Strong Gravity

Current waveform models of binary black hole mergers incorporate a large amount of analytical information during the inspiral phase. In contrast, post-merger descriptions typically rely on phenomenological ansätze informed by numerical relativity, with the quasi-normal mode frequencies providing the only direct analytical input. In this seminar, I will first give a gentle introduction to black hole perturbation theory and the Green’s function approach, which is useful for describing the time-domain response of a black hole to a dynamical source, e.g., an infalling test particle. I will then present an analytical framework to study the dynamical excitation of quasi-normal modes during the plunge–merger–ringdown stages for generic orbits, including highly eccentric ones, and discuss its implications for ringdown modeling. I will conclude by presenting novel insights into the modelling of the prompt response, fundamental to correctly capture the signal emitted during the merger at the linear level. 

Gravitational waves (GW) offer a new and independent window onto the Universe. However, as they propagate across cosmological distances, they are inevitably affected by the intervening matter distribution. In this talk, I present a comprehensive overview of my work on the gravitational lensing of GWs, spanning weak lensing by the large-scale structure of the Universe, strong lensing by galaxies, and microlensing by stars and compact objects, and covering lensing effects from the geometric-optics to the wave-optics regimes. I highlight how gravitational lensing can serve not only as a systematic effect in GW observations, but also as a novel probe of cosmic structure and fundamental physics. Finally, I discuss future directions of GW lensing studies, inspired by advances in electromagnetic lensing.

Overtones are known to improve the performance of fits to the ringdown, both in numerical-relativity simulations and gravitational-wave observations. Although the overtone frequencies are a concrete prediction of general relativity, it remains an open question whether they are excited to the extent that fits to data would suggest. In this talk, after a brief introduction to the ringdown, overtones, and black-hole spectroscopy, I will present work where we take a pragmatic approach and investigate the practical utility of each additional overtone in extracting information from the ringdown. We look at the dependence of the ringdown start time on the number of overtones, and the feasibility of detecting deviations from general relativity in the ringdown frequencies. We suggest that there is no clear highest-excited overtone, but rather the utility of each additional overtone decreases compared to the one before. Finally, we perform Bayesian parameter estimation (as opposed to least-squares fits) to obtain posterior distributions on the overtone amplitudes and phases, allowing us to investigate their correlation structure. Due to strong correlations it becomes increasingly hard to measure individual amplitudes and phases for the highest overtones. However, we find that the joint measurement of overtone amplitudes (i.e., the correlation structure itself) is sensitive to the frequencies and decay times of even the highest overtones, possibly offering an avenue to perform consistency tests with general relativity.

Uncovering the origins of binary black holes through gravitational-wave detections provides a powerful probe of the evolution, interactions, and environments of massive stars. Stellar theory predicts a forbidden range of black-hole masses between 50-130Msun due to pair-instability supernovae, but evidence for such a gap in the mass distribution in previous gravitational-wave studies has proved elusive. In this talk, I will present our latest observational evidence of the pair-instability gap in the distribution of black hole masses. While the gap is not present in the distribution of primary masses (the bigger of the two black holes in a binary system), it appears unambiguously in the distribution of secondary masses. The location of the gap lines up well with a previously identified transition in the binary black-hole spin distribution; binaries with primary components in the gap tend to spin more rapidly than those below the gap. We interpret these findings as evidence for a subpopulation of hierarchical mergers: binaries where the primary component is the product of a previous black-hole merger and thus populates the gap. If time permits, I will also highlight the signatures of hierarchical mergers that not only populate the mass gap but also present at lower mass.

The recent detection of a low frequency gravitational wave background by pulsar timing arrays provides solid evidence that the observable universe contains a population of in-spiralling supermassive black hole binaries. Such binaries likely form as a result of collisions between galaxies and can offer clues as to how black holes and galaxies grow over time. Moreover, since galactic centers are often densely populated by gas and stars, these systems are much more likely to be actively accreting and radiating compared to stellar mass-sized black hole binaries. This makes them promising candidates for multi-messenger detection (combining electromagnetic and gravitational wave information), particularly when LISA comes online or as pulsar timing arrays improve their sensitivity. In order to facilitate this goal, it is important that, in addition to predictions for the possible gravitational wave signals from numerical relativity, we also develop predictions for electromagnetic signals from simulations of black hole binary accretion. In this talk I will present some of our recent results from 3D general relativistic magnetohydrodynamic simulations that utilize a strong-field approximation to the in-spiralling spacetime metric. Specifically, I will showcase several possible distinctive electromagnetic signals from black hole binaries, including quasi-periodic emission, jet precession, and two processes analogous to flaring activity frequently observed in the outer layer of the Sun.