arXiv subjects

Anish Das

Publications and source records attributed to Anish Das.

6 recordsLinked to original sources

arXiv2023

Stability, quasinormal modes in a charged black hole in perfect fluid dark matter

In this work, we study time-like and null geodesics in a charged black hole background immersed in perfect fluid dark matter (PFDM). Using the condition for circular geodesics, we evaluate the energy ($E$) and angular momentum ($L$) in terms of the radius ($r_c$) of the circular orbits. The existence and finite-ness of $E$ and $L$ constrain the possible range of PFDM parameter ($\chi$) and the radius of the circular orbit ($r_c$). We then use the Lyapunov exponent ($\lambda$) to study the stability of the geodesics. Then we analyze the critical exponent ($\gamma$) useful for determining the possibility of detection of gravitational wave signals. After that, we study the perturbation due to a massless scalar field in such a background and calculate the quasinrmal mode (QNM) frequencies and their dependence on PFDM parameter $\chi$ and black hole charge $Q$. Also, we compare the obtained QNM frequencies both in the exact case and in the eikonal limit. We also calculate the quality factor of the oscillating system and study its dependence on $\chi$ and $Q$. Finally, we evaluate the black hole shadow radius $R_s$ and graphically observe the effect of $\chi$ and $Q$ on it.

gr-qc↗

arXiv2022

Shadow of Kottler black hole in the presence of plasma for a co-moving observer

We consider a spherically symmetric black hole metric in (3+1)-dimensions in presence of a positive cosmological constant $Λ$. We use a general approach as proposed in \cite{1} to transform the metric in co-moving coordinates. Then for a general metric we have evaluated the angular shadow size both in absence and presence of plasma. By using the aberration relation, we then find the corresponding shadow radius as viewed by a co-moving observer for Schwarzschild de-Sitter or Kottler spacetime. The effect of cosmological constant $Λ$ and plasma parameter $k$ on the angular size of black hole shadow has been observed in detail. Finally, by using the observed angular size of M87$^*$ and Sgr A$^*$ black hole, we constrain the value of the plasma parameter $k$ with a specific observational value of the cosmological constant $Λ$ (or the Hubble constant $H_0$).

gr-qc↗

arXiv2022

Enabling On-Device Smartphone GPU based Training: Lessons Learned

Deep Learning (DL) has shown impressive performance in many mobile applications. Most existing works have focused on reducing the computational and resource overheads of running Deep Neural Networks (DNN) inference on resource-constrained mobile devices. However, the other aspect of DNN operations, i.e. training (forward and backward passes) on smartphone GPUs, has received little attention thus far. To this end, we conduct an initial analysis to examine the feasibility of on-device training on smartphones using mobile GPUs. We first employ the open-source mobile DL framework (MNN) and its OpenCL backend for running compute kernels on GPUs. Next, we observed that training on CPUs is much faster than on GPUs and identified two possible bottlenecks related to this observation: (i) computation and (ii) memory bottlenecks. To solve the computation bottleneck, we optimize the OpenCL backend's kernels, showing 2x improvements (40-70 GFLOPs) over CPUs (15-30 GFLOPs) on the Snapdragon 8 series processors. However, we find that the full DNN training is still much slower on GPUs than on CPUs, indicating that memory bottleneck plays a significant role in the lower performance of GPU over CPU. The data movement takes almost 91% of training time due to the low bandwidth. Lastly, based on the findings and failures during our investigation, we present limitations and practical guidelines for future directions.

cs.LG↗

arXiv2022

Study of circular geodesics and shadow of rotating charged black hole surrounded by perfect fluid dark matter immersed in plasma

In this work, we consider a rotating charged black hole surrounded by perfect fluid dark matter. We consider the system to be immersed in non-magnetised, pressureless plasma. First, we evaluate the null geodesics in order to study the co-rotating and counter rotating photon orbits. Further, we analyse the null geodesics to calculate the celestial coordinates ($α, β$). The celestial coordinates are used to determine the black hole shadow radius ($R_s$). Thereafter, we observe and analyse the effects of black hole spacetime, perfect fluid dark matter and plasma parameters ($a$, $Q$, $χ$, $k$) on the black hole shadow in detail. Finally, we study the effect of plasma distribution on the effective potential ($V_{eff}$) of the black hole spacetime as encountered by the photons. We also present bounds on the plasma parameter from the observational data from $M87^{*}$ central supermassive black hole.

gr-qc↗

arXiv2021

Investigation of circular geodesics in a rotating charged black hole in the presence of perfect fluid dark matter

In this work we have obtained a charged black hole solution in the presence of perfect fluid dark matter (PFDM) and discuss its energy conditions. The metric corresponding to the rotating avatar of this black hole solution is obtained by incorporating the Newman-Janis algorithm. We then compute two types of circular geodesics, namely, the null geodesics and time-like geodesics for this rotating spacetime geometry. For the case of time-like geodesics, we consider both neutral as well as charged massive particles. The effective potentials of the corresponding circular geodesics has also been studied. We then present our results by graphically representing the collective effects of the black hole parameters, namely, the charge of the black hole ($Q$), spin parameter ($a$) and the PFDM parameter ($α$) on the energy ($E$), angular momentum ($L$) and effective potential ($V_{eff}$) of the concerned particle. Finally, we discuss the Penrose process in order to study the negative energy particles having possible existence within the ergosphere, and which in turn leads to the energy gain of the emitted particle.

gr-qc↗

arXiv2019

Shadow of charged black holes in Gauss-Bonnet gravity

In this paper, we investigate the effect of higher curvature corrections from Gauss-Bonnet gravity on the shadow of charged black holes in both $AdS$ and Minkowski spacetimes. The null geodesic equations are computed in $d=5$ spacetime dimensions by using the directions of symmetries and Hamilton-Jacobi equation. With the null geodesics in hand, we then proceed to evaluate the celestial coordinates ($α, β$) and the radius $R_s$ of the black hole shadow and represent it graphically. The effects of charge $Q$ of the black hole and the Gauss-Bonnet parameter $γ$ on the radius of the shadow $R_s$ is studied in detail. It is observed that the Gauss-Bonnet parameter $γ$ affects the radius of the black hole shadow $R_s$ differently for the $AdS$ black hole spacetime in comparison to the black hole spacetime which is asymptotically flat. In particular the radius of the black hole shadow increases with increase in the Gauss-Bonnet parameter in case of the $AdS$ black hole spacetime and decreases in case of the asymptotically flat black hole spacetime. We then introduce a plasma background in order to observe the change in the silhouette of the black hole shadow due to a change in the refractive index of the plasma medium. Finally, we study the effect of the Gauss-Bonnet parameter $γ$ on the energy emission rate of the black hole which depends on the black hole shadow radius and represent the results graphically.

gr-qc↗