arXiv subjects

Rupam Jyoti Borah

Publications and source records attributed to Rupam Jyoti Borah.

5 recordsLinked to original sources

arXiv2026

Perturbative Renormalisation Group Improved Black Hole Solution and its Quasinormal Modes

In this work, we construct a perturbative black hole (BH) solution motivated by renormalization group (RG) improvement and investigate the quasinormal modes (QNMs) of the BH under scalar field perturbations in both Schwarzschild-de Sitter (SdS) and Schwarzschild-anti-de Sitter (SAdS) backgrounds. To compute the QNMs in the SdS spacetime, we employ the 6th-order Padé-averaged WKB approximation method, while for the SAdS background we utilize the direct shooting method. We examine the dependence of the QNM frequencies on the free parameter of the solution. Furthermore, we analyze the time evolution of a scalar field perturbation around the BH and present the corresponding time-domain profiles. The QNMs are also extracted from the time-domain data using the matrix pencil method. Using the extracted QNM frequencies, we reconstruct the waveform and compare it with the original time-domain profile, finding good agreement between the two. The QNM frequencies obtained from the 6th-order Padé-averaged WKB method and the time-domain analysis in the SdS background, as well as those obtained from the direct shooting method and time-domain analysis in the SAdS spacetime, show very good consistency.

gr-qc↗

arXiv2026

Traversable wormholes in $\boldsymbol{f(Q)}$ gravity: Energy conditions, stability and quasinormal modes

We investigate static and spherically symmetric traversable wormhole solutions in the framework of $f(Q)$ gravity by considering a power-law model of the form $f(Q)=γ(-Q)^m$. By adopting an anisotropic matter distribution and imposing an equation of state relating the radial pressure and energy density, we obtain an analytic shape function that satisfies the geometric requirements for a traversable wormhole. The model parameter is constrained to $0<m<1/2$, corresponding to a quintessence-like regime with $-1<ω<-1/3$. The energy conditions are analyzed in detail, showing that violations of the null and weak energy conditions are unavoidable but remain localized near the wormhole throat. The anisotropy parameter is positive throughout the spacetime, indicating that repulsive anisotropic stresses play a key role in sustaining the wormhole. The equilibrium configuration is examined using the generalized Tolman-Oppenheimer-Volkoff (TOV) equation for both zero and logarithmic redshift functions, where a consistent force balance is achieved with anisotropic effects providing the dominant outward support. Dynamical stability is studied through scalar perturbations, leading to a Schrödinger-like wave equation with a single-peak effective potential. The quasinormal modes are computed using the sixth-order WKB method with Padé approximation. The resulting frequencies possess negative imaginary parts, indicating stable damping of perturbations. Time-domain simulations further confirm the stability of the solutions and show good agreement with the WKB results, with small deviations in the damping rates. Thus, these results establish that $f(Q)$ gravity admits traversable wormhole solutions that are geometrically consistent and dynamically stable, with $f(Q)$ gravity effects effectively regulating the required matter content.

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arXiv2026

Black Hole Quasi-Periodic Oscillations in the Presence of Gauss-Bonnet Trace Anomaly

We investigate the effects of the Gauss-Bonnet (GB) gravitational trace anomaly on the circular motion of test particles around black holes (BHs) and its implications for quasi-periodic oscillations (QPOs) in various theoretical models. Beginning with the equations of motion, we study the effects on effective potential, angular momentum, specific energy, and the innermost stable circular orbit (ISCO) induced by the anomaly parameter $α$. The fundamental frequencies are calculated. Moreover, we examine several QPO models, including PR, RP, WD, TD, and ER2-ER4, and study the relationship between the upper and lower QPO frequencies as well as the corresponding resonance radii for frequency ratios of 1:1, 3:2, 4:3, and 5:4. Our results show that increasing $α$ leads to deviations from the Schwarzschild case in both upper and lower QPO frequencies correlations and QPO orbital radii, with model-dependent trends. Further, we constrain the BH parameters using the observational data using MCMC analysis. Finally, we calculate the upper and lower QPO frequencies for a few BH candidates on the basis of the RP model using the constrained parameter values and find a good agreement with the observed results.

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arXiv2025

Quasinormal Modes and Shadows of Black Holes in Infinite Derivative Theory of Gravity

In this work, we study the quasinormal modes (QNMs) and shadow of a Schwarzschild black hole (BH) with higher-order metric corrections, in the framework of the Infinite Derivative theory of Gravity (IDG). We study the effects of corrections to the BH's metric, which arises from the IDG's corrections, on the QNMs and shadow of the BH. We used the 6th-order Padé averaged WKB approximation method to study the QNMs of the BH perturbed by a scalar field. The dependence of the amplitude and damping of QNMs with respect to the free parameters has been analysed. It is found that the BH system becomes unstable for some values of the free parameters. We also studied the time evolution of a scalar field around the BH spacetime. The QNMs have also been calculated from the time profile of the evolution, which show good agreement with the values obtained from the WKB method. The variation of the shadow radius of the BH due to the inclusion of higher-order corrections has been studied. Finally, we constrain the free parameters associated with the correction terms using the data from the Keck and VLTI observation, and we obtain some bounds on the parameters.

gr-qc↗

arXiv2025

Strong Lensing Effect and Quasinormal Modes of Oscillations of Black Holes in $\boldsymbol{f(R,T)}$ Gravity Theory

In this work, we analyze the strong lensing phenomenon and quasinormal modes (QNMs) in the case of black holes (BHs) surrounded by fluids within the framework of $f(R,T)$ gravity, adopting a minimally coupled model of the theory. Our analysis is conducted for three surrounding fields corresponding to three different values of the parameter $ω$ of the equations of state, each representing a unique class of BH solutions. A universal method developed by V.~Bozza is employed for strong lensing analysis and the WKB approximation method to compute the QNMs of oscillation of the BHs. The influences of the model parameters $β$ and $c_2$ on the deflection angle and associated lensing coefficients are analyzed. Our findings on lensing reveal that smaller values of $β$ and $c_2$ cause photon divergence at larger impact parameters as well as the lensing results converge to the Schwarzschild limit. Extending the analysis to the supermassive BH Sgr A*, we examine the outermost Einstein rings, estimate three lensing observables: angular position $\vartheta_{\infty}$, angular separation $s$ and relative magnification $r_\text{mag}$ for the BHs. For a specific values of $β$ and $c_2$, BHs with different field configurations exhibit substantial variations in their observable properties. The variation of amplitude and damping of QNMs with respect to the model parameter $β$ and $c_2$ is analyzed for the BHs. We found that the $β$ parameter has a direct correlation with the amplitude and an inverse relation with the damping of the QNMs, while $c_2$ has direct correlation with amplitude as well as damping. Further, we use the time domain analysis to verify the results and found a good match between the two methods.

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