36th Annual Report (2023-24)

178 Areasofresearch: Cosmology: The Cosmology group is currently actively involved in the issues relates to Cosmological Tensions, e.g the Hubble Tension, S_{8} tension and anomalies in JWST observations for abundances of massive galaxies at high redshifts compared to concordance LCDM model. Black Hole Shadow and Event Horizon Telescope (EHT) Results: Research in this area centres on understanding theoretical predictions of black hole shadows and comparing them with EHT observational data. Constraints derived from EHT results on deviation parameters offer crucial insights into the validity of black hole and gravity theories. Gravitational Lensing by Black Holes: Extensive studies explore gravitational lensing in black hole contexts, aiding in inferring black hole properties such as mass, spin, and accretion disk structure. This phenomenon also serves as a tool to probe dark matter and test alternative gravity theories. Black Hole Thermodynamics: Interest in black hole thermodynamics spans entropy, temperature, and evaporation processes, with recent advances shedding light on entropy behaviour in diverse gravitational theories. These findings impact fundamental physics, including the holographic principle and quantum spacetime nature. Quasinormal Modes (QNMs): QNMs are pivotal in understanding black hole stability, structure, and interactions with matter and gravitational waves. Ongoing research refines QNM calculations for various black hole types and explores their connections with broader aspects of black hole physics. ResearchWorkandCollaborations 1. We have proposed a model with ADS behaviour in the Dark Energy Sector, an interesting model for resolving the Hubble tension. Subsequently, we study the abundance of high redshift galaxies in such modes in light of JWST observations. We also studied the forecast of such models for post- ionisation 21cm signals in future SKA- Mid observations. 2. We have also studied the role of GRB observations in the context of cosmological tensions due to low- redshift and CMB observations. We propose a technique using EHT observables to estimate parameters of SMBHs described by the Kerr metric, explicitly accounting for measurement uncertainties. Modelling Kerr–Newman and three rotating regular spacetimes for M87* and Sgr A*, we estimate charge parameters and spin. Our method aligns with existing formalisms and applies to general, non-circular shadowshapes. 3. We also investigate gravitational lensing in the strong deflection regime by loop quantum gravity (LQG)- motivated rotating black hole (LMRBH) metrics, characterised by parameters $l$, mass $M$, and rotation $a$. Using SMBHs Sgr A* and M87* as lenses, we compare LMRBH signatures with Kerr black holes. For Sgr A*, the angular position $\theta_{\infty}$ ranges from 16.4 to 39.8 $\mu$as, and for M87* from 12.33 to 29.9 $\mu$as. Angular separation $s$ ranges from 0.008 to 0.376 $\mu$as for Sgr A* and from 0.006 to 0.282 $\mu$as for M87*. Deviations in observables $\Delta \theta_{\infty}$ and $\Delta s$ for LMRBH ($a=0.80,l=2.0$) from Kerr black holes can reach up to 10.22 $\mu$as and 0.241 $\mu$as for Sgr A*, and 7.683 $\mu$as and 0.181 $\mu$as for M87*. Relativemagnitude $r_{mag}$ ranges from 0.047 to 1.54. EHT results for Sgr A* impose stricter limits on LMRBH parameter space than those for M87*. 4. We analysed the restricted phase space thermodynamics (RPST) of Kerr-Sen- AdS black holes with the central charge C and its conjugate chemical potential μ but excluded the familiar PdV term in the first law of black hole thermodynamics. That gives rise t o a new pe r s pe c t i v e on t he thermodynamics of black holes. We investigate the first law and the corresponding Euler formula using the scaling properties. Such formalism has its beauty; for example, the mass is considered to be a homogeneous function of the extensive variables in the first order. In contrast, the intensive variables are of zeroth order. Because of the complicated expressions of the metric, we numerically calculate the critical values of the thermodynamic quantities. We find the phase transition behaviour of the free energy and other thermodynamic conjugate variables that appear in the first law. 5. We analysed the restricted phase space thermodynamics (RPST) of Kerr-Sen- AdS black holes, focusing on the central charge C and its conjugate chemical potential μ , excluding the PdV term. This provides a novel perspective on black hole thermodynamics. By examining scaling properties, we derive the first law and Euler formula, treating mass as a homogeneous function of extensive variables of the first order. In contrast, intensive variables are of zeroth order. Due to complex metric expressions, we numerically calculate critical thermodynamic values, revealing phase transition behaviour in free energy and other thermodynamic variables. Coordinator: Professor Sushant G. Ghosh ICARD - Centre for Theoretical Physics, Jamia Millia Islamia, New Delhi (Activities from 01 April 2023 to 31 March 2024)