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Anisur Rahaman

Publications and source records attributed to Anisur Rahaman.

At least 19 recordsLinked to original sources
arXiv2025

Universality of Quasinormal-Mode Shifts from Small Nonlocal Effective Couplings

We investigate perturbative quasinormal-mode (QNM) shifts of black holes arising from fractional, nonlocal modifications to the wave operator. Starting from a scalar master equation corrected by a small fractional Laplacian term $(-Δ)^{s}$ with $0<s<1$, we derive an analytic expression for the complex frequency shift at first order in the nonlocal coupling $\varepsilon$. Evaluation of the fractional operator in both coordinate and momentum representations reveals a universal scaling law $δω/ω\propto \varepsilon/M^{2s}$, largely independent of the field spin, with an additional $\ell^{2s}$ enhancement in the eikonal regime $\ell \gg 1$. Applying the formalism to Schwarzschild, slowly rotating Kerr, Hayward regular, and LQG-corrected black holes, we demonstrate that the leading-order fractional QNM shift is universal, with geometric details entering only through overlap integrals of the mode functions. This universality provides a model-independent signature of nonlocality in strong-gravity ringdown spectra and offers a potential observational window into quantum-gravity-inspired modifications.

gr-qc↗
arXiv2025

Radiative transition of an atom falling into spherically symmetric Lorentz violating black hole background

In this work, we explore the intriguing phenomenon of acceleration radiation exhibited by an atom falling into a black hole, as previously studied in Phys. Rev. Lett. 121, 071301 (2018) . Our investigation focuses on examining the impact of Lorentz violation within the framework of the bumblebee gravity model on this phenomenon. We observe that the excitation probability although acquires Planck-like factor the exponential part of it acquires the Lorentz violation factor dependent frequency. However, equivalence principle is not violated. Then we calculate the horizon brightened acceleration radiation (HBAR) entropy for this black hole geometry. We observed that the HBAR entropy has the form similar to that of Bekenstein-Hawking black hole entropy however it has been observed that it is also influenced by Lorentz violation associated to the Bumblebee theory. Additionally, we note that the Lorentz violation effect and conformal symmetry both affect the transition probabilities of a two-level atomic detector.

gr-qc↗
arXiv2025

An atom in front of Lorentz violating Kalb-Ramond black hole background

We investigate the role of Lorentz violation in the acceleration radiation produced when an atom falls into a Kalb-Ramond (KR) black hole and observe that the amplitude and an exponential (Planck-like) factor are both shaped by the Lorentz-violating parameter, indicating a breach of the equivalence principle and resembling characteristics observed in bumblebee gravity models. We further investigate how Lorentz violation and conformal symmetry work together to determine the thermodynamic behavior of the system and the implications for the equivalence principle by looking at the transition probabilities of a two-level atomic detector interacting with the black hole. These findings provide new information about the interaction of black hole entropy, symmetry breaking, and possible observational probes of novel physics beyond general relativity. The horizon brightening acceleration radiation (HBAR) entropy in the KR black hole spacetime is also calculated in detail. Although the corrections are very different from those in bumblebee gravity, our study demonstrates that even though Lorentz-violating events alter the entropy, it nevertheless maintains a structural resemblance to the ordinary Bekenstein-Hawking entropy.

hep-th↗
arXiv2025

Non-Minimal RT Coupling and its Impact on Inflationary Evolution in f(R, T) Gravity

We examine inflationary models in the $f(R, T)$ gravity framework where we have a conformal constant and an $RT$-mixing term apart from an R term. The RT-mixing term introduces non-minimal coupling between gravity and matter. We consider the exponential SUSY potential $V(\p)=M^4 \lt(1-e^{-ł\p/\mp}\rt)$ and a novel potential $V(\p)=ł\mp^{4-2\a} \p^{2\a} \sin^2\lt(\frac{\b \mp^\a}{\p^\a}\rt)$. With the help of COBE normalization, we constrain values of different parameters and extract the field value at the time of Hubble crossing. The end of inflation is marked by $\tep(\p_i)=1$ where $\p_i$ is the field value at the end of inflation. Equipped with these values, we then move on to calculate values of spectral index $n_s$ and tensor-to-scalar ratio $r$. Our predicted values of $n_s$ and $r$ fall within their observed values from the Planck 2018 survey and BICEP/Keck array measurement for both potential, making them plausible candidates for the inflationary model. We also display the variation of the tensor-to-scalar ratio and spectral index with the coefficient of RT-mixing term for fixed values of e-fold number. There, we find the existence of two local maxima of $n_s$, which occur at a negative and a positive value of $\x$, the coefficient of $RT$-mixing term. Our analysis finds a significant impact of $\x$ on values of observables

gr-qc↗
arXiv2024

Constrain from shadows of $M87^*$ and $Sgr A^*$ and quasiperiodic oscillations of galactic microquasars on a black hole arising from metric-affine bumblebee model

We examine a static spherically symmetric black hole metric that originates from the vacuum solution of the traceless metric-affine bumblebee model in which spontaneous Lorentz symmetry-breaking occurs when the bumblebee fields acquire a non-vanishing vacuum expectation value. A free Lorentz-violating parameter enters into the basic formulation of the metric-affine bumblebee model. In this study, we use observations from the Event Horizon Telescope (EHT) collaboration on $M87^*$ and $SgrA^*$ to analyse the shadow of the black hole and an attempt has been made to constrain that free Lorentz-violating parameter. We also investigate particle motion over time-like geodesics and compute the corresponding epicyclic frequencies. We further constrain the Lorentz-violating parameter by using the reported high-frequency quasi-periodic oscillations (QPOs) of microquasars, offering new insights into its possible impact on astrophysical phenomena.

gr-qc↗
arXiv2024

GUP corrected black holes with cloud of string

We investigate shadows, deflection angle, quasinormal modes (QNMs), and sparsity of Hawking radiation of the Schwarzschild string cloud black hole's solution after applying quantum corrections required by the Generalised Uncertainty Principle (GUP). First, we explore the shadow's behaviour in the presence of a string cloud using three alternative GUP frameworks: linear quadratic GUP (LQGUP), quadratic GUP (QGUP), and linear GUP. We then used the weak field limit approach to determine the effect of the string cloud and GUP parameters on the light deflection angle, with computation based on the Gauss-Bonnet theorem. Next, to compute the quasinormal modes of Schwarzschild string clouds incorporating quantum correction with GUP, we determine the effective potentials generated by perturbing scalar, electromagnetic and fermionic fields, using the sixth-order WKB approach in conjunction with the appropriate numerical analysis. Our investigation indicates that string and linear GUP parameters have distinct and different effects on QNMs. We find that the greybody factor increases due to the presence of string cloud while the linear GUP parameter shows the opposite. We then examine the radiation spectrum and sparsity in the GUP corrected black hole with the cloud of string framework, which provides additional information about the thermal radiation released by black holes. Finally, our inquiries reveal that the influence of the string parameter and the quadratic GUP parameter on various astrophysical observables is comparable, however the impact of the linear GUP parameter is opposite.

gr-qc↗
arXiv2024

Hairy black hole, Fermionic greybody factors, Quasinormal modes, Hawking radiation, Power spectrum and sparsity

A hairy black hole (HBH) emerges due to matter surrounding the Schwarzschild metric when using the Extended Gravitational Decoupling (GD) approach. The fermionic greybody factors (GFs) and quasinormal modes (QNMs) as well as Hawking spectra and sparsity of HBH solutions are investigated. We consider massive and massless spin- 1/2 fermions, along with massless spin- 3/2 fermions. The equations of the effective potential for fermions with different spins are derived in HBH spacetime. Then, the rigorous bound method is used to calculate the fermionic spin- 1/2 and spin- 3/2 GFs. With the time domain integration method at our disposal, we illustrate the impact of additional parameters on the ringdown waveform of the massless fermionic spin -1/2 and spin -3/2 fields and, in turn, on their quasinormal modes. We then delve into investigating the Hawking spectra and sparsity of the radiation emitted by an HBH. Hairy parameters significantly affect the sparsity of Hawking radiation as well. We observe that the total power emitted by the BH increases both with $α$ and $Q$ but decreases with $l_{0}$. Our study conclusively shows the significant impact of the additional parameters on important astrophysical phenomena such as quasinormal modes, Hawking spectra, and sparsity.

gr-qc↗
arXiv2024

Study of quasinormal modes, greybody bounds, and sparsity of Hawking radiation within the metric-affine bumblebee gravity framework

We consider a static and spherically symmetric black hole metric that emerges from the vacuum solution of the traceless metric-affine bumblebee model. Our study focuses on the possible implications of the modifications induced by the model on various astrophysical observables that include quasinormal modes, ringdown waveforms, Hawking radiation spectrum, sparsity of that radiation, and the lifetime of a black hole. We explore the impact of the Lorentz symmetry-breaking parameter $α$ on the quasinormal modes with the help of the $6th$-order WKB method. Our inquisition reveals that the emission frequency and decay rate initially decrease with $α$ and then grow up. As a result, the LSB becomes critically important for maintaining the stability of the system after being exposed to perturbation. The convergence of the WKB method for various orders is also studied here. We then analyze the Hawking temperature, radiation spectrum, and sparsity in this modified gravity framework that provides valuable insights into the thermal radiation emitted by black holes. It points out that the Hawking temperature, the peak of the power spectrum, and the total power emitted initially decreases and then increases with $α$. However, The variation of the sparsity with $α$ follows a reverse trend. Finally, we obtain the analytical expression of the 'lifetime' of black holes and scrutinize the effect of $α$ on it.

gr-qc↗
arXiv2024

Accretion, greybody factor, quasinormal modes, power spectrum, sparsity of Hawking radiation, and weak gravitational lensing of a minimum measurable length inspired Schwarzchild black hole

In this manuscript, we delve into an analytic and numerical probe of shadow with different accretion models, quasinormal modes, Hawking radiation, and gravitational lensing to study observational impacts of quantum effect introduced throughh linear-quadratic GUP(LQG). Our investigation reveals that the shadows of LQG modified black holes are smaller and brighter than Schwarzschild black holes. To examine the impact of the quantum correction on the quasinormal mode, linear-quadratic GUP modified black holes are explored under scalar and electromagnetic field perturbation. Here, linear-quadratic GUP is used to capture quantum corrections. It is observed that the incorporation of quantum correction by linear-quadratic GUP alters the singularity structure of the black hole. To compute the quasinormal modes of this linear-quadratic GUP-inspired quantum-corrected black holes, we compute the effective potential generated under the perturbation of scalar and electromagnetic field, and then we use the sixth-order WKB approach in conjunction with the appropriate numerical analysis. We find that the greybody factor decreases with the GUP parameter $α$ implying that the probability of transmission decreases with the GUP parameter. The total power emitted by LQG modified black hole is found to be greater than that emitted by Schwarzschild black hole. Finally, we study weak gravitational lensing and make a comparison with quadratic GUP and linear GUP modified black holes.

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

Impact of generalized uncertainty principle on the accretion process within the asymptotically safe ambiance

We investigate the impact of quantum gravity on accretion onto a modified Schwarzschild black hole within the context of the generalized uncertainty principle (GUP). The minimal measurable length connected to GUP modifies the Schwarzschild black hole, giving it the capacity to accommodate the correction due to quantum gravity. We look at potential critical point locations and calculate the critical speed of the matter accreting. We determine the temperature and total integrated flux correction at the event horizon for the polytropic matter using the least measurable length conjecture offered by the GUP. We note that quantum gravity has a significant impact on the accretion process Additionally, the quantum gravity regime also maintains an asymptotically safe ambiance.

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

Thermodynamics of a Schwarzschild-like black hole with a minimum observable length and the radiation process of a thin accretion disc around it

We study quantum gravity effects on the thermodynamic character and the radiation process of the thin accretion disks around Schwarzschild-like black hole. The quantum gravity correction is invoked through the framework of generalization of uncertainty which is equivalent to the renormalization group improved quantum gravity and maintain the limit of the asymptotically safe preposition of gravity. It admits a free parameter that encodes the quantum effects on the spacetime geometry. It allows us to study how the thermal properties of the black hole itself and the the accretion around it disk are modified in the quantum regime. We computed explicitly the entropy, temperature, free energy, and enthalpy of the modified black hole and show its variation with with the free parameter that encodes the quantum effects. We explicitly make estimations of quantum correction to the time averaged energy flux, the temperature of the disk, the differential luminosity, and the conversion efficiency of accreting mass into radiation. We observe a conspicuous shifting of the radius of the innermost stable circular orbit (ISCO) toward small values together with an enhancement of the maximum of the values of the average thermal radiation and greater conversion efficiency of accreting mass into radiation compared to the classical gravity scenario.

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

A precisely feasible gauged model of chiral boson with its BRST cohomological perspectives

We find that Siegel type chiral boson with a parameter-dependent Lorentz non-covariant masslike term for the gauge fields to be equivalent to the chiral Schwinger model with one parameter class of Faddeevian anomaly if the model is described in terms of Floreanini-Jackiw type chiral boson. By invoking the Wess-Zunino field gauge-invariant reformulation is made. It has been shown that the gauge-invariant model has the same physical content as its gauge non-invariant ancestor had. The BRST invariant effective action corresponding to this model has also been constructed. All the nilpotent symmetries associated with the BRST symmetry along with the bosonic, ghost, and discrete symmetries have been systematically studied. We establish that the nilpotent charges corresponding to these symmetries resemble the algebra of the de Rham cohomological operators in differential geometry. In the environment of conserved charges associated with the models, we study the Hodge decomposition theorem on the compact manifold.

hep-th↗
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↗