arXiv subjects

Majid Karimabadi

Publications and source records attributed to Majid Karimabadi.

5 recordsLinked to original sources

arXiv2026

Analytical Criteria for Black Hole Instability in Non-Minimally Coupled Scalar-Tensor Theories

In this paper, we show that the robustness of black hole stability is not preserved when perturbations are subjected to critical values of the coupling constant in two non-minimally coupled scalar-tensor models, particularly for several regular black holes. Our main result is the derivation of a general near-horizon analytical criterion for the onset of instability, which yields analytical expressions for the critical coupling constants in both coupling models. Numerical time-domain analysis shows that these critical values are coincident with the threshold points of the field perturbation profiles. At the critical coupling, the effective potential of the Regge-Wheeler equation develops an extremum exactly at the location of event horizon. We further show that, in the near-horizon regime, the real parts of the quasi-normal frequencies vanish, giving rise to purely imaginary modes. Finally, we recover the universal area quantization of spherically symmetric black holes at the critical coupling without resorting to the highly damped-mode approximation, and show that the result is independent of the particular non-minimal coupling model.

gr-qc↗

arXiv2026

Scalar field perturbations in Non-commutative Schwarzschild spacetime: Comparative analysis and Upper bound on non-commutativity

This work presents a comparative analysis of the quasi-normal modes and ringdowns of scalar field perturbations in the non-commutative Schwarzschild black hole spacetime, focusing on two distinct non-minimal curvature couplings: in the first, the scalar field is coupled directly to the Ricci scalar of the background geometry, while in the second, its derivatives are coupled to the Einstein tensor. We show that the spectra of frequencies in the two models are nearly identical at the low overtone numbers, in particular for the fundamental modes. Time-domain profiles further reveal that, as the value of the coupling constant increases, the tensor-coupled model exhibits greater stability at low multipolar numbers, whereas the scalar-coupled model becomes more stable at high multipolar numbers. Finally, using the critical values of the coupling constants from the stability condition of the ringdown profiles, we provide a comparable upper bound on the non-commutative parameter.

gr-qc↗

arXiv2021

Quasinormal modes for non-minimally coupled scalar fields in regular black hole spacetimes: Grey-body factors, Area spectrum and Shadow radius

Black hole perturbation theory is a useful approach to study interactions between black holes and fundamental fields. A particular class of black hole solutions arising out of modification of Einstein's general theory of relativity are regular black holes (RBHs) which can be constructed using a nonlinear electrodynamic Lagrangian. Because of their importance, we are interested in studying the behavior of three kinds of such RBHs under perturbations generated by an external field. Indeed, we investigate the quasinormal modes (QNMs) of a massive scalar field propagating near the RBHs which is non-minimally coupled to the Ricci scalar tensor of background geometry. We will attempt to find the low-lying quasinormal frequencies of the perturbations by using WKB approximation. We shall also study the relationship between the QNMs and some characteristic properties of black holes such as grey-body factors, area quantization, and shadow radius.

hep-th↗

arXiv2021

Gravitational measurements in higher dimensions

We attempt to study three significant tests of general relativity in higher dimensions both in commutative and non-commutative spaces. In the context of non-commutative geometry, we will consider a solution of the Einstein equation in higher dimensions with a source given by a static, spherically symmetric Gaussian distribution of mass. The resulting metric would describe a regular or curvature singularity free black hole in higher dimensions. The metric should smoothly interpolate between Schwarzschild geometry at large distance and de-Sitter spacetime at short distance. We will consider gravitational redshift, lensing, and time delay in each sector. It will be shown that, compared to the four-dimensional spacetime, there can be significant modifications due to the presence of extra dimensions and the non-commutative corrected black holes. Finally, we shall attempt to obtain a lower bound on the size of the extra dimensions and on the mass needed to form a black hole in different dimensions.

gr-qc↗

arXiv2020

Non-commutative effects on gravitational measurements

The general theory of relativity is currently the accepted theory of gravity and as such, a large repository of test results has been obtained since its inception in 1915. However, in this paper we only focus on what are considered as the main tests but in non-commutative (NC) geometries. Using the coordinate coherent state formalism, we calculate the gravitational redshift, deflection, and time delay of light, separately, for NC inspired Schwarzschild and Riessner-Nordstrom black holes. We show the NC predictions have different behavior than the general relativity. We also provide an upper bound on the NC parameter by comparing the NC corrections with the accuracies of gravitational measurements in the case of typical primordial black holes produced in the early universe. In this regard, we use observational data for the scale factor $a$ at the end of inflation to obtain physical satisfactory results.

gr-qc↗