My research focuses on the following aspects of Gravitational-Wave astronomy and beyond GR theories:
  • Numerical Relativity; Surrogate waveform modeling of gravitational-wave sources;
  • Understanding interplay between Numerical Relativity and black-hole perturbation theory;
  • Gravitational-wave data analysis;
  • Tests of general relativity (GR) using gravitational wave observations; and
  • Astrophysical tests for beyond GR theories.

1. Gravitational Wave Astrophysics

When two compact objects (e.g. binary black-holes, binary neutron stars or neutron-star and black-hole pairs) merge, they emit gravitational waves (GWs). GWs then travel through the space-time and could be detected by ground-based or space-borne fancy detectors (such as LIGO or LISA). The existence of GWs is a natural outcome in Einstein's general relativity (GR) and had finally been detected in 2015 for the first time. My research interests in GW astronomy are diverse. Below I mention some of my works in waveform modelling, data analysis and testing the nature of gravity.

1.1. Waveform Modelling

I have contributed to the development of the following gravitational waveform models developed using reduced order surrogate techniques:

  • Minor Contributor to EMRISur1dq1e4, a surrogate waveform model for extreme-mass-ratio-inspirals. The model is available through Black-Hole Perturbation Toolkit (BHPTK) and gwsurrogate python package. The data for the model is hosted here.
  • Lead Contributor to NRSur2dq1Ecc, a numerical relativity based surrogate waveform model for eccentric non-spinning binaries with equal masses. The model would soon be available through gwsurrogate.
  • Lead Contributor to NRSur2dq1Ecc+, a customized surrogate waveform model for eccentric non-spinning binaries with comparable masses up-to mass ratio q<=3. This model too will soon be available through gwsurrogate.
  • Lead Contributor to NRSur2dq1EccRemnant, a surrogate model for the remant properties of eccentric equal mass non-spinning binaries. This model will soon be publicly available through surfinBH.

Image: LIGO-VIRGO Collaboration

Eccentric NR-Surrogate Waveform Model

Eccentric binary black hole surrogate models for the gravitational waveform andremnant properties: comparable mass, nonspinning case.
Publications: arXiv.2101.11798;

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NR-Calibrated Pertubation theory based Non-spinning Waveform Model

Surrogate model for gravitational wave signals from non-spinning, comparable- to large-mass-ratio black hole binaries built on black hole perturbation theory waveforms calibrated to numerical relativity
Publications: arXiv.2204.01972;

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Remnant properties from NR-Calibrated Pertubation theory

Remnant black hole properties from numerical-relativity-informed perturbation theory and implications for waveform modelling
Publications: arXiv.2301.07215;

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1.2. INterplay between NR and perturbation theory

NR and Perturbation theory at the high mass ratios

Interplay between numerical-relativity and black hole perturbation theory in the intermediate-mass-ratio regime
Publications: arXiv.2306.08771;

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NR and Perturbation theory : finite size effect

Interplay between numerical relativity and perturbation theory : finite size effects
Publications: arXiv.2306.08767;

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Scaling between NR and Perturbation Waveforms

Approximate relation between black-hole perturbation theory and numerical relativity waveforms & its regime of validity
Publications: arXiv.2307.03155;

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Scaling between NR and Perturbation Fluxes

Mapping between black-hole perturbation theory and numerical relativity: gravitational-wave energy flux
Publications: arXiv.2310.05743;

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1.3. Gravitational Waves Data Analysis

Once these surrogate models are available, I use Bayesian Inference to analyze the detected GWs signals and extract properties of the black-holes. My recent work has shown that using these surrogate models provide more information about the black hole progenitor system than what other models are able to offer.

Analysis of GWTC-3 with numerical relativity surrogate models

Evidence of large recoil velocity from a black hole merger signal
Publications: arXiv.2309.14473;

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Large Kick Velocity in GW200129

Evidence of large recoil velocity from a black hole merger signal
Publications: arXiv.2201.01302;

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Reanalyzing GW190412 with NRSurrogate

Improved analysis of GW190412 with aprecessing numerical relativity surrogate waveform model.
Publications: arXiv.2010.04848;

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Source characterization of intermediate mass-ratio inspirals (IMRIs) black hole

Detectability and source characterization of intermediate mass-ratio black hole coalescences with gravitational waves: The importance of higher-order multipoles
Publications: arXiv.2105.04422;

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GW Memory in Intermediate mass ratio inspirals (IMRIs)

Survey of gravitational wave memory in intermediate mass ratio binaries.
Publications: arXiv.2109.00754

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1.4. Developing Rapid Parameter Estimation Framework

Estimating the source properties of the GW signal is generally done through Bayesian Inference using either Monte-Carlo-Markov-Chain (MCMC) or nested sampling. Such methods are extremely expensive and take days (weeks) to analyze a binary-black-hole signal (binary-neutron-star signal). This is a bottleneck for possible electromagnetic follow-ups which requires accurate sky localization of the binary. Here, with my collaborators, I work on building rapid and accurate parameter estimation framework.

Removing degeneracy and multimodality in gravitational wave source parameters

Removing degeneracy and multimodality in gravitational wave source parameters.
Publications: arXiv.2207.03508;

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Factorized Rapid Paramater Estimation Framework

Framework to analyze an aligned-spin quadruplar BBH signal in ~200 seconds and BNS signal in ~250 seconds using one computing core.
Publications: arXiv.2210.16278;

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1.5. Testing the nature of gravity using GWs

An important next step is to use the extracted information about the merging black-holes to test Einstein’s general relativity (GR). This constitutes the third front of my research. With my collaborators, I work on developing efficient tests that can identify any departure from GR.

Testing GR with higher modes

Testing the “no-hair” nature of binary black holesusing the consistency of multipolar gravitational radiation.
Publications: arXiv.1910.14259;

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Applying higher-modes consistency test on GW190814

Applying higher-modes consistency test on GW190814 : lessons on no-hair theorem, nature of the secondary compact object and waveform modeling
Publications: arXiv.2111.00111;

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2. Beyond General Relativity

My secondary direction of research constitutes the exploration of beyond GR theories.

The validity of GR is well-established in solar-system neighborhood and binary pulsar systems. The detection of gravitational waves from the mergers of binary black holes (BBHs) has further allowed us to test GR in the previously unexplored highly relativistic strong field regime of gravity. So far all GW observations are consistent with GR. However, GR is plagued by an observed ‘mass-discrepancy’ in galaxies and clusters. One way to reconcile GR with observations is to invoke exotic dark matters. No-show of dark matter particles in ambitious experiments so far though has encouraged researchers to look for alternative explanation which considers the ‘mass-discrepancy’ as a signature of the breakdown of GR at galactic and cosmological scales.

Many alternative or modified gravity theories try to achieve these through replacing Einstein's GR with new laws of gravity. My research explores the ability of motivated alternative gravity theories (Weyl Conformal gravity, Chern-Simons gravity, f(R) gravity, Scalar-Tensor-Vector gravity) in providing acceptable fits to the dynamics of self gravitating objects of a wide range of masses, namely globular clusters, galaxies and clusters of galaxies.

Image: arXiv:1807.09241 (Ezquiaga & Zumalacarregui 2018)

White dwarf mass-radius relation in theories beyond general relativity

White-dwarf mass-radius relation in Scalar-Tensor-Vector Gravity and beyond Horndeski theories of G3 type.
Publications: arXiv.2301.08677;

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NGC1052-DF2 and the fate of modified gravity

Modified Gravity Theoriesin Light of the Anomalous Velocity Dispersion of NGC1052-DF2.
Publications: arXiv.1908.07160;

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Ultre-diffuse galaxies

Enigmatic Velocity Dispersions of Ultra-Diffuse Galaxiesin Light of Modified Gravity Theories and Radial Acceleration Relation.
Publications: arXiv.1910.09726;

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Milky Way as a testing bed for gravity

Acceleration Relations in the Milky Way as Differentiators of Modified Gravity Theories.
Publications: arXiv.1911.11836;

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Globular Clusters as a probe for gravity

Globular clusters as a probe for Weyl Conformal Gravity.
Publications: arXiv.1811.00065;

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Viability of conformal gravity

Testing Weyl Gravity at Galactic and Extra-galactic Scales.
Publications: arXiv.1808.06923;

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