Strong Gravity

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.

As the sensitivity of gravitational-wave detector networks increases, high-fidelity signal recovery from the most prominent events becomes possible. The strongest signals present both opportunities and challenges for determining the properties of their sources. Focusing on the problem of inferring the nuclear equation of state from neutron star mergers, I will discuss how systematic errors from waveform modeling have the potential to dominate the inference of matter properties in near-future observations. If properly addressed, loud signals may reveal subdominant effects on the source dynamics, providing novel opportunities to learn about neutron star matter beyond the equation of state. I will outline a data-driven approach to interpreting gravitational-wave observations with phenomenological signal corrections, including some first results from work at CSUF on binary black hole systems. I’ll also discuss methods for connecting unmodeled waveform effects to the energetics of the source system. Finally, I will relate these uncertainty quantification methods to goals for instrumental design, demonstrating how next-generation sensitivities will translate into improved scientific potential.

In this talk, I will present a proof that extremal Reissner-Nordström arises on the black hole formation threshold for the Einstein equations coupled to a self-gravitating plasma. This constitutes the first rigorous result on critical collapse. I will then discuss a formulation of the stability problem for extremal black holes, and present a positive resolution in the case of extremal Reissner-Nordström perturbed by a self-gravitating neutral scalar field in spherical symmetry, despite the presence of the celebrated Aretakis instability. This is based on joint work with Yannis Angelopoulos (Caltech) and Christoph Kehle (MIT) (2402.10190, 2410.16234).