I will present general arguments, based on fundamental physics principles, as to why we should expect a significant, horizon scale, departure from semiclassical gravity inside astrophysical black holes. Then, I will present a simple model which realizes this idea: the frozen star model. I will show that a static frozen star looks exactly like a…

In four-dimensional asymptotically flat spacetimes, the infinite tower of soft graviton modes is known to generate a w(1+infinity) symmetry algebra at tree-level. In this talk, I will review this symmetry and demonstrate that it acts non-trivially on massive scalar particles. The symmetry action is derived from previously-discovered…

The towers of massive states in quantum gravity are ubiquitous and highly constrained. I will review some of the Swampland conjectures, known as distance conjectures, that quantify the universal behaviors of such towers in string theory. Due to the strong implications of distance conjectures, connecting them to more fundamental principles…

The growing catalog of gravitational-wave signals from compact object mergers has allowed us to study the properties of black holes and neutron stars more precisely than ever before and has opened a new window through which to probe the earliest moments in our universe’s history. In this talk, I will demonstrate how current and future…

Gravitational waves provide a unique observational handle on the properties of strong, dynamical gravity. Black hole ringdowns, in particular, cleanly encode information about the structure of black holes, allowing us to test fundamental principles like the no-hair theorem and the area law. In this talk, I will review the status of this effort,…

Following the first detection of a binary black hole merger in 2015, the number of gravitational wave events has grown to almost one hundred, including binary neutron star mergers and mixed black hole, neutron star mergers. Going forward, the next few years should bring a wealth of new discoveries, including several more neutron star mergers…

This year, NASA is beginning its search for a Probe-class ($1B) mission to fly in ~2032. This once-per-decade opportunity was endorsed by astro2020 as a way to achieve some of the science promised by the more complex and expensive far-infrared and X-ray missions that were prepared for the decadal survey. SALTUS, latin for leap, is a far…

Black holes in the Universe do not exist in isolation but, rather, they are surrounded by matter. It is therefore important to study the stability properties of black holes under matter field perturbations. In this talk we will discuss the stability properties under classical field perturbations of several rotating (Kerr) black hole spacetimes…

"On a background Minkowski spacetime, the relativistic Euler equations are known, for a relatively general equation of state, to admit unstable homogeneous solutions with finite-time shock formation. By contrast, such shock formation can be suppressed on background cosmological spacetimes whose spatial slices expand at an accelerated rate. The…

I will introduce the gravitational path integral and discuss some of its modern applications. Our first application will be to the black hole information paradox, where we will see that nontrivial gravitational saddles are important in exhibiting unitarity of black hole evaporation. Our second application will be to cosmological spacetimes,…

Magnetohydrodynamics (MHD) is a theory with a broad range of applications, from plasma physics to heavy-ion collisions and astrophysics. In this talk, I will show how methodologies in formal hydrodynamics allows to reformulate MHD in terms of symmetry considerations. In particular, I will show that MHD can be viewed as a theory of…

Observations have shown that nearly all galaxies harbor massive or supermassive black holes at their centers. Gravitational wave (GW) observations of these black holes will shed light on their growth and evolution, and the merger histories of galaxies. Pulsar timing arrays (PTAs) use observations of millisecond pulsars…

Abstract: One can paraphrase Penrose’s strong cosmic censorship conjecture as stating that general relativity is generically a deterministic theory. While the full conjecture remains wide open there has long been evidence pointing towards its validity at least for small perturbations of exact rotating Kerr black holes. In this talk I will give…

What happens when two masses scatter? This deceptively simple question has produced a series of surprises that touch on deep issues in general relativity, such as the localizability of gravitational field energy, the notion of asymptotic flatness at timelike infinity, and observability of particle position. Most recently, we found that the…

Abstract:

Guided by the principles of effective field theory, I will discuss three consecutive avenues to constrain physics beyond General Relativity with black-hole observations.

1) Shadows: Without specifying any particular gravitational dynamics, I will discuss image features of black-hole shadows in general…

Over last six years, the LIGO-Virgo detections of gravitational waves from compact binary mergers have underlined their promise as an independent and powerful tool to explore the Universe. Nevertheless, the electromagnetic observations of the counterparts of gravitational-wave sources provide different and complementary information about the…

In this talk, we introduce and solve the characteristic gluing problem for the Einstein vacuum equations. We prove that obstructions to characteristic gluing come from an infinite-dimensional space of conservation laws along null hypersurfaces for the linearized equations at Minkowski. We show that this obstruction space splits into an infinite…

Abstract: The AdS instability conjecture provides an example of weak turbulence appearing in the dynamics of the Einstein equations in the presence of a negative cosmological constant. The conjecture claims the existence of arbitrarily small perturbations to the initial data of Anti-de Sitter spacetime which, under evolution by the vacuum…

Abstract:

As was realized by Bondi, Metzner, van der Burg, and Sachs (BMS), the symmetry group of asymptotic infinity is not the Poincaré group, but an infinite-dimensional group called the BMS group. Because of this, understanding the BMS frame of the gravitational waves produced by numerical relativity is crucial for ensuring that…

Abstract: Over the past years, electromagnetic and gravitational observations have been used to understand the nature of compact objects and the matter around them. However, our ability to learn about the underlying fundamental physics depends heavily on our understanding of the theory of gravity that describes the geometry around these compact…

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