arXiv subjects

Anshuman Baruah

Publications and source records attributed to Anshuman Baruah.

7 recordsLinked to original sources

arXiv2025

Quasinormal Modes, Grebody Factors, and Hawking Radiation Sparsity of Black Holes Influenced by a Global Monopole Charge in Kalb-Ramond Gravity

Kalb-Ramond (KR) gravity is an intriguing model incorporating local Lorentz violation, and black hole (BH) solutions are known to exist. In this study, we investigate some crucial aspects of BHs endowed with a global monopole charge in the self-interacting KR field. Specifically, we study the quasinormal modes (QNMs) corresponding to scalar, electromagnetic, and gravitational perturbations; derive rigorous bounds for the greybody factors (GBFs); and examine the sparsity of Hawking radiation. The effects of the model parameters $\ell$ (Lorentz-violating parameter in KR gravity) and $η$ (monopole charge) on these phenomena are elaborated. First, QNMs are evaluated with high precision using the 13\textsuperscript{th}-order Padé-averaged WKB method and cross-examined via time-domain analyses within an acceptable parameter space. The results show that the estimated QNMs are more sensitive to $\ell$; however, both model parameters influence the frequency spectra. The derived bounds on the GBFs aid in further constraining the parameter space. It is shown that $\ell$ and $η$ have a similar effect on the greybody bounds. Furthermore, positive and negative values of $\ell$ have opposing effects in that the bounds are reversed for the two cases. The analyses of the Hawking radiation sparsity highlight the effect of $\ell$, and two scenarios are noted: either the radiation emitted is less sparse than Hawking radiation, or it is more sparse during the evaporation phase. Thus, this work presents a comprehensive account of BHs in KR gravity with a global monopole charge.

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arXiv2023

Traversable wormholes in $f(R)$ gravity sourced by a cloud of strings

Wormhole solutions in General Relativity (GR) require \textit{exotic} matter sources that violate the null energy condition (NEC), and it is well known that higher-order modifications of GR and some alternative matter sources can support wormholes. In this study, we explore the possibility of formulating traversable wormholes in $f(R)$ modified gravity, which is perhaps the most widely discussed modification of GR, with two approaches. First, to investigate the effects of geometrical constraints on the global characteristics, we gauge the $rr$-component of the metric tensor, and employ Padè approximation to check whether a well-constrained \textit{shape function} can be formulated in this manner. We then derive the field equations with a background of string cloud, and numerically analyse the energy conditions, stability, and amount of exotic matter in this space-time. Next, as an alternative source in a simple $f(R)$ gravity model, we use the background cloud of strings to estimate the wormhole shape function, and analyse the relevant properties of the space-time. These results are then compared with those of wormholes threaded by normal matter in the simple $f(R)$ gravity model considered. The results demonstrate that wormholes with NEC violations are feasible; however, the wormhole space-times in the simple $f(R)$ gravity model are unstable.

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arXiv2023

Quasinormal Modes and Bounding Greybody Factors of GUP-corrected Black Holes in Kalb-Ramond Gravity

The vacuum expectation value of the non-minimally coupled Kalb-Ramond (KR) field leads to spontaneous local Lorentz symmetry violation, and static spherically symmetric solutions exist. In this study, we study the quasinormal modes (QNMs) of modified black holes in non--minimally coupled KR gravity. We employ a higher-order Padé averaged WKB method to compute the QNMs for scalar, electromagnetic, and gravitational perturbations. In order to account for quantum corrections, we examine the geometric characteristics of the horizon and QNMs by introducing the generalized uncertainty principle (GUP). Additionally, we shed light on the impact of the Lorentz violating parameters on our findings and estimate QNMs for different perturbations. Further, we estimate bounds on the greybody factors for the modified and GUP-corrected black holes. Our findings reveal the influence of the Lorentz violating parameters in the model on the QNM frequencies and their reliance on the GUP parameters.

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arXiv2023

Spherically symmetric wormholes in General Relativity and modified gravity with a Kalb-Ramond background

Among the several modified/extended gravity paradigms, the concept of antisymmetric connections leading to space-time torsion can be traced back to Cartan. More recently, developments in string theory have suggested the existence of a rank-2 self-interacting tensor field called the Kalb-Ramond field with similar outcomes, the field strength of which can support analytic wormhole-like solutions. However, detailed analyses of the physical properties of interest of such solutions are lacking. In this study, we comprehensively probe the properties of traversable Morris-Thorne like wormhole solutions sourced by the Kalb-Ramond field strength in both General Relativity (GR) and $f(R)$ and $f(R,T)$ modified gravity. We also analyze the coupling of the field strength in GR via a novel non-minimal interaction term in the action. Using suitable parametric constraints in all cases, we evaluate wormhole shape functions, numerically analyze the energy conditions near the throat, check the stability using the generalized Tolman-Oppenheimer-Volkov equation, and demonstrate the possibility of minimum exotic matter by estimating the volume integral quantifier. Our results show the existence of stable wormhole solutions in GR and a simple $f(R,T)$ gravity model, and unstable ones in a power-law type $f(R)$ gravity model.

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arXiv2022

New Wormhole Solutions in a Viable $f(R)$ Gravity Model

Traversable wormhole solutions in General Relativity require exotic matter sources that violate the null energy condition (NEC), and such behavior maybe avoided in modified gravity. In this study, we analyze the energy conditions for static, spherically symmetric traversable Morris-Thorne wormholes in a recently proposed viable $f(R)$ gravity model. We numerically analyze solutions considering both constant and variable redshift functions, and present wormhole space-times respecting the NEC, supported by a phantom energy-like equation of state for the source. Moreover, we analyze the stability of the space-times using the generalized Tolman-Oppenheimer-Volkov equation. We demonstrate the effects of certain parameters in the $f(R)$ model in determining energy condition violations, and establish that stable wormholes can be formulated only at the expense of violating the NEC.

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arXiv2022

Non-commutative Wormholes in $f(R)$ Gravity satisfying the Energy Conditions

Traversable wormholes in General Relativity (GR) require exotic matter sources that violate the null energy condition (NEC), and such behavior may be avoided in modified gravity. Moreover, the concept of non-commutative geometry as a gravitational source can be leveraged both in GR and modified gravity to realize non-trivial space-time configurations. In this study, we use $f(R)$ gravity in conjunction with non-commutative geometry to analyze spherically symmetric traversable Morris-Thorne wormhole solutions from the aspect of energy condition violation, considering both constant and variable red-shift functions. First, we use well constrained metric and model parameters in a viable $f(R)$ gravity model to demonstrate that wormholes respecting the NEC can be obtained with suitable choices of parameters. Additionally, we check the strong and dominant energy conditions to further validate our results. We then leverage non-commutative geometry in the framework of $f(R)$ gravity to show that wormholes respecting the different energy conditions with a phantom-like source can be realized with suitable choices of model parameters. Our comprehensive analyses using well-constrained model parameters show that wormholes satisfying the NEC can be realized in the framework of non-commutative geometry with modified gravity.

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arXiv2019

Traversable Wormholes in Higher Dimensional Theories of Gravity

Wormhole solutions in classical General Relativity are unstable and hence non traversable. Morris and Thorne discovered a traversable wormhole solution that required the energy momentum tensor of matter sources to violate various energy conditions and are out of the purview of the standard model of particle physics. The search for traversable wormhole solutions in Modified Theories of Gravity has been of significant interest in the decades after Morris and Thorne first published their results as such violations may be avoided in such theories. This work comprehensively reviews traversable wormhole solutions in modified theories of gravity with extra dimensions that satisfy the various energy conditions with an in depth look at the matter sources and the various constraints on the parameters of the theory to make the energy momentum of the matter sources respect the energy conditions.

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