arXiv subjects

Nashiba Parbin

Publications and source records attributed to Nashiba Parbin.

5 recordsLinked to original sources

arXiv2024

Investigating the effects of gravitational lensing by Hu-Sawicki $\boldsymbol{f(R)}$ gravity black holes

In this work, gravitational lensing in the weak and strong field limits is investigated for black hole spacetime within the framework of Hu-Sawicki $f(R)$ gravity. We employ the Ishihara et al. approach for weak lensing and adopt Bozza's method for strong lensing to explore the impact of Hu-Sawicki model parameters on lensing phenomenon. The deflection angles are computed and analyzed in both the field limits. Our investigation in the weak as well as the strong lensing reveals that in the case of Hu-Sawicki black holes, photons exhibit divergence at smaller impact parameters for different values of the model parameters compared to the Schwarzschild scenario and the photon experiences negative deflection angle when impact parameter moves towards the larger impact parameter values. Additionally, by calculating strong lensing coefficients we study their behavior with model parameters. The strong lensing key observables associated with the lensing effect viz. the angular position $\vartheta_{\infty}$, angular separation $s$ and relative magnification $r_\text{mag}$ are estimated numerically by extending the analysis to supermassive black holes $\text{SgrA}^*$ and $\text{M87}^*$ and analyzed their behavior concerning the parameters for each black hole. The analysis shows that $\text{SgrA}^*$ demonstrates larger values of $\vartheta_{\infty}$ and $s$ relative to $\text{M87}^*$.

gr-qc↗

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↗

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↗

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.

gr-qc↗