AstroSat, India's first multi-wavelength space observatory is an unique platform to observe cosmic X-ray sources, especially to probe the strong gravity of 'compact' objects. I will discuss the capabilities of AstroSat and present some interesting results to decipher the environment around the 'compact' objects.
In this talk, I will provide a brief overview of regular spacetimes in theories of gravity. In particular, I will primarily review some of the well--known regular black holes and also discuss wormholes and bubble spacetimes. Some recent results, as well as physical effects in such spacetimes will be mentioned. We will end with
general comments and open questions.
A rapidly emerging messenger in astrophysics is gravitational waves (GWs). A new window in the GW spectrum was recently opened when emerging evidence for ultra-long wavelength or nanoHertz frequency GWs was reported by four major pulsar timing array experiments (PTAs). These experiments use a collection of widely separated pulsars in the sky to look for a characteristic spectrum and spatial...
In this talk I will discuss the challenges and prospects of Gravitational-wave Paleontology: studying massive stars from their `remnants’ as compact object coalescences, with the goal to answer the key questions in gravitational-wave astronomy today: What can we learn from these gravitational-wave sources about the formation, lives, and explosive deaths of massive stars across cosmic time?...
Since the Nobel winning discovery of gravitational waves (GWs) by the LIGO-Virgo-Kagra (LVK) detectors from merging compact object binaries, understanding the various astrophysical formation channels of these sources has come to sharp focus. While qualitatively, the processes involved in producing these astrophysical systems are well understood, cutting-edge research is underway to put better...
The physics of gravitational waves is well understood for asymptotically flat space-times. Asymptotic flatness presumes a vanishing cosmological constant. However, cosmological observations over the decades have indicated that our universe is undergoing an accelerated expansion, which is most simply modelled by a de Sitter universe or equivalently by a positive cosmological constant. Even a...
Temporal evolution in low-energy fundamental constants such as the fine structure constant and the proton-electron mass ratio is a generic prediction of theories that attempt to unify the Standard Model of particle physics and general relativity. The exciting possibility of low-energy tests of such unification theories has inspired a number of methods to probe fundamental constant evolution on...
Compact binaries observed in gravitational waves (GWs) are standard distance indicators or standard sirens. This has opened up a novel path to measuring cosmological parameters such as the Hubble constant. In this talk we give a brief overview of the current results in this context from the LIGO-Virgo-KAGRA detector network, some of the near-future prospects, and finally move over to the...
The rising concern in the Hubble constant tension ($H_0$ tension) of the cosmological models motivates the scientific community to search for alternative cosmological scenarios that could resolve the $H_0$ tension. In this regard, we aim to work on a torsion-based modified theory of gravity which is an alternative description to the coherence model. We solve numerically for the Hubble...
Observations of the Cosmic Microwave Background (CMB) radiation have made significant contributions to our understanding of cosmology. While temperature observations of the CMB have greatly advanced our knowledge, the next frontier lies in detecting the elusive B-modes and obtaining precise reconstructions of the CMB's polarized signal in general. In anticipation of proposed and upcoming CMB...
We present the results of a comprehensive long-term spectro-temporal analysis of eight ultra-luminous X-ray sources (ULXs) with the central object being a black hole, using XMM-Newton monitoring of about a decade. Temporal studies reveal the existence of short-term variability in each sources with fractional variance varying in the range of $1.42-27.28$ per cent. Five sources of the sample are...
We use cosmological N-body simulations to study a local Hubble constant measurement and study the uncertainty introduced by our lack of peculiar velocities. We consider observers to be located in dark matter halos and target galaxies to be distributed amongst dark matter halos. We average over all observers. Our findings show a trend where local measurements have a significant dispersion...
A permanent offset caused by the passage of gravitational waves, known as the memory effect, is under active research in both theoretical and observational aspects of gravitational physics. Due to its relation to asymptotic symmetries and soft theorems, the memory effect has received considerable attention for asymptotically flat spacetimes in general relativity (GR). As a result, the memory...
The astrophysical Stochastic Gravitational Wave Background (SGWB) is the superposition of all the compact binary merger events that go undetected as individual events along with other sources such as supernovae, magnetars, etc. The individual gravitational wave (GW) signals from these events vary over time, depending on the source parameters. The timescale of the individual events along with...
The light-cone (LC) effect arises from the cosmological evolution of the redshifted 21-cm signal along an observer's line of sight (LoS), which is the frequency axis. It is particularly pronounced during the Epoch of Reionization (EoR) when the mean neutral hydrogen fraction (x_HI) and statistical properties change rapidly. The 3D power spectrum only quantifies the ergodic part of the signal...
The count-in-cells (CIC) is a one point spatial statistic that is used to describe the spatial distribution of galaxies in the Universe. Besides the computational simplicity, it can be modelled theoretically to allow estimation of the parameters describing the large-scale structure of the Universe, such as the $\sigma_8$ and bias. In this work, we measure the galaxy CIC distribution at high...
Massive neutrinos are well-known to cause a characteristic suppression in the growth of structures at scales below the neutrino free-streaming length. A detailed understanding of this suppression is essential in the era of precision cosmology we are entering into, enabling us to better constrain the total neutrino mass and possibly probe LCDM cosmological model(s) and beyond. In this talk, I...
In this article, we analyze a class of compact object in spheroidal geometry described by Vaidya–Tikekar model and MIT bag equation of state considering the finite value of strange quark mass $(m_s)$. The maximum mass and radius is evaluated by maximizing the radial sound velocity $(v_r^2)$ at the centre of the star. For monotonically decreasing nature of the sound velocity, it is noted that...
Among various properties of black holes studied so far, their response to an external tidal field remains an especially interesting topic. In our recent work [[arXiv:2306.13627 [gr-qc]][1]], we presented an analytic method for calculating the tidal response function of non-rotating and slowly rotating black holes from the Teukolsky equation in the small frequency and the near horizon limit....
We investigate the cosmological applications of new gravitational scalar-tensor theories and we
analyze them in the light of H0 tension. In these theories the Lagrangian contains the Ricci scalar
and its first and second derivatives in a specific combination that makes them free of ghosts, thus
corresponding to healthy bi-scalar extensions of general relativity. We examine two specific...
In this work we have analytically deduced the frequency dependent expression of conductivity and the band gap energy in AdS4 Schwarzschild background for p-wave holographic superconductors considering Einstein-Yang-Mills theory. We also used the self consistent approach to obtain the expressions of conductivity for different frequency ranges at low temperature. We then compared the imaginary...
At all Universe scales, there is a detectable amount of magnetic field. This observed magnetic field has several probable origins, including the possibility that it was produced during the early Universe. There are several models for primordial magnetogenesis, and if the inflationary background is taken into account, breaking conformal symmetry is required to generate a sufficient amount of...
Einstein equations projected on Black Hole horizons give rise to the equations of motion of a viscous fluid. This suggests a way to understand the microscopic degrees of freedom on the Black Hole horizon by focusing on the physics of this fluid. In this talk, we shall approach this problem by building a crude model for the Horizon-fluid(HF) corresponding to asymptotically flat Black Holes in...
Gravitational wave(GW) astronomy has been maturing rapidly since the first detection of gravitational waves. Already, the current GW detectors have the sensitivity to detect gravitational waves emitted from neutron stars. Next-generation detectors will improve on this, resulting in a golden age of GW astronomy for Neutron Stars(NS). The observed mass of NS is limited to around two times that...
The uneasiness associated with the notion of a quantum state of a universe presents challenges not only on the interpretational front but on the phenomenological front as well. A reductionist approach that somewhat circumvents this issue is to consider sharply peaked states on the classical trajectory and introducing a quantum-corrected spacetime arising from a quantum gravity model. This...
Quantum entanglement harvesting in the relativistic setup attracted a lot of attention in recent times. Acquiring more entanglement within two qubits may be very desirable to establish fruitful communication between them. On the other hand use of reflecting boundaries in a spacetime has close resemblance to the cavity quantum optomechanical systems. Here, in presence of two reflecting...
Testing transitivity in quantum field theory is a fundamental aspect of understanding the consistency of the theory and its predictions. In our paper, we considered a vacuum state of the massless scalar field in Minkowski spacetime and two Rindler wedges, Rindler-1 and Rindler-2, separated by a distance. From a set-theoretic view, this setup assumes the picture where the Minkowski spacetime...
Tidal disruption of white dwarfs (WD) is an astrophysical transient phenomenon capable of revealing significant information on the presence of intermediate-mass black holes (IMBH). Using hydrodynamical simulations, we explore the observable properties of these astrophysical events. We accurately calculate the fallback rate of the tidal debris during its accretion onto an IMBH. We find the...
AstroSat, India's first multi-wavelength space observatory is an unique platform to observe cosmic X-ray sources, especially to probe the strong gravity aspects of 'compact' objects. I will discuss the capabilities of AstroSat and present some interesting results to decipher the environment around the 'compact' objects.
Gravitational waves (GW) from the inspiral of binary compact objects offer a one-step measurement of the luminosity distance to the event, which is essential for estimating the Hubble constant, $H_{0}$, that characterizes the expansion rate of the Universe. However, unlike binary neutron stars, the inspiral of binary black holes is not expected to be accompanied by electromagnetic radiation...
The recent observations of the shadows and images of the supermassive compact objects Sgr A and M87 at the hearts of Our Galaxy and the nearby galaxy M87, respectively, by The Event Horizon Telescope (EHT) collaboration have opened up a new window in observational astronomy to probe and test gravity and fundamental physics in the strong-field regime. It is commonly believed that the...
From electromagnetic observations, we know that the supermassive black holes at galactic centers are surrounded by dark and baryonic matter. With the advent of gravitational wave (GW) astronomy, we are poised to probe the details of geometry from where GW gets generated. GWs from binaries containing at least one supermassive black hole will be observed with space-based detector LISA. In such a...
Boundary term and Brown-York (BY) formalism, which is based on the Hamilton-Jacobi principle, are complimentary of each other as the gravitational actions are not, usually, well-posed. In scalar-tensor theory, which is an important alternative to GR, it has been shown that this complementarity becomes even more crucial in establishing the equivalence of the BY quasi-local parameters in the two...
Dark Matter (DM) is ubiquitous and thus has been proposed to be probed by several terrestrial and celestial detectors. DM particles from the galactic halo can accumulate in Neutron Stars (NS) and transmute them into sub-2.5 solar mass black holes (BH) if the DM particles are heavy, stable, and have feeble but sufficient interactions with nucleons. These BHs are named Transmuted Black Holes...
The quest to understand the properties of matter at high density has intrigued physicists for more than a few decades. The problem is complicated, as having a proper theory describing it is challenging. Also, earth-based experiments to probe them have yet to materialize. One of the naturally occurring laboratories where such matter exists is the cores of a neutron star. Recent precise mass and...
In this paper, we analyze the causal aspects of evolving marginally trapped surfaces in a D dimensional
spherically symmetric spacetime, sourced by perfect fluid with a cosmological constant. The norm of the
normal to the marginally trapped tube is shown to be the product of lie derivatives of the expansion
parameter of future outgoing null rays along the incoming and outgoing null...
The Israel-Carter theorem (famously known as ``no-hair theorem'') puts a restriction on the existence of parameters other than mass, electric charge, and angular momentum of a black hole. On the other hand, Bekenstein showed the possibility of existence of scalar hair by considering a massless conformal scalar field non-minimally coupled to gravity. The Einstein-Maxwell-scalar solution for a...
The primary objective of this research is to examine the potential for collapse in the generalized emergent Vaidya spacetime, utilizing the theoretical framework of $f(\bar{R}, \bar{T})$ gravity, with a special emphasis on the K-essence theory. In this study, the non-standard Lagrangian of the Dirac-Born-Infeld type is employed to ascertain the emergent metric denoted as $\bar{G}_{\mu\nu}$. It...
Primordial non-Gaussianity has set strong constraints on models of the early universe. Studies have shown that Loop Quantum Cosmology (LQC), which is an attempt to extend inflationary scenario to planck scales, leads to a strongly scale dependent and oscillatory non-Gaussianity. In particular, the non-Gaussianity function $f_{_{\rm NL}}(k_1,\, k_2,\, k_3)$ generated in LQC, though similar to...
Galactic spinning compact objects (COs) with non-zero ellipticity are expected to be sources of continuous gravitational waves (CGWs). Certain classes of hypothetical compact objects, such as neutron stars with quark cores (hybrid stars), and quark stars, are thought to have large ellipticities from theoretical considerations. These should enable such COs to produce CGWs detectable by the...
The era of Reheating is an interesting phase of inflationary Universe and it can be parameterized by various parameters like reheating temperature $T_{\text{re}}$, reheating duration $N_{\text{re}}$ and average equation of state parameter $\overline{\omega }_{\text{re}}$, which can be constrained by observationally viable values of scalar power spectral amplitude $A_{\text{s}}$ and spectral...
In this work, we try to compare the cosmological perturbation in the Einstein frame and the Jordan frame for the two-field model. Here we match the metric potentials in both frames to find the equivalence. Further, we evolve the perturbations numerically for selected models.
The study of cosmological perturbations and quantization during inflation has predominantly focused on the Friedmann-Lemaitre-Robertson-Walker (FLRW) spacetime. Still, there are very few non-FLRW spacetime cosmological perturbation studies. Among these, the Bianchi I model is one of the simplest anisotropic spacetime models, featuring varying scale factors (a(t)) in orthogonal directions. This...
Motivated by the effectiveness of $f(Q)$ gravity models in fitting observational data at both background and perturbation levels, our study employs a comprehensive dynamical system analysis to independently validate these findings. We focus on two well-studied $f(Q)$ models, specifically the power-law and exponential variants. Our analysis reveals a matter-dominated saddle point in both cases,...
Early data from the James Webb Space Telescope (JWST) has uncovered the
existence of a surprisingly abundant population of very massive galaxies at extremely high
redshift, which are hard to accommodate within the standard ΛCDM cosmology. We explore
whether the JWST observations may be pointing towards more complex dynamics in the dark
energy (DE) sector. Motivated by the ubiquity of...
The study of the interior regions of a neutron star is one of the active areas of research
and gravitational wave astronomy is one of its critical tools. Currently, astronomers
from across the spectrum are detecting different neutron star systems and it has
become essential to consistently combine this information. We perform Bayesian
inference to constrain the equation of state of the...
A class of relativistic compact objects is analysed in modified Finch-Skea geometry described by modified MIT bag model equation of state of the interior matter. The bag constant B plays an important role in determining the physical features of strange star. In this work we have considered the effect of finite strange quark ($m_{s}≠ 0$) on the stability of quark matter inside a star. We have...
We investigate the non-Gaussian three-point cross-correlation function between the primordial curvature perturbation and the primordial magnetic field generated via direct gauge-inflaton coupling, for generic non-standard initial vacua. Among the possible triangular configurations of the resulting cross-bispectrum, we find that the squeezed limit leads to a product form decomposition in terms...
In the past decades, several neutron stars (NSs), particularly pulsars, with mass $M > 2M_\odot$ have been observed. Hence, there is a generic question of the origin of massive compact objects. Here we explore the existence of massive, magnetized, rotating NSs with various equation of states (EoSs) using XNS code, which solves axisymmetric stationary stellar equilibria in general relativistic...
Matter inside a neutron star is composed of nucleons in β-equilibrium up to
2-3 times saturation density(ρ0). The equation of state(EOS) of such matter can
be expressed using iso-scalar and iso-vector nuclear matter parameters (NMPs)
which characterize the symmetric nuclear matter (SNM) and density-dependent
symmetry energy, respectively. The tidal deformation of a neutron star in...
In this talk, I discuss the evolution of various measures of quantumness of the curvature perturbation by integrating out the inaccessible entropic fluctuations in the multi-field models of inflation. In particular, I discuss the following measures of quantumness, namely purity, entanglement entropy and quantum discord. The models being considered in this talk are ones that produce large scale...
In this paper, we perform the dynamical system analysis of the cosmological models framed in the extended teleparallel gravity, the $f (T, B)$ gravity. We use the mapping, $f(T, B)=-T+f(T, B)$, and define the dynamical variables to form the autonomous dynamical system. The critical points are obtained in two well-motivated forms of $f (T, B)$, one that involves the logarithmic form of the...
The present work deals with a dynamical systems study of quintessence potentials leading to the present accelerated expansion of the universe. The principal interest is to check for late time attractors which give an accelerated expansion for the universe. Two examples are worked out, namely the exponential and the power-law potential. Furthur we can encountered with the other type of...
Abstract : We study evolution of the universe in modified theories of gravity. The dynamics of the universe is explored in the following theories (i) $f(R)= R + \gamma R^2 + \beta R^{\delta}$ where $\beta$, $\gamma$, $\delta$ are arbitrary constants, (ii) $f(R, GB) = R+ f(GB) $, GB is the Gauss-Bonnet term : $GB=R^{\alpha \beta \gamma \delta} R_{\alpha \beta \gamma \delta} -4...
Although the sources of ultra-high energy cosmic rays (UHECRs) are unknown, the high-quality data acquired by the most modern CR observatories indicate that these CRs are of extragalactic origin. As the intergalactic media are predicted to be filled with turbulent magnetic fields (TMFs), these intergalactic magnetic fields may profoundly impact how UHECRs travel throughout the expanding...
Trapped surfaces are the basic building blocks of a black hole region. Marginally trapped surfaces, which are trapped surfaces with vanishing value of the outward null expansion scalar, foliate the null horizon of a black hole in equilibrium. Using the intrinsic geometry of trapped surfaces, it shall be argued that the algebra of classical charges follow a simple algebra. The representation of...
This abstract is based on our recent papers PRD 105 (2022) 024028 and PRD 107 (2023) 044072. Recent observations of several peculiar over- and under-luminous type Ia supernovae infer indirect evidences for the violation of the Chandrasekhar mass-limit by suggesting the existence of super- and sub-Chandrasekhar limiting mass white dwarfs (WDs). In an attempt to explain these phenomena in the...
Quantum chromodynamics predicts that at high enough temperature/density, hadronic matter (HM) deconfines to quark-gluon matter. it is conjectured that the deconfinement transition from HM to quark matter (QM) takes place at an intermediate density range (a few times nuclear matter density). However, there is no ab-initio calculation, nor are there any earth-based experiments. The only...
$\mathbb{Z}_3$ symmetric dark matter models have demonstrated remarkable potential in addressing various (astro-)particle physics challenges. In this presentation, I will discuss the diverse ways in which this model can successfully explain the different cosmological observations. We have considered two such promising models: semi-annihilating dark matter (SADM) and Co-SIMP $2\rightarrow 3$...
We develop a static charged stellar model in $f(R,T)$ gravity where the modification is assumed to be linear in $T$ which is the trace of the energy momentum tensor. The exterior spacetime of the charged object is described by the Reissner-Nordstr\"om metric. The interior solution is obtained by invoking the Buchdahl-Vaidya-Tikekar ansatz, for the metric potential $g_{rr}$, which has a clear...
Recently, a novel modification in the conventional area-entropy relation has been introduced by Barrow, as $S=(A/A_{0})^{1+\Delta/2}$, by taking account of the quantum gravitational deformation effects on the black hole's surface. Recent literature has adopted this horizon entropy to the cosmological domain, leading to significant insights. In this line of thought, we formulated the law of...
We study here the unhindered gravitational collapse of spatially homogeneous (SH) scalar fields $\phi$ with a potential $V_{s}(\phi)$, as well as vector fields $\tilde{A}$ with a potential $V_{v}(B)$ where $B=g(\tilde{A},\tilde{A})$ and $g$ is the metric tensor. If the past end-point of a causal geodesic is a singularity, then this singularity is said to be naked. Such a singularity is strong...
The General Theory of Relativity(GTR), Quantum Field Theory(QFT) and Newtonian Quantum Gravity(NQG) are three alternative approaches to study Full Quantum Gravity(FQG). We investigate a system of self-gravtitating fermionic particles with small but significant charges using the NQG model. We derive the equation for ground state energy by adding the energy resulting from charges to the total...
We have investigated the detection probability of continuous Gravitational Wave (CW) signals from the spinning neutron stars in our galaxy across a wide range of their spin frequency. For this purpose, we use observed neutron stars' spin frequencies and spin-down rates, observed with radio telescopes as provided in the Australia Telescope National Facility (ATNF) pulsar catalog. We model the...
The study investigates the evolution of the global 21 cm temperature, a pivotal cosmological probe for understanding the early Universe, employing a novel approach rooted in scalar field dynamics.In this research, we use scalar field models to understand its impact on 21 cm cosmology, aiming to uncover the unexplored nuances in the temperature fluctuations. Furthermore, we try to understand...
Quantum gravity has been studied using various approaches and all of these approaches introduce a fundamental length scale in the theory. Non-commutative space-time is an approach which incorporates this fundamental minimum length scale naturally and provides a testing ground to build quantum gravity models. Thus, it is of paramount interest to study how non-commutative space-time will affect...
Gravitational waves (GWs) emitted from astrophysical sources can get lensed on their way to Earth, similar to electromagnetic waves. There are claims that detections made by LIGO and Virgo in earlier observational runs show evidence of lensing. Lensing has been invoked to explain the discovered high mass events, the bimodal mass function distribution of black holes, and for the objects in the...
The Jordan and Einstein frame representations of scalar-tensor theories of gravity are mathematically equivalent; however, the cosmological evolutions therein can be drastically different. An interesting example is an expansion-collapse duality -- a continually expanding Einstein frame universe can have a dual Jordan frame description that is always contracting. The scenario eventually runs...
We delve into the intricacies of the Bianchi-I cosmological model, driven by the intriguing backdrop of anisotropic cosmic microwave background (CMB) observations. In our pursuit to understand the underlying anisotropy of this model, we introduce anisotropic sources into the framework, augmenting the isotropic matter with dust-like dark matter and dark energy in the form of a cosmological...
In this talk, I will explore a dynamic wormhole solution featuring the Modified Chaplygin Gas (MCG) as the matter at the throat, characterized by an equation of state (EOS): p = Aρ - B/ρ^α.Its global properties, its traversability and the necessary energy conditions to maintain it within the framework of the standard Big Bang cosmological model will be explored.
Further, I will investigate...
We study linear scalar perturbations of black holes in two-dimensional (2D) gravity models with a particular emphasis on Jackiw-Teitelboim (JT) gravity. We obtain an exact expression of the quasinormal mode frequencies for single horizon black holes in JT gravity and then verify it numerically using the Horowitz-Hubeny method. We shall also consider the dimensionally reduced...
The first serendipitous detection of a highly magnified star in a spiral galaxy (z=1.49) lensed by a galaxy cluster, MACS1149 (z=0.54), has opened a new window to observe stars at cosmological distances. Since then, several other lensed stars have been detected in HST imaging of various galaxy clusters, and nearly all galaxy clusters observed by JWST revealed lensed stars candidates. Observing...
In this work, we present the environmental effects on wormhole geometries residing in a galaxy through a fully relativistic analysis. In particular, we consider two wormhole spacetimes classes: the Damour-Solodukhin wormhole and the braneworld wormhole. While there is no classical matter model for the Damour-Solodukhin wormhole, the braneworld wormhole, on the other hand, is supported by a...
In the paradigm of hierarchical structure formation, galaxies are thought to form and evolve inside a potential well environment of 'collisionless' and 'only gravitationally interacting' form of matter; the dark matter halos. These dark halos have formed at the peaks of initial density fluctuations due to gravitational instability and as observations have revealed, are the sites of most of the...
Pre-merger gravitational-wave (GW) sky-localisation of binary neutron star (BNS) and neutron star black hole (NSBH) coalescence events, would enable telescopes to capture precursors and electromagnetic (EM) emissions around the time of the merger. We propose an astrophysical scenario that could provide early-warning times of hours to days before coalescence with sub-arcsecond localisation,...
A class of new Gravastar solution is presented here following the Mazur and Mottola model in gravitational Bose-Einstein condensate (GBEC) star in the cylindrically symmetric space-time. A stable gravastar with three distinct regions namely, (i) interior de-Sitter space, (ii) intermediate thin shell with a slice of finite length and (iii) exterior vacuum region. The interior region is...
The rate of energy loss and orbital period decay of quasi- stable compact binary systems is a useful tool to constrain theories of gravity. In this talk, we present exact expressions for energy loss and orbital period decay are in three $f (R)$ theories derived using the method of a single vertex graviton emission process from a classical source. After linearising the f (R) action written in...
An investigation has been done on the gravitational red-shift and blue-shift of photons emitted by time-like geodesics orbiting in the regular Bardeen black hole in the presence of clouds of strings. As the particles/gas are orbiting in a black hole in space-time, they emit light that travels along null geodesics towards the point of detection (observer). In terms of killing vector formalism...
Gravitational wave (GW) memory is studied in the context of a certain class of braneworld wormholes. Unlike other wormhole geometries, this novel class of wormholes do not require any exotic matter fields for its traversability. First, we study geodesics in this wormhole spacetime, in
the presence of a GW pulse. The resulting evolution of the geodesic separation shows the presence of...
This abstract is primarily based on my recent paper MNRAS 520 (2023) 3742. Since the discovery of fast radio bursts (FRBs), researchers have proposed several theories and models to explain their characteristics. One of the most recent models takes into account the Gertsenshtein-Zel'dovich (GZ) phenomenon, which suggests that a portion of gravitational radiation is converted into...
We explore the formulation of Gravitational Waves(GWs) in the modified f(R) gravity model given by f(R)=\frac{R^{1+\delta}}{R^\delta_c}. We introduce the weak field approximation and study polarization of GWs. The Gravitational Waves carry an extra mode of polarization beyond the TT mode for the weak field approximation. We discuss the dependence of the polarization of these waves on the...
One of the most revolutionary outcomes of Einstein's general theory of relativity is the Black Hole (BH). In 1974 Stephen Hawking showed that BHs can also emit particles called Hawking radiation. The spectrum of the emitted particles is a black body spectrum. Till now we have considered the BH an isolated one and the particle emission spectrum is a Planckian. But we are interested if the BHs...
We present our Monte-Carlo simulation of galaxy properties: optical and HI. We use this simulation to study the expected number of direct detections in upcoming surveys (MIGHTEE with MeerKAT and WALLABY with ASKAP). We also study the expected detections in redshifted 21cm line emission with HI stacking. We propose that with these surveys HI stacking can be done in bins with a given range of...
The knowledge of what entered them is completely lost as black holes evaporate. This contradicts the unitarity principle of quantum mechanics and is referred to as the information loss paradox. Understanding the end stages of black hole evaporation is key to resolving this paradox. As a first step, we need to have exact models that can mimic 4-D black holes in General relativity in classical...
Cosmology today, although precise, is perhaps not accurate. The so-called tensions in the standard $\Lambda$CDM model have been cited as a crisis in the field. With no clear evidence of systematical errors, nor a foolproof alternative theory as of now, the crisis is often attributed to the insufficiency of data at hand, that calls for future missions. In this presentation, I would investigate...
The fluctuations in the 21-cm radiation emitted by the neutral hydrogen(HI) in the Intergalactic medium(IGM) during different stages of the Epoch of reionization (EoR) is expected to be highly non-Gaussian. The degree of non-Gaussianity varies with the nature of the ionizing sources, state of the IGM and the underlying physical processes within the IGM. One of the crucial observable statistic...
We study a global, two-dimensional (2D) general relativistic magnetohydrodynamics (GRMHD) simulation of an accreting torus around a non-rotating black hole. Our initial configuration is threaded with a net-vertical magnetic flux. This study investigates the effects of initial field strength onto the disk dynamics. We find that the initial net vertical magnetic field significantly enhances its...
We investigate accretion flows around rotating black holes (BHs) and obtain self-consistent transonic solutions in full general relativistic prescription. The flow is assumed to be viscous and radiative. Viscosity helps in the removal of angular momentum outwards, allowing matter to get accreted inwards. In addition, viscous heat dissipated makes the matter hotter. On the other hand, radiation...
In general relativity, eccentricity is not defined uniquely. Different waveform models rely on gauge-dependent definitions of eccentricity and other orbital elements, which leads to incompatibility between different models. We employ a [recently proposed gauge invariant eccentricity definition][1] to eliminate this ambiguity in our PN-NR comparisons. We also present an eccentric GW model by...
In the domain of gravitational wave exploration, the swift prediction of waveforms holds immense importance for real-time and computational scenarios. Our research revolves around the development of efficient surrogate models, encompassing a three-step process to engineer accurate representations of true waveforms. This endeavour brings about transformative enhancements in waveform prediction,...
Most galaxies host a supermassive black hole (SMBH) at their centers, yet a large fraction of SMBHs are quiescent and only observable through their transient interactions with stellar objects. As a star approaches an SMBH, the tidal gravitational forces can overwhelm the star's self-gravity, resulting in a tidal disruption event (TDE). The disrupted debris accretes onto the black hole,...
Minimally Modified Gravity (MMG) theories are gravity theories that differ from Einstein's General Relativity (GR) but do not propagate any additional degree of freedom in the gravity sector. Such theories have given rise to a new direction to the study of modified gravity theories and their cosmological implications are being investigated enthusiastically. In my talk, I will briefly outline...
We use the PLANCK 2018 and the WMAP data to constraint inflation models driven by a scalar field $\phi$ in the presence of the non-minimal scalar-curvature mixing term $\frac{1}{2}\xi R \phi^2$. We consider four distinct scalar field potentials $\phi^p e^{-\lambda\phi},~(1 - \phi^{p})e^{-\lambda\phi},~(1-\lambda\phi)^p$ and $\frac{\alpha\phi^2}{1+\alpha\phi^2}$ to study inflation in the...
The quantum speed limit (QSL) specifies the shortest amount of time required for a quantum system to evolve from an initial to a final state. In this work, we look into QSL for the unitary evolution of neutrinos and antineutrinos in the presence of a gravitational field. Since the transition probabilities between neutrino and antineutrino in the framework of one and two flavors depend on the...
The Galactic Center (GC) hosting a supermassive black hole, Sgr A* is
surrounded by a population of S-stars. The orbit of these S-stars is
used as a probe for understanding the nature of gravity in such an
extreme environment. In dynamic interaction between stars and a
supermassive black hole, tidal interaction plays an important role in
determining the fate of the interacting system....
We explore the behaviour of barotropic and irrotational fluids with a small viscosity under the effect of first-order acoustic perturbations. We discuss, following the extant literature, the difficulties in gleaning an acoustic geometry in the presence of viscosity. In order to obviate various technical encumbrances, when viscosity is present, for an extraction of a possible acoustic geometry,...
GRS 1716-249 is a transient galactic black hole that experienced nine months of outburst activity in 2016–17. During its outbursts, Astrosat observed it at three different epochs with a fair amount of exposure time. We investigate whether quasi-periodic oscillation (QPO) exists and how it has evolved throughout the three epochs. We also explore the energy-dependent nature of QPOs. We model the...
LIGO-Aundha is expected to join the network of terrestrial broadband gravitational wave (GW) detectors and begin operations in the early 2030s. We study the impact of this additional detector on the accuracy of determining the direction of incoming transient signals from coalescing binary neutron star sources. Our study involves performing a full Bayesian parameter estimation (PE) over a...
Recently, there have been efforts to examine the contributions to the scalar power spectrum due to the loops arising from the cubic order terms in the action describing the perturbations, specifically in inflationary scenarios that permit a brief epoch of ultra slow roll (USR). A phase of USR inflation leads to significant observational consequences, such as the copious production of...
Magnetic fields are observed throughout the universe on different length scales having different strengths. Galaxies and galaxy clusters have a magnetic field strength of $\sim 10^{-9}~ G$ coherent over a 1KPc scale. We find magnetic fields at $\sim 1 MPc$ scale having a strength of $\sim10^{-16}~ G$ in the intergalactic medium (IGM). Inflationary magnetogenesis is the most profound theory of...
In this article we have analyzed a class of strange star described in terms of CFL phase equation of state. The results obtained by considering CFL equation of state is then compared with those obtained from MIT bag model equation of state. It is noted that if we consider the CFL phase equation of state in which the quarks are assumed to form cooper pair, the maximum mass of strange star takes...
In this article, we set up a variational problem to arrive at the equation of a maximal hypersurface inside a spherically symmetric evolving trapped region. In the first part of the article, we present the Lagrangian and the corresponding Euler-Lagrange equations that maximize the interior volume of a trapped region that is formed dynamically due to infalling matter in D-dimensions, with and...
Measurements of the Cosmic Microwave Background Radiation indicate the presence of a dipole anisotropy in the sky distribution of temperature fluctuations of the CMB photons. It is believed that the CMB dipole arises because of the earth's motion with respect to the cosmic rest-frame; hence, the strength of the dipole provides an estimate of the earth's speed. Similar measurements recently...
Dark matter haloes in a given mass range are expected to cluster differently based on secondary halo properties such as concentration or spin, a behaviour known as secondary halo bias. While secondary halo bias has been quantified in simulations, observational uncertainties in halo properties make it difficult to measure the signal in data using only two-point clustering. The $k$-Nearest...
Searches for exotic compact objects (ECOs) from gravitational wave data require a thorough understanding of their signatures during the inspiral and the ringdown. ECOs are motivated by quantum gravity extensions of general relativity and are characterized by the absence of a horizon and partial reflectivity. In the ringdown, which can serve as a fingerprint of an ECO, it is essential to...
In this paper we study the memory effect produced in pp-wave spacetimes due to the passage of gravitational wave pulses. We assume the pulse profile in the form of a ramp (which may be considered as an appropriate representation of burst gravitational waves), and analyse its effects on the evolution of nearby geodesics. For a ramp profile, we are able to determine analytical solutions of the...
We demonstrate a general relativistic approach to model dark matter halos using the Einstein cluster, with the matter stress-energy comprising collisionless particles moving on circular geodesics in all possible
angular directions and orbital radii. Such matter, as is known, allows an anisotropic pressure profile with non-zero tangential but zero radial pressure. We use the Einasto density...
We study the equilibrium configurations and the radial stability of spherically symmetric relativistic Neutron Stars(NS) with a polytropic equation of state (EoS) in a modified $f (R, T )$ gravity framework by introducing a quadratic term in $T$ (where $T$ is the trace of the conserved energy-momentum tensor $T_{\mu \nu}$ of the matter-energy) for the functional form of $f(R, T)$ with $f (R, T...
This research paper examines the Ricci scalar R and the Gauss-Bonnet invariant G to characterize a cosmological model in flat space-time via $f(R,G)$ gravity. Our model assumes that $f(R,G)$ is an exponential function of $G$ combined with a linear combination of $R$. We scrutinize the observational limitations under a power law cosmology that relies on two parameters - $H_0$, the Hubble...
Accurate component separation of full-sky maps in the radio and microwave frequencies, such as the cosmic microwave background (CMB), relies on a thorough understanding of the statistical properties of the Galactic foreground emissions. These Galactic emissions include Galactic synchrotron, free-free, thermal dust emissions, Anomalous Microwave Emission (AME), etc. Extracting the morphological...
The wideband timing technique enables the high-precision simultaneous estimation of Times of Arrival (ToAs) and Dispersion Measures (DMs) while effectively modeling frequency-dependent profile evolution. We present two novel independent methods that extend the standard wideband technique to handle simultaneous multi-band pulsar data incorporating profile evolution over a larger frequency span...
The stability of an asymptotically flat, static, spherically symmetric naked singularity spacetime
in the novel four-dimensional Einstein-Gauss-Bonnet (EGB) gravity has been studied. Such a naked singularity is obtained from the four-dimensional EGB black
hole for large enough values of the coupling parameter. The stability and
the response of the spacetime are studied against the...
Abstract: We study the properties of the neutrino-dominated accretion flow (NDAF) around the rotating black holes. The accretion flow of this kind involves hyper-accretion rate (${\dot m} \sim 0.001-10$ $M_\odot~{\rm s}^{-1}$) and because of this, the disk becomes geometrically as well as optically thick that makes it difficult for photons to escape. On the contrary, neutrinos easily move out...
A new two-parameter, static and spherically symmetric regular geometry is proposed, which, for specific parameter choices, represents a geodesically complete, regular black hole. However, unlike most regular black holes which have Schwarzschild spacetime as their singular limit, our spacetime reduces to a singular, mutated Reissner–Nordström geometry, for a particular choice of parameters. The...
In this work we have discussed the origin of cosmic viscosity with the in of deformed cosmic phase space geometry. We have introduced the Non-Commutative(NC) deformed geometry and shown that the transformation from non-Riemannian geometry to Riemannian geometry (From Non-Commutative to Commutative(C) geometry) can provide the dissipation in cosmology. A single scalar field in deformed...
While fitting a non-linear model to data, it is common to consider errors only in the dependent variable and treat other variables as perfectly measured. A more flexible model fitting considering errors in independent variables is expected to better estimate the parameters of the model from the same data. We employ a Bayesian method to consider redshift errors in the Pantheon sample of Type-Ia...
Abstract: Production of gravitationally coupled light moduli fields must be suppressed in the early universe, so that its decay products do not alter Big Bang Nucleosynthesis (BBN) predictions for light elements. On the other hand, the moduli quanta can be copiously produced non-thermally during preheating after the end of inflation. In this work, we study the production of moduli in the...
In Loop Quantum Cosmology (LQC), a quantum bounce precedes inflationary epoch. The presence of a quantum bounce leads to a departure from scale invariance of the spectra of primordial perturbations. Studies conducted mostly at the level of the primordial power spectrum show that this departure from scale invariance is a remnant of the bounce and is largely independent of the form of the...
In this paper we apply the Regge-Wheeler formalism to study the propagation of axial and polar gravitational waves in matter-filled Bianchi I universe. Assuming that the expansion scalar Θ, of the background space-time is proportional to the shear scalar σ, we solve the background field equations in the presence of matter (found to
behave like a stiff fluid). We then derive the linearised...
We investigate the time evolution of the transonic-viscous accretion flow around a non-rotating black hole. The input parameters used for the simulation are obtained from semi-analytical solutions. This code is based on the TVD routine and correctly handles the angular momentum transport due to viscosity. The thermodynamic properties of the flow are described by an equation of state with a...
We attempt a computation of the spectrum of scalar particles produced in the background of a spherical gravitational wave. The idea was adopted form the great work by Parker in 1976 where he showed the phenomenon of particle creation in the background of an early expanding universe, the spectrum of which was found to be thermal. In fact, any dynamical spacetime, which is a spacetime having no...
In this talk, the particle motion around the naked singularity and black hole of Kerr-Newman spacetime will be discussed with a special attention on the closed timelike orbits. For KN black hole, the Cauchy surface is always located inside the inner horizon where particles with positive angular momentum that co-rotate with the spacetime can only pass through. It is found that in both the naked...
In this talk, we aim to address the question of whether the quasi-normal modes, which represent the characteristic frequencies associated with perturbed black hole spacetimes and are central to the stability of these black holes, are themselves stable. We begin by presenting a general method for transforming to a hyperboloidal coordinate system in both asymptotically flat and asymptotically de...
Abstract: The primary ingredient for studying the phases of a quantum field theory is the effective action, which to the leading order involves computation of one-loop determinants. In this talk which is based on our papers [1] and [2], I will describe a method for computing one-loop partition functions for scalars and fermions on AdS$_{d+1}$ for zero and finite temperature for arbitrary...
Lensing due to intervening matter such as clusters or galaxies can (de)magnify a gravitational-wave (GW) event, leading to a biased measurement of the source mass and redshift. Hierarchical inference on the detected GW events can be performed to estimate the population properties of binary black holes, such as their mass and redshift distributions. Currently, it is assumed that the current...
In this study, we investigate a cosmological model involving a negative cosmological constant (AdS vacua in the dark energy sector). We consider a quintessence field on top of a negative cosmological constant and study its impact on cosmological evolution and structure formation. We use the power spectrum of the redshifted HI 21 cm brightness temperature maps from the post-reionization...
In this study, The power-law solution for an isotropic and homogeneous universe under $f(R, T)$ gravity is examined by taking into account its functional form, $f (R, T) = R + \xi RT$, where $\xi$ is a positive constant. We have constructed the field equation in f(R, T) gravity for homogeneous and isotropic space-time. The solution of the constructed model is given by the $a = \alpha...
The observations of gravitational waves (GW) have proved to be a probe for the physics of celestial objects like black holes (BH) and neutron stars (NS). Such detections have the potential to unravel the mysteries of cosmic origins, equations of state of compact objects and will prove to be a test of the theory of general relativity. To successfully achieve these scientific goals, it is...
The Cosmic Microwave Background continues to be a cornerstone of precision cosmology. It has provided the most accurate parameter constraints for the widely used $\Lambda$ cosmological constant and Cold Dark Matter ($\Lambda$CDM) model, via missions like COBE, WMAP, and Planck. In precision regimes, accounting for the weak lensing of the CMB photons by the large structure gravitational...
We compare the dark matter(DM) production processes and its parameters space in the background of reheating obtained from two chief systems in the early Universe: the inflaton $\phi$ and the primordial black holes (PBHs). We concentrated on the mechanism where DMs are universally produced only from the PBH decay and the generation of the standard model plasma from both inflton and PBHs....
Analysis of Planck data has revealed to us the presence of several anomalies in the Cosmic Microwave Background (CMB). Many investigations have revealed to us the interconnection between several such anomalies. In this talk, we highlight the primordial connection of some of these anomalies. In particular, we analyse certain templates of the primordial power spectrum and explain how features in...
It is widely postulated that the high-energy radiation in X-ray binary systems arises from the upscattering of photons originating from the accretion disk, likely due to interactions with an electron cloud or a corona. Nevertheless, our understanding of the exact geometry and orientation of these coronal structures remains limited. In recent times, spectro-polarimetry studies have emerged as a...
The high-redshift observations of galaxies with instruments like the JWST can be complemented with the upcoming SKA, which will map the early IGM via intensity mapping of the redshifted 21cm signal with improved sensitivity over the current generation of radio interferometers. These star-forming galaxies are expected to emit most of the bulk of ionizing photons during the Epoch of Reionization...
We study the relativistic, inviscid, advective accretion flow in a stationary axisymmetric Kerr-like wormhole spacetime characterized by the spin parameter ($a_{\rm k}$) and the dimensionless parameter ($\beta$). While doing this, we solve the governing equations that describe the relativistic accretion flow in a Kerr-like wormhole and calculate all types of accretion solutions, including...
In recent years, there has been a growing interest in understanding the General Theory of Relativity (GTR) in several ways for the construction of stellar modeling. One of such modification is the inclusion of higher order curvature terms in the Lagrangian. By introducing a quadratic form of the Riemann tensor to the standard Einstein-Hilbert action Lovelock extended GTR in higher dimensions,...
The lensing of gravitational waves occurs when it passes near massive objects like galaxies and clusters that bends its path. The detection of the first lensed gravitational wave is expected within the next few years. Decihertz detectors such as Lunar Gravitational Wave Antenna (LGWA) are expected to detect gravitational waves from intermediate mass blackhole mergers and white dwarf binaries....
Understanding the emergence of classical behavior from a quantum theory is vital towards establishing the quantum origin for the temperature fluctuations observed in the Cosmic Microwave Background (CMB). This talk presents how a real-space approach can comprehensively address this problem even in the leading order of curvature perturbations. Spatial bipartitions of quantum fluctuations are...
One of the key aims of next-generation gravitational wave detectors is to test General Relativity (GR) in the strong gravity regime. It is expected that gravity is modified in the strong gravity regime. Hence, it is imperative to obtain rotating black hole solutions in modified theories of gravity, look at their quasinormal mode (QNM) signatures, and obtain the difference between the new...
We investigate the radiative processes involving two entangled Unruh-DeWitt detectors that are moving on circular trajectories in (2+1)-dimensional Minkowski spacetime. We assume that the detectors are coupled to a massless, quantum scalar field, and calculate the transition probability rates of the detectors in the Minkowski vacuum as well as in a thermal bath. We also evaluate the transition...
The present work deals with the classical and quantum aspects of the Raychaudhuri equation in the framework of $f(T)$-gravity theory. In the background of homogeneous and isotropic Friedmann–Lemaitre–Robertson-Walker space-time, the Raychaudhuri equation has been formulated and used to examine the focusing theorem and convergence condition for different choices of $f(T)$. Finally in quantum...
The intrinsic angular momentum of fermions can generate torsion in spacetime. This gives rise to an effective four-fermi interaction that fermions experience within a fermionic distribution. This interaction is expected to become significant when densities start to grow. In this contribution, I will discuss some findings from our ongoing exploration regarding the role of this interaction in a...
I shall discuss the prospects of machine learning algorithms, namely Gaussian processes and neural networks, to reconstruct the evolutionary history of the Universe with present available observational data independent of any cosmological model. Through this reconstruction, one can constrain different cosmological parameters, which can serve as a promising tool in addressing the rising...
Kofman, Linde and Starobinsky had showed that preheating can occur due to a huge amount of particle creation by parametric resonance after the inflationary era of the Universe. In the $\phi^2$ model, they confirmed the existence of narrow and broad resonances as well as stochastic resonance in the presence of an expanding background, by means of exact numerical computation of the corresponding...
We have proposed a new phase space coordinate system for scalar field theory that can provide a detailed analysis of the cosmological evolutionary phases in a more generalized manner. We have studied \textcolor{red}{(how many)} intermediate states of cosmic inflations. We have discussed fixed point analysis for these phases. The new phase space dynamics discussed here provide a new family of...
Next generation gravitational wave (GW) detectors are expected to detect $10^4 \mbox{--} 10^5$ binary black holes (BBHs) per year. Understanding the formation pathways of these binaries is an open question. Orbital eccentricity can be used to distinguish between the formation channels of compact binaries as different formation channels are expected to yield distinct eccentricity distributions....
We have examined data from 852 Coma cluster galaxies from three different catalogs and grouped them according to their velocity and magnitude in two fields to survey the core (Coma1) and the periphery (i.e. southwest of the core and centered on NGC4839; Coma3). We have identified a group of these galaxies that appear to be a dense cluster (main cluster) of 545 galaxies (430 in Coma1 and 115 in...
A plethora of knowledge about the universe’s chronology may be found in the spectral properties of the CMB energy spectrum. The spectral aberrations of the CMB complement all other cosmological investigations. According to the astonishing measurement of the CMB spectrum by COBE/FIRAS, the CMB spectrum resembles a blackbody with a temperature of TCMB = 2.72548 ± 0.00057 K. This paper aims to...
If the dark matter is a Dirac fermion($\chi$) and it has an electric and magnetic dipole moment, it can couple with the Standard Model photon through a dimension five effective operators, suppressed by a New Physics(NP) scale $\Lambda$. We probe the parameter space in terms of Dark Matter mass ($m_\chi$) and the New Physics scale $\Lambda$ via a mono-photon signal at the upcoming...
We revisit the black hole X-ray binary source XTE J1859+226 during its outburst phase in 1999-2000 and carry out the spectral and timing analyses using RXTE observations. Over the course of outburst, type-B QPO is observed multiple occasions and the combined spectro-temporal results reveal enhanced hard X-ray contributions as ${\rm QPO}_{\rm rms}\% \sim 1-3$, covering fraction $\sim 0.4-0.6$...
We investigate the generalised radial Rindler trajectories and their corresponding Rindler horizons in the background of the Schwarzschild spacetime. In a curved spacetime, a covariant definition for Rindler trajectories is provided in terms of the generalised Letaw-Frenet equations. A generalized Rindler trajectory remains linearly uniformly accelerated throughout its motion with constant...
We demonstrate an equivalence between the Minkowski photon emission rate in the inertial frame for an accelerating charge moving on a Rindler trajectory with additional transverse drift motion and the combined Rindler photon emission and absorption rate of the same charge in the Rindler frame in the presence of the Davies-Unruh thermal bath. We further show that the equivalence can be extended...
The cold dark matter (CDM) paradigm provides a remarkably good description of the Universe’s large-scale structure. However, some discrepancies exist between its predictions and observations at very small sub-galactic scales. To address these issues, the consideration of a strong interaction between dark matter particles and dark radiation emerges as an intriguing alternative. In this talk, we...
The study aimed to examine the connection between radio pulsars and ultra-high energy neutrinos using the IceCube catalog of point source neutrino events. For this purpose we use the unbinned maximum likelihood method to search for a statistically significant excess from each of the pulsars in the ATNF catalog.
Next, we performed a Stacked search to further *enhance signal to noise...
In this paper, We have studied the Shadow cast by a rotating Bardeen
black hole in the background of asymptotically safe gravity. Using Hamilton-
Jacobi variable separation method we have derived the null geodesics and the
shadow observables. We have found that the size of the shadow decreases with
an increase in ASG parameter (ω) and gets more distorted with an increase
in spin parameter...
Under the assumption of thermal equilibrium between the horizon and the fluid inside, we conducted a thermodynamic stability analysis on a model designed to mimic the characteristics of the $\Lambda$CDM model, which is the prevailing framework for portraying the cosmic acceleration. The scale factor for this model is defined as $a\sim \sinh^{2/3}(t/t_0)$. The Hayward-Kodama temperature is...
In this work, we present singular bounce scenario
in the framework of the generalised Brans-Dicke (GBD)theory where an evolving BD parameter along with a self-interacting potential is considered. The GBD field equations are derived for an anisotropic space time to provide a more general approach to the cosmic expansion. The evolutionary behaviour of the Brans-Dicke scalar field, dynamical...
We present flat emergent universe with a dynamical wormhole with a modified matter described by nonlinear Equation of state (nEoS) in Einstein’s gravity. The Emergent universe (EU) is free from initial singularity accommodating late accelerating universe satisfactorily. The basic assumption of the original EU model is that the present universe emerged out from an initial Einstein static...
We obtain the global properties of static and slowly rotating self-gravitating Bose-Einstein condensate (BEC) stars and study the effect of temperature on the stellar structural properties. For this we consider a recently developed temperature dependent equation of state of BEC stars formed due to Cooper pairing of nucleons. We use the Hartle-Thorne slow rotation approximation equations to...
We study the finite temperature effects on the soft graviton theorem and the gravitational memory effect using the thermofield dynamics formalism. The soft factor depends on the nature of the scatterers at finite temperatures. Thus, the universal behaviour of the soft factor is lost. However, the universality in the scattering cross-section of the soft processes is observed at low...
Recently we have shown that Ellis-Bronnikov wormholes embedded in warped background do satisfy energy conditions. We present analysis of particle trajectories, geodesic congruences in such wormhole spacetimes and their quasinormal modes. We emphasize on distingushing signatures of the wormhole geometry and the warped extra dimension.
In this study, we analyse the quasinormal modes of black holes occurring within the framework of degenerate gravity. We investigate the properties of the asymptotically flat spacetimes introduced recently in [JCAP 02(2022)02] which are solutions to the degenerate Einstein Gauss-Bonnet(dEGB) action and belong to a much larger class of solutions which include cosmological constant. These...
We report the analysis of the Z-track neutron star (NS) low-mass X-ray binary (LMXB) GX 17+2 using the simultaneous data from the AstroSat (LAXPC/SXT) and NICER mission data. On segmenting the hardness intensity diagram (HID) into three slices—horizontal branch (HB), hard apex (HA), and normal branch (NB)- we investigate the variability of the source and its spectral state evolution throughout...
The phenomena of Squeezing and chaos have recently been studied in the context of inflation. We apply this formalism in the post-inflationary preheating phase. During this phase, inflaton field undergoes quasi-periodic oscillation, which acts as a driving force for the resonant growth of quantum fluctuation or particle production. Furthermore, the quantum state of the fluctuations is known to...
I will present a new upcoming theme of research based on stochastic aspects of the cosmic fluids. The aim is to develop new foundations for a mesoscopic intermediate scale theory, which helps to probe nature and evolution of dense matter in compact stars and early universe cosmology around the era of decoherence of the inflaton field, at intermediate sub-hydro scales. Connections of...
We investigate the affine perturbation series of the deflection angle of a ray near the photon sphere of by kazakov-solodukhin black hole . The values of strong field parameters calculated and analyzed its variation with deformation parameter. With the help of lens equation the expression for angular position of innermost image, the angular separation of outermost image with the remaining...
Superradiance in spinning black holes is an intriguing phenomenon through which the black hole loses its energy and angular momentum over time. In this work, we explore the transient effect of the superradiance process in active galactic nuclei (AGN). We aim to see the spin-down effect on the accretion disk using an analytic model named Shakura Sunayev. Considering this model, we show how...
Within the framework of static-charged and rotating black hole spacetime, an extensive amount of work suggests that the bosonic fields exhibit superradiant scattering. In this work, we have investigated the scattering of scalar waves and electromagnetic (EM) waves for Schwarzschild black hole in the ring down phase (we refer to this black hole as the ``ringing black hole"), which is the last...
Orbital eccentricity of coalescing compact binaries produces a strong imprint in the gravitational waves (GWs) emitted by these systems. Its presence indicates at dynamically assembled binaries in dense stellar environments like globular clusters, nuclear star clusters, etc. Hence, detecting an eccentric merger will significantly enhance our knowledge about the formation channels of these...
A remote observer in black hole spacetime sees the creation of a pair of particles. Now, one can use Bell’s operator to test whether the two spacelike-separated particles (one outside the horizon and one inside the horizon) are quantum mechanically entangled or not. Also, we describe a prescription to check entanglement of particles created outside the horizon and inside the horizon using...
The presence of compact stellar orbits near the Galactic Center (GC) black hole presents a magnificent opportunity for testing modified theories of gravity as the gravitational potential ($GM/c^2r$) is equal to or more than 100 times the one encountered in the solar system. In this work, we study the effect of f(R) gravity near the GC black hole using both model dependent and independent...
Asymmetric emission of gravitational waves during a compact binary coalescence results in the loss of linear momentum and a corresponding `kick' or recoil on the binary's center of mass. This leads to a direction-dependent Doppler shift of the ringdown gravitational waveform. We quantify the measurability of the kick imparted to the remnant black hole in a binary black hole merger. Future...
Gravitational-wave observations are unique means to test general relativity (GR) in the strong-field regime. Parametrized tests of post-Newtonian theory have been very efficient in testing GR in the inspiral phase of compact binary dynamics. In this test, one introduces deformation coefficients at each post-Newtonian order in the inspiral phase of the gravitational wave which by definition are...
In this project we focus on testing statistical isotropy and Gaussianity of CMB lensing convergence maps using recently released data of the cosmic microwave background (CMB) radiation from the Atacama Cosmology Telescope
(ACT). The CMB photons are lensed by gravitational potential wells of the
large-scale matter distribution. This CMB temperature data is converted to convergence map by...
We investigate the status of the gravitational arrow in the case of spherical collapse of a fluid which conducts heat and radiates energy. In particular, we examine the results obtained by W. B. Bonnor in his 1985 paper, where he found that the gravitational arrow was opposite to the thermodynamic arrow. The measure of gravitational epoch function used by Bonnor was given by the ratio of the...
We investigate the prospect of performing binary black hole (BBH) nature tests using spin-induced multipole moment (SIQM) measurements when the binary is fully precessing. As SIQM is strongly degenerate with spin parameters, we are interested in the degeneracies the SIQM parameter has with spin precession. We extend the previous SIQM-based BBH nature tests by incorporating two-parameter...
The Galactic Center supermassive black hole, Sgr A* provides an ideal laboratory for testing general relativity (GR) and constraining its alternatives. In this work, we search for GR breaking points by estimating the pericenter shift of stellar orbits having a semimajor axis in the range of (45 - 1000)au. We work with theoretical scalaron field amplitude and coupling. The scalaron mass is...
In my talk, I will explore the intriguing aspects of ghost-free dRGT massive gravity, which introduces two additional characteristics scales, $\gamma$ and $\Lambda$, representing non-zero graviton masses. I will delve into how these parameters influence wormhole solutions, ultimately leading to a loss of asymptotic flatness near the throat region. This inconsistency arises from the induced...
Pulsars are fast spinning neutron stars that lose their rotational energy via various processes such as gravitational and magnetic radiation, particle acceleration and mass loss processes. This dissipation can be quantified by a spin-down equation that measures the rate of change of the frequency as a function of the rotational frequency itself. We explore the pulsar spin-down and consider the...
The talk will present a summery of the main results within the Scale Invariant Vacuum (SIV) paradigm as related to the Weyl Integrable Geometry (WIG) as an extension to the standard Einstein General Relativity (EGR). After a short sketch of the mathematical framework, the main results until 2023 [1] will be highlighted in relation to: the inflation within the SIV [2], the growth of the density...
We study the possibility of detecting dark radiation (DR) produced by a combination of interactions with the thermal bath and ultra-light primordial black hole (PBH) evaporation in the early universe. We show that the detection prospects via cosmic microwave background (CMB) measurements of the effective relativistic degrees of freedom ${\rm \Delta N_{eff}}$ get enhanced in some part of the...
It is widely believed that in the post-Newtonian approach, the asymptotic gravitational fields of non-spinning black holes do not deform under the influence of its companion. Would their horizons deform? In this talk, we present an alternate approach to the problem of tidal deformations of black holes in binary mergers using the source multipole moments of their dynamical horizons and...
The Hubble tension refers to the discrepancy in the value of the Hubble constant $H_0$ inferred from the cosmic microwave background observations, assuming the $\Lambda$CDM model of the universe, and that from the distance ladder and other direct measurements. In order to alleviate this tension, we construct a plausible dark energy scenario, within the framework of Horndeski gravity which is...
Ultra-low mass primordial black holes (PBHs), which evaporate far before Big Bang Nucleosynthesis (BBN), are unconstrained to dominate the energy density of our universe for a short duration. We analyze the stochastic gravitational wave background (SGWB) signals from the domination of ultra-low mass PBHs to explain this recent discovery of SGWB from Pulsar Timing Array collaborations at...
The Infrared (IR) triangle, famously portrayed by Strominger, highlights the unity between soft theorems, infinite-dimensional asymptotic symmetries, and Memory Effects within a single framework. In the realm of Gravity, this is known as BMS symmetry, with a corner relating to measurable classical Gravitational Memory Effects. The revolutionary detection of gravitational waves sets the stage...
Ultra-high energy cosmic rays (UHECRs) beyond the Greisen-Zatsepin-Kuzmin (GZK) cut-off provide us with a unique opportunity to understand the universe at extreme energies. Secondary GZK photons and GZK neutrinos associated with the same interaction are indeed interconnected and render access to multi-messenger analysis of UHECRs. The GZK photon flux is heavily attenuated due to the...
Intermediate-mass black holes (IMBHs) are considered to be seeds of supermassive black holes (SMBHs). Knowledge of the formation and the growth of IMBHs can lead to a better understanding of SMBH formation and galaxy evolution. In recent years, gravitational waves (GWs) have opened a new window to observe and study IMBHs. The advanced ground-based GW detectors, such as Advanced LIGO and...
The two-level particle detector models, such as Unruh-DeWitt detectors(UDD), play a significant role in understanding quantum effects in different frames of reference such as the Unruh effect. These two-level quantum probes are used to study quantum field theory for different observers in flat spacetime as well as in curved spacetime. In recent years, there has been an interest in relativistic...
The line of sight velocity dispersion of the ultra-diffuse galaxies (UDGs) NGC1052-DF2 and NGC1052-DF4 have been reasonably explained only with the baryonic matter, without requiring any dark matter contribution.
The comparable ratio between the baryonic and halo mass also ascertain the above claim for the two dark matter deficit galaxies. This paves the way for analyzing alternative gravity...
This abstract is primarily based on ApJ 949 (2023) 62. Understanding various physical mechanisms requires an understanding of fundamental constants, however, measurements of these constants are subject to error due to experimental constraints. Researchers have proposed several bounds on fundamental constants based on a variety of experiments and observations. These constraints are different...
The network of LIGO-Virgo detectors has detected nearly 100 compact binary mergers in their three observing runs, among which most of the merger events are from quasicircular orbits. Though binaries tend to circularize when they enter the LIGO band, binaries formed via dynamical interactions in dense stellar clusters or through Kozai-Lidov processes can have large residual eccentricities. As...
