Despite its elegance, the theory of General Relativity is subject to experimental, observational, and theoretical scrutiny to arrive at tighter constraints or an alternative, more preferred theory. In alternative gravity theories, the macroscopic properties of neutron stars, such as mass, radius, tidal deformability, etc. are modified. This creates a degeneracy between the uncertainties in the...
Intermediate mass black holes (IMBH) with mass ranges between $100 M_\odot$ to $10^5 M_\odot$ provide a missing link between stellar mass and supermassive black holes. Understanding them provides an insight into galaxy formation as they are considered to be the precursors of supermassive black holes. The high mass of IMBH binaries leads to a short gravitational wave signal duration and...
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...
Binary black hole mergers produce a remnant black-hole in a perturbed state. This then relaxes to form a stable Kerr black hole by emitting gravitational waves, which we call as the ringdown. Ringdowns contain imprints of both strong field linear and non-linear dynamics predicted by the general theory of relativity. Traditionally, we have been using ringdown to test strong-field linear...
Being subject to the tidal field of its companion, each component of a coalescing binary suffers a slow change in its mass (tidal heating) and spin (tidal torquing) during the inspiral and merger. For black holes, these changes are associated with the energy and angular momentum fluxes down their horizons. This effect modifies the inspiral rate of the binary, and consequently, the phase and...
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....
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...
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...
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...
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...
In the standard Big Bang model, Reheating follows inflation but precedes Big Bang Nucleosynthesis (BBN), marking the transition from the inflationary epoch to the hot Big Bang. During inflation, the universe expands exponentially due to the inflation field's energy. After inflation, as the field oscillates, it transfers energy to other particles, forming a hot, dense plasma. This phase depends...
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...
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...
So far, the post-Newtonian technique has generated spin-less gravitational waveforms of binaries in quasi-circular orbits to a very high degree of precision, or to a high post-Newtonian order. Including spins and eccentricity in the system brings challenges that have not been fully tackled to a high degree of accuracy. The inclusion of spins leads to characteristic effects on the orbital...
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...
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...
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...
In this talk we will explore the late inspiral and then the transition regime to the plunge phase of a secondary, less massive compact object into a more massive deformed Kerr black hole. We will show how one can find fluxes such as the energy and the angular momentum and henceforth use them to infer about their detectibility from a GW standpoint. We also explore how the different deviation...
The detection and parameter estimation of compact binary coalescences (CBC) through the analysis of gravitational wave signals have been revolutionized by the advent of deep learning neural networks. Conventionally, curriculum learning, a method that progressively exposes a neural network to more challenging examples during training, has emerged as the de facto procedure for training these...
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,...
We investigate the influence of tidal deformability and strong magnetic fields on the generation of gravitational waves during the inspiral of binary neutron stars in eccentric orbits. Although the current number of identified neutron stars exhibiting strong magnetic fields ($10^{14-15}$ G) remains limited, the maximum allowed magnetic fields in these stars is $10^{18}$ G. Neutron star...
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...
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,...
Post-detection analyses targeting inferences of source properties are often time-consuming and computationally expensive. Assumptions concerning source properties, such as the circularity of binary orbits or the absence of precessing component spins, are routinely made to reduce analysis costs. We shall present a method that may be used to infer the presence (or the lack thereof) of physical...
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...
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...
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...
GR tests used by the LIGO-Virgo-KAGRA collaboration probes various parts of the gravitational-wave signal. However, a comprehensive understanding of the tests requires accounting for potential biases introduced by unmodeled physical effects like eccentricity, spin precession, or lensing. In this talk, we delve into the intricate influence of microlensing and millilensing on the IMR consistency...
Most of the gravitational wave (GW) signals detected so far by LIGO and Virgo detectors consist of comparable mass binary black holes (BBHs). Another interesting source of GWs is highly eccentric intermediate mass ratio inspirals (IMRIs). GW signals from IMRI sources are expected to be highly eccentric when they enter the detection band of the space based detectors such as LISA and DECIGO....
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...
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...
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 accelerated motion of binary black holes (BBHs) can be constrained by the corresponding gravitational waves (GWs) they emit. We investigate the prospects of detecting this acceleration in future third-generation and proposed space-based GW detectors. Since this acceleration could be indicative of the binary's formation channel, we also forecast accelerations of BBHs in dense stellar...
Third generation gravitational wave (GW) detectors are expected to detect millions of binary black hole (BBH) mergers during their operation period. A small fraction of them (∼1%) will be strongly lensed by intervening galaxies and clusters, producing multiple observable copies of the GW signals. The expected number of lensed events and the distribution of the time delay between lensed images...
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....
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...
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....
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...
The parametrized post-Newtonian (PN) test of general relativity (GR) currently assumes binary black holes (BBHs) in quasi-circular orbits. However, population simulations predict that a subpopulation of BBHs retain residual eccentricity in the frequency-band of ground-based detectors. To perform robust parametrized tests of GR with eccentric binaries, corrections due to orbital eccentricity...
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...
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...
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...
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...
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 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...
