Strong Gravity

I will revisit the canonical quantization of General Relativity in the self-dual formulation in the presence of a cosmological constant, discussing the normalizability of graviton states over the de Sitter spacetime. Time permitting, I will explain how the physics of the Hamiltonian constraint is analogous to the quantum Hall effect, with the cosmological constant playing the role of a quantum gravitational Hall resistivity.

Gravitational waves (GW) from compact binaries provide excellent opportunities for testing general relativity (GR) in the strong- and dynamical-field regime. So far, inspiral-based tests have mostly assumed smooth deviations from GR within a post-Newtonian description, thereby overlooking certain beyond-GR effects that can arise in well-motivated extensions including quantum gravity, dark energy, and dark matter. In this talk, I will present an example of such an overlooked effect arising from massive scalar fields nonminimally coupled to gravity, and discuss recent constraints on this scenario using GW observations. I will then introduce a recently developed AI-based approach that incorporates both smooth and nonsmooth deviations within a unified, theory-agnostic parametrization for inspiral tests of GR. 

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.