Research

2024

• Inferring small neutron star spins with neutron star-black hole mergers: The precise measurement of neutron star (NS) spins can provide important insight into the formation and evolution of compact binaries containing NS. While traditional methods of NS spin measurement rely on pulsar observations, gravitational wave detections offer a complementary avenue. However, determining component spins with gravitational waves is hindered by the small dimensionless spins of the NS and the degeneracy in the mass and spin parameters. This degeneracy can be addressed by the inclusion of higher-order modes in the waveform, which are important for systems with unequal masses. This study shows the suitability of neutron star-black hole mergers, which are naturally mass-asymmetric, for precise NS spin measurements. We explore the effects of the black hole masses and spins, higher-mode content, inclination angle, and detector sensitivity on the measurement of NS spin. We find that networks with next-generation observatories like the Cosmic Explorer and the Einstein Telescope can distinguish NS dimensionless spin of 0.04 (0.1) from zero at 1−σ confidence for events within ∼350 (∼1000) Mpc. Networks with A+ and A♯ detectors achieve similar distinction within ∼30 (∼70) Mpc and ∼50 (∼110) Mpc, respectively.

  • • • Publication: Accepted in ApJ.

      • Presentation: APS April Meeting 2024, Sacramento, USA. Session P13: Gravitational Wave Parameter Estimation II: Eccentricity and Spins

      • Preprint: arXiv:2402.07075

• Cosmography with next-generation gravitational wave detectors: Advancements in cosmology through next-generation ground-based gravitational wave observatories will bring in a paradigm shift. We explore the pivotal role that gravitational-wave standard sirens will play in inferring cosmological parameters with next-generation observatories, not only achieving exquisite precision but also opening up unprecedented redshifts. We examine the merits and the systematic biases involved in gravitational-wave standard sirens utilizing binary black holes, binary neutron stars, and neutron star-black hole mergers. Further, we estimate the precision of bright sirens, golden dark sirens, and spectral sirens for these binary coalescences and compare the abilities of various next-generation observatories (A♯ , Cosmic Explorer, Einstein Telescope, and their possible networks). When combining different sirens, we find sub-percent precision over more than 10 billion years of cosmic evolution for the Hubble expansion rate H(z). This work presents a broad view of opportunities to precisely measure the cosmic expansion rate, decipher the elusive dark energy and dark matter, and potentially discover new physics in the uncharted Universe with next-generation gravitational-wave detectors.

  • • • Collaborators: Dr. Hsin-Yu Chen (UT Austin, USA), Dr. Jose M. Ezquiaga (Niels Bohr Institute, Denmark)

      • Publication: Invited Review in Focus Issue: Focus on the Science Case for Next Generation (XG) Ground-Based Gravitational Wave Detectors. . Class. Quantum Grav. 41, 125004.

      • Preprint: arXiv:2402.03120

• Characterizing gravitational-wave detector networks: From A♯ to Cosmic Explorer: Gravitational-wave observations by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo have provided us a new tool to explore the Universe on all scales from nuclear physics to the cosmos and have the massive potential to further impact fundamental physics, astrophysics, and cosmology for decades to come. In this paper we have studied the science capabilities of a network of LIGO detectors when they reach their best possible sensitivity, called A#, given the infrastructure in which they exist and a new generation of observatories that are factor of 10 to 100 times more sensitive (depending on the frequency), in particular a pair of L-shaped Cosmic Explorer observatories (one 40 km and one 20 km arm length) in the US and the triangular Einstein Telescope with 10 km arms in Europe. The presence of one or two A# observatories in a network containing two or one next generation observatories, respectively, will provide good localization capabilities for facilitating multimessenger astronomy and precision measurement of the Hubble parameter. Two Cosmic Explorer observatories are indispensable for achieving precise localization of binary neutron star events, facilitating detection of electromagnetic counterparts and transforming multimessenger astronomy. Their combined operation is even more important in the detection and localization of high-redshift sources, such as binary neutron stars, beyond the star-formation peak, and primordial black hole mergers, which may occur roughly 100 million years after the Big Bang. The addition of the Einstein Telescope to a network of two Cosmic Explorer observatories is critical for accomplishing all the identified science metrics. For most metrics the triple network of next generation terrestrial observatories are a factor 100 better than what can be accomplished by a network of three A# observatories.

  • • • Collaborators: The full list of collaborators can be found in the manuscript.

      • Publication: Accepted in the Class. Quant. Grav. Focus Issue: Focus on the Science Case for Next Generation (XG) Ground-Based Gravitational Wave Detectors.

      • Related publication: The results of this work were also used in Matthew Evans et al. (Ish Gupta) and Alessandra Corsi et al. (Ish Gupta).

      • Preprint: arXiv:2307.10421

2023

• Detectability of QCD Phase Transitions in Binary Neutron Star Mergers: Bayesian Inference with the Next Generation Gravitational-wave Detectors: In the high-density regimes, the hadronic material comprising the neutron star can convert to deconfined quarks by undergoing QCD phase transition. This can happen for a more massive neutron star, especially one that may be formed as a result of the merger of two neutron stars. The presence of quarks will increase the compactness of the star, changing the value of the peak frequency of the (2,2) mode (f2) measurable from the post-merger signal of the hypermassive neutron star. We study the detectability of postmerger QCD phase transitions in neutron star binaries with next-generation gravitational-wave detectors Cosmic Explorer and Einstein Telescope. We perform numerical relativity simulations of neutron star mergers with equations of state that include a quark deconfinement phase transition through either a Gibbs or Maxwell construction. These are followed by Bayesian parameter estimation of the associated gravitational-wave signals using the NRPMw waveform model, with priors inferred from the analysis of the inspiral signal. We assess the ability of the model to measure f2 and identify aspects that should be improved in the model. We show that, even at postmerger signal to noise ratios as low as 10, the model can distinguish (at the 90% level) f2 between binaries with and without a phase transition in most cases. Phase-transition induced deviations in the f2 from the predictions of equation-of-state insensitive relations can also be detected if they exceed 1.6 σ. Our results suggest that next-generation gravitational wave detectors can measure phase transition effects in binary neutron star mergers. However, unless the phase transition is “strong”, disentangling it from other hadronic physics uncertainties will require significant theory improvements.

  • • • Collaborators: Dr. Aviral Prakash (lead author), Dr. Rahul Kashyap, Prof. David Radice, Prof. Bangalore Sathyaprakash (Penn State, USA), Dr. Matteo Breschi, Prof. Sebastiano Berzuzzi (Theoretisch-Physikalisches Institut, Germany), Prof. Domenico Logoteta (Università di Pisa, Italy)

      • Publication: Phys. Rev. D 109, 103008

      • Preprint: arXiv:2310.06025

• Detecting Neutron star-black hole Mergers with Next-generation Gravitational-wave Detectors: Although tens of exciting compact binary mergers have been observed, neutron star-black hole (NSBH) mergers remained elusive until they were first confidently detected in 2020. The number of NSBH detections is expected to increase with sensitivity improvements of the current detectors and the proposed construction of new observatories over the next decade. In this work, we explored the NSBH detection and measurement capabilities of these upgraded detectors and new observatories using the following metrics: network detection efficiency and detection rate as a function of redshift, distributions of the signal-to-noise ratios, the measurement accuracy of intrinsic and extrinsic parameters, the accuracy of sky position measurement, and the number of early-warning alerts that can be sent to facilitate the electromagnetic follow-up. Additionally, we evaluated the prospects of performing multi-messenger observations of NSBH systems by reporting the number of expected kilonova detections with the Vera C. Rubin Observatory and the Nancy Grace Roman Space Telescope. 

  • • • Collaborators: Dr. Ssohrab Borhanian (Theoretisch-Physikalisches Institut, Germany), Dr. Arnab Dhani, Dr. Rahul Kashyap, Dr. Ashley Villar, Prof. Bangalore Sathyaprakash (Penn State, USA), Dr. Debatri Chattopadhyay (Cardiff University, UK)

      • Presentation: APS April Meeting 2022, New York, USA. Session Z01: Next-generation Gravitational Wave Detectors, Methods, and Forecasts

      • Publication: Phys. Rev. D 107, 124007

      • Preprint: arXiv:2301.08763

• Using Gray Sirens to Resolve the Hubble-Lemaître Tension: The measurement of the Hubble-Lemaître constant H0 from the cosmic microwave background and the Type IA supernovae are at odds with each other. One way to resolve this tension is to use an independent way to measure H0. This can be accomplished by using gravitational-wave (GW) observations. Previous works have shown that with the onset of next-generation of GW detector networks, it will be possible to constrain H0 better than 2% (enough to resolve the tension) with binary black hole systems, also called dark sirens. Bright sirens like binary neutron star systems can also help resolve the tension if both the GW and the following electromagnetic counterpart are detected. In this work, we assess the potential of using neutron star-black hole (NSBH) mergers to measure the Hubble-Lemaître constant, both as dark sirens and bright sirens, thus, assigning them the term gray sirens. We find that the Voyager network might be able to resolve the tension using NSBH mergers in an observation span of 5 years, whereas next-generation networks which include the Cosmic Explorer detectors and the Einstein Telescope will be able to measure the H0 to sub-percent level.

  • • • Presentation: APS Mid-Atlantic Section 2022 Meeting, Penn State, USA. Session E03: Gravitational Physics and Cosmology II

      • Publication:  Mon. Not. R. Astron. Soc. 524, 3537–3558 

      • Preprint: arXiv:2212.00163

      • Related publication: The methods used in this work were also applied for a population of binary black hole and NSBH mergers in Sec. 6.4.3 in Marica Branchesi et al. (Ish Gupta), JCAP 07 (2023) 068.

2022

• Testing general relativity using higher-order modes of gravitational waves from binary black holes: Recently, strong evidence was found for the presence of higher-order modes in the gravitational wave signals GW190412 and GW190814, which originated from compact binary coalescences with significantly asymmetric component masses. This has opened up the possibility of new tests of general relativity by looking at the way in which the higher-order modes are related to the basic signal. Here we further develop a test which assesses whether the amplitudes of sub-dominant harmonics are consistent with what is predicted by general relativity. To this end we incorporate a state-of-the-art waveform model with higher-order modes and precessing spins into a Bayesian parameter estimation and model selection framework. The analysis methodology is tested extensively through simulations. We investigate to what extent deviations in the relative amplitudes of the harmonics will be measurable depending on the properties of the source, and we map out correlations between our testing parameters and the inclination of the source with respect to the observer. Finally, we apply the test to GW190412 and GW190814, finding no evidence for violations of general relativity.

  • • • Collaborators: Anna Puecher, Chinmay Kalaghatgi, Soumen Roy, Yoshinta Setyawati, Prof. Chris Van Den Broeck (Utrecht University), Prof. Bangalore Sathyaprakash (Penn State, USA)

      • Publication: Phys. Rev. D 106, 082003

      • Preprint: arXiv:2205.09062

2021

• Gravitational Wave Data Analysis of the Ringdown Signal with Mirror Modes: In the past few years, there have been attempts to improve the accuracy of waveforms in describing the ringdown signal of a binary black hole merger. Some recent attempts involve including higher, positive-frequency overtones in the waveform to achieve better results. In arXiv:2010.08602, the role of negative-frequency modes, called mirror modes, has been emphasized in obtaining better constraints for remnant parameters at times earlier than the peak of the signal. In this work, we perform data analysis on the ringdown signals of two gravitational wave events, GW150914 and GW190521, and show that the remnant parameters are better estimated at earlier times on the inclusion of mirror modes.

  • • • Collaborators: Arnab Dhani, Ssohrab Borhanian, Prof. Bangalore Sathyaprakash (Penn State, USA), Gregorio Carullo (University of Pisa, Italy)

      • Presentation: APS April Meeting 2021, Session Y16: Gravitational Wave Observations

2020

• Testing General Relativity using Gravitational Wave Observations: As a part of my Master's Thesis, I worked towards using gravitational wave observations to test general relativity. As a part of the same, I implemented two tests of general relativity:  first, by using the multipole structure of gravitational waves from compact binaries, and second, by implementing a "no hair" type test for gravitational waves from binary black hole systems where we check the consistency in values of chirpmass and mass ratio obtained using the 22-mode and the higher modes. Further, we applied these tests to a subset of events from the first and second observing runs of LIGO and Virgo.

  • • • Collaborators: Prof. Bangalore Sathyaprakash (Penn State, USA), Dr. Anuradha Gupta (University of Mississippi, USA), Dr. Abhirup Ghosh, Dr. Ajit Mehta (Albert Einstein Insititute, Germany)

      • Presentation: APS April Meeting 2020, Volume 65, Number 2. Session R05: Tests of General Relativity with Gravitational Waves II 

2019

• Cosmological Aspects of an Asymmetric Metric with Torsion: I worked on the cosmological aspects of spacetime with an asymmetric metric, where the anti-symmetric part is present due to nonzero torsion. We reviewed Hammond's formulation for torsion and showed that it does not reduce to the Einstein-Cartan formalism when torsion is absent. So, we proposed a modified action, derived the modified Einstein's equations and showed that our formalism does reduce to Einstein-Cartan. 

  • • • Project Supervisor: Prof. Tejinder Pal Singh (TIFR Mumbai, India)

      • Collaborators: Guru Kalyan, Shubham Kadian (IIT Bombay, India)

• Mathematical Aspects of General Relativity: In this study project, I was involved in learning about the basic concepts of topology and differential geometry. We started with the use of differential geometry in classical mechanics, touched upon symplectic geometry, moved to the applications of differential forms and also covered the tetrad formalism in general relativity.

  • • • Project Supervisor: Dr. Kinjal Banerjee (BITS Pilani Goa Campus, India)

2018

• Quantum to Classical Transition: As a part of the Summer Fellowship Program- 2018 hosted by IIT Madras, I worked on understanding the measurement problem and quantum to classical transitions. Our main emphasis was on the theory of decoherence and how it helps in solving major parts of the measurement problem. We explored the theory of decoherence from the basics to the master equation formalism, using Zurek's initial papers and Schlosshauer's text on the subject. We also looked at weak continuous measurements and their application in explaining the quantum-to-classical transition for chaotic systems.

  • • • Project Supervisor: Dr. Vaibhav Madhok (IIT Madras, India)

• Chaos Theory and Hamiltonian Systems: As a study project, I studied the classical chaos theory where I looked at the properties of Hamiltonian systems. Then, using the KAM (Kolmogorov–Arnold–Moser) perturbation theory, I studied the the effects of perturbations to integrable systems and simulated its application using standard maps.

  • • • Project Supervisor: Prof. Gaurav Dar (BITS Pilani Goa Campus, India)

• Cosmological Perturbation Theory: I did a short term study project on cosmological perturbation theory as a part of my course in General Relativity. As a part of the same, I studied about scalar perturbations and their application to explore the Cosmological Microwave Background Radiation.

  • • • Project Supervisor: Dr. Kinjal Banerjee (BITS Pilani Goa Campus, India)

2017

• Simulating the magnetic field of the Sun during solar cycle: The analysis involves decomposing the solar magnetic field into toroidal and polar components which are calculated using linear combinations of spherical harmonics. We use this treatment in order to simulate the characteristic nature of the solar  magnetic field during different phases of the solar cycle. This involves simulating the magnetic field during the initial part of the cycle characterized by a typical dipole structure, the phase in which the north pole and the south pole of the Sun have the same polarity, and during the maximum solar activity characterized by occurrence of a large number of sunspots.

  • • • Project Supervisor: Prof. Mahendra K. Verma (IIT Kanpur, India)

      • Project Collaborator: Roshan J. Dattatri (BITS Pilani Goa Campus, India)

• Special Relativistic Analysis of Refraction: We began with a study of Einstein's mirror, i.e. how reflection is effected when special relativity comes into picture. Then, we moved on to analyzing the consequences of special relativistic treatment on refraction. Finally, we attributed the observed changes to change in the refractive index of the medium and formulate a relationship between the velocity of the observing inertial frame and the refractive index of the observed medium.

  • • • Project Supervisor: Dr. Kinjal Banerjee (BITS Pilani Goa Campus, India)