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Public viva-voce Notification Date: Friday, 6 September 2024 Time: 03:30 PM Venue: Lecture Hall 202 Implications of gravitational recoil for astrophysics and fundamental physics Parthapratim Mahapatra Chennai Mathematical Institute. 06-09-24 Abstract 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. We study the astrophysical and fundamental physics implications of binary black hole kicks. Black holes between 50-130 solar mass were thought to be prohibited due to pair-production instabilities in massive stars. Observations of binary black holes with component masses in this range challenge this notion and suggest hierarchical mergers in dense star clusters as a possible origin of these high-mass black holes. When binary black holes merge in dense star clusters, their remnants can pair up with other black holes in the cluster, forming heavier and heavier black holes through the hierarchical merger process. Black hole kick plays a decisive role in the hierarchical merger process, which cannot occur if a merger remnant is ejected from its dense stellar environment. Using the publicly available gravitational-wave event database, we infer the magnitudes of kick velocities imparted to the remnant black holes and use that to identify the most promising astrophysical sites for repeated mergers. Next, we construct a simple parametric model for hierarchical mergers in dense star clusters. Our model relies on pairing probability and initial mass functions for the black hole population, along with numerical relativity fitting formulas for the mass, spin, and kick speed of the merger remnant. We assess the efficiency of hierarchical mergers as a function of merger generation and derive the mass distribution of black holes. Further, we propose a Bayesian inference framework to predict the merger history of LIGO-Virgo binary black holes, whose binary components may have undergone hierarchical mergers in the past. This proposed framework computes the masses, spins, and kicks imparted to the remnant of the parent binaries, given the initial masses and spin magnitudes of the binary constituents. We validate our approach by performing an ``injection study'' based on a constructed sequence of hierarchically-formed binaries. This method is then applied to three GWTC-3 events: GW190521, GW200220_061928, and GW190426_190642. We use the backward evolution framework to predict the parameters of the parents of the primary companion of these three binaries. This approach can be readily applied to future high-mass gravitational wave events to predict their formation history under the hierarchical merger assumption. Finally, we look into the signatures of recoil in the gravitational-wave signal and discuss how to directly measure (rather than infer) black hole kicks. Gravitational recoil 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 with future detectors. This direct measurement of black hole kicks can also facilitate a novel test of general relativity based on linear momentum balance. We formulate this kick consistency test via the measurement of a null variable that quantifies the difference between the inferred kick (using numerical relativity) and that observed via the Doppler-shifted ringdown signal. Weak ringdown signals limit current means to perform this test, making it an important science driver for future detectors (which can yield constraints with impressive precision). All are invited to attend the viva-voce examination.
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