Lorentz symmetry violation has noticeable impacts on the quasinormal frequencies and the decay rate. We have studied the greybody factors, power spectrum and sparsity of the black hole with the vanishing effective cosmological constant for all the three perturbations. The presence of Lorentz symmetry breaking and the GUP parameter $α$ decrease, while other GUP parameter $β$ and the presence of global monopole increase the probability of Hawking radiation to reach the spatial infinity. The presence of Lorentz violation can make the black holes less sparse, while the presence of a global monopole can increase the sparsity of the black holes. Moreover, we have seen that the black hole area quantization rule is modified by the presence of Lorentz symmetry breaking.

gr-qc↗

arXiv2022

Radial oscillations and gravitational wave echoes of strange stars with nonvanishing lambda

We study the effect of the cosmological constant on radial oscillations and gravitational wave echoes (GWEs) of non-rotating strange stars. To depict strange star configurations we used two forms of equations of state (EoSs), viz., the MIT Bag model EoS and the linear EoS. By taking a range of positive and negative values of cosmological constant, the corresponding mass-radius relationships for these stars have been calculated. For this purpose, first we solved the Tolman-Oppenheimer-Volkoff (TOV) equations with a non-zero cosmological constant and then we solved the pressure and radial perturbation equations arising due to radial oscillations. The eigenfrequencies of the fundamental $f$-mode and first 22 pressure $p$-modes are calculated for each of these EoSs. Again considering the remnant of the GW170817 event as a strange star, the echo frequencies emitted by such stars in presence of the cosmological constant are computed. From these numerical calculations, we have inferred relations between cosmological constant and mode frequency, structural parameters, GWE frequencies of strange stars. Our results show that for strange stars, the effective range of cosmological constant is $10^{-15}\, \mbox{cm}^{-2} \le\,Λ\, \le 3\times 10^{-13}\, \mbox{cm}^{-2}$.

gr-qc↗

arXiv2022

Cosmology with a new f(R) gravity model in Palatini formalism

One of the most favourable extensions of General Relativity is the $f(R)$ gravity. $f(R)$ gravity in Palatini formalism can be a realistic alternative to the dark energy problem. In this work we study a recently introduced dark energy $f(R)$ gravity model along with two other models in cosmological perspectives under the Palatini formalism. First, we study the cosmic expansion history of these models with the help of the important cosmographic parameters, such as the Hubble parameter, luminosity distance, effective equation of state etc. This study shows that the new model behaves similarly with the other two models as well as with the $Λ$CDM model in some respects in the early or very early phases of the universe. It could predict the present accelerated expansion of the universe somewhat differently from the other models with a peculiar future history of the universe. Within a constrained range of parameters all models show a good agreement with the Union2.1 luminosity distance data. However, the new model shows a quite satisfactory agreement in the whole range of its allowed parameters than that of the other two models. We also obtain cosmological constraints on these models from the Observed Hubble Data. Further, models have been tested by using $Om(z)$ test and statefinder diagnostics. These diagnostics especially, the statefinder diagnostic shows that the evolutionary differences between these models are distinct. The evolutionary trajectories of the new model are completely different from the other two models we have considered.

gr-qc↗

arXiv2021

Scalarons mimicking Dark Matter in the Hu-Sawicki model of f(R) gravity

In this paper, we conduct a study on the scalar field obtained from $\mathit{f(R)}$ gravity via Weyl transformation of the spacetime metric $g_{μν}$ from the Jordan frame to the Einstein frame. The scalar field is obtained as a result of the modification in the geometrical part of Einstein's field equation of General Relativity. For the Hu-Sawicki model of $\mathit{f(R)}$ gravity, we find the effective potential of the scalar field and calculate its mass. Our study shows that the scalar field (also named as scalaron) obtained from this model has the chameleonic property, i.e.\ the scalaron becomes light in the low-density region while it becomes heavy in the high-density region of matter. Then it is found that the scalaron can be regarded as a dark matter (DM) candidate since the scalaron mass is found to be quite close to the mass of ultralight axions, a prime DM candidate. Thus the scalaron in the Hu-Sawicki model of $\mathit{f(R)}$ gravity behaves as DM. Further, a study on the evolution of the scalaron mass with the redshift is also carried out, which depicts that scalaron becomes light with expansion of the Universe and with different rates at different stages of the Universe.

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arXiv2021

Quasinormal Modes of Black Holes with Non-Linear-Electrodynamic sources in Rastall Gravity

One of the notable modifications of General Relativity (GR) is the Rastall gravity. We have studied the polarization modes of Gravitational Waves (GWs) in this gravity. It is found that at a very far distance away from the source of GWs, Rastall gravity behaves identically to GR. That is, we get only two massless tensor polarization modes of GWs in the theory. Afterwards, we have extended our study to quasinormal modes of black holes in Rastall gravity in presence of non-linear electrodynamic sources. Here the impacts of cosmological field, dust field, phantom field, quintessence field and radiation field on the quasinormal modes in presence of electrodynamic sources have been investigated. Apart from this, we have also checked the dependency of quasinormal modes with the Rastall parameter $λ$, black hole structural constant $N_s$ and charge of the black hole $Q$. The study shows that the quasinormal modes corresponding to the black hole with non-linear electrodynamic sources show significant deviations from a general charged black hole in Rastall gravity under certain conditions. Further, the behaviour of black holes and hence the quasinormal modes depend on the type of surrounding field considered.

gr-qc↗

arXiv2021

Signature of Anomalous Andreev bound states in magnetic Josephson junction of noncentrosymmetric superconductor on a topological insulator

We study the Josephson effect in a clean noncentrosymmetric superconductor/half-metal/noncentrosymmetric superconductor junction, which is grown on the surface of a three-dimensional Topological Insulator (TI) in the ballistic limit. We find the signature of anomalous Andreev Bound States (ABS) and band splitting for a spin-active barrier whose barrier magnetic moment is misaligned with the bulk moment. The chiral Majorana mode and 4$π$ periodic ABS are found to exist on the surface of TI for parallel orientation of the moments in the normal incidence condition. But for anti-parallel misalignment, we observe the 2$π$ periodic ABS. There exist a gap in ABS for oblique incidence. We find the splitting of Andreev levels in the presence of RSOC and also for unequal mixing of singlet-triplet correlations present in NCSC. The Majorana mode, ABS and Josephson supercurrent can be controlled by the ratio of barrier magnetic and non-magnetic moments. The critical current is found to be maximum for singlet or triplet dominated NCSC, while it is minimum in the equal mixing condition. The ABS is found to be barrier thickness dependent and is suppressed for an opaque barrier. We observe a monotonic decay in critical current with a finite length of the junction for all the singlet-triplet mixings and magnetic moments. The current-phase relation is found to be sinusoidal with no phase shift in the half-metallic limit, however, for different orientations of the bulk moment, an anomalous characteristic is also observed.

cond-mat.supr-con↗