Strong Gravity

GW250114, with a signal-to-noise ratio of ∼80, enables the most precise single-event tests of strong-field gravity to date: setting the tightest constraints on deviations from general relativity, delivering the cleanest ringdown measurement yet, and placing stringent bounds on compact object structure. The one thing it withholds is Love: no tidal deformability is detected, with a 90% upper limit of $\tilde{\Lambda} < 34.8$, fully consistent with the Kerr prediction of vanishing tidal deformability.

Within the Effective Field Theory approach to gravity, deviations from General Relativity can be systematically described by higher-curvature operators. However, computing the resulting corrections to black hole quasinormal mode spectra remains challenging in the rapidly rotating regime, where perturbative expansions in the spin break down. Making use of recently constructed numerical rotating black hole solutions, we computed scalar quasinormal mode frequency shifts at leading order for scalar Gauss-Bonnet, dynamical Chern-Simons and Cubic Riemann corrections. We employed a pseudo-spectral collocation method to solve the perturbation equations on these backgrounds, enabling accurate computation across a broad parameter range. We observed that corrections to certain modes grow significantly as the spin approaches the near-extremal regime.

Gravitational-wave observations by LIGO and Virgo are revealing a population of merging stellar-mass binary black holes. Thus far, the broad connection between the inferred population and the physics that underlies black hole formation remains elusive. The O4a run of the LVK has more than doubled the number of binary black hole detections, and this new data is revealing new features in the population and elucidating old ones. In this talk, I survey several interesting and sometimes perplexing features and subpopulations in the population distribution. I show how they let us glean some information about the underlying formation channels and connect with transient and compact object astrophysics. As the number of detections balloons in the future, these subpopulations will be footholds to a more global understanding of binary black hole formation and the rich physics behind it. 

Neutron stars are ultra-dense remnants of massive stellar cores, observable across the electromagnetic spectrum. Their emission reflects a delicate interplay of magnetic, thermal, and structural processes under extreme conditions of density, temperature, and magnetic fields—regimes unattainable in terrestrial laboratories. Among neutron stars, magnetars stand out, hosting the strongest magnetic fields in the Universe. Their intense X-ray activity, recurrent outbursts, and unusually slow rotation point to powerful interior dynamics, yet the origin of the large-scale dipolar fields that drive their long spin periods remains a longstanding mystery. In this talk, I will introduce MATINS, a new open-access 3D code for modeling the coupled magneto-thermal evolution of isolated neutron stars. Focusing on the evolution of magnetic fields in magnetar interiors, I will discuss the key mechanisms operating under these extreme conditions, in particular the chiral magnetic effect—a phenomenon arising from the chiral anomaly that enables the conversion between magnetic helicity and electron chiral asymmetry. I will present simulations for both non-zero and vanishing initial magnetic helicity, the latter reflecting realistic conditions at the birth of a proto–neutron star. Our results show that the chiral magnetic effect can naturally transfer energy from small-scale magnetic fields at birth into the large-scale dipolar fields observed in magnetars without requiring external energy input. This provides a compelling mechanism for magnetar formation.

Tidal disruption events — where an unfortunate star is destroyed by a previously quiescent supermassive black hole — offer a unique probe of the low mass end of the supermassive black hole population. Recent observational advances have lead to the discovery of ~100 such events, many of which are comprehensively followed up across the entire electromagnetic spectrum. I will discuss how time-dependent relativistic accretion theory can be used to understand this multi wavelength emission from optical through X-ray energies, and how this theory allows tidal disruption events to act as probes of the massive black hole population. I will present current constraints on the occupation fraction of intermediate mass black holes, and look forward to what can be expected in the LSST era.

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.