Gravity Seminars This Term

The existence of black holes is among the most fascinating predictions of Einstein's theory of General Relativity. Black holes are thought to be ubiquitous throughout the Universe. Their immense gravitational pull enables them to accrete surrounding matter, forming an accretion disk and creating what are known as black hole-disk (BH-disk) systems. These systems provide ideal laboratories for testing theories of gravity in the regime of strong spacetime curvature, where deviations from Einstein's General Relativity are most likely to manifest.

In this talk, I will discuss how BH-disk systems can be employed to test theories of gravity using reflection spectra from the disk, a method known as X-ray reflection spectroscopy. I will also address the systematic uncertainties inherent in the reflection models commonly used to study the properties of these systems.

Neutron star mergers are Nature's ultimate supercolliders, where two massive objects—each containing around 10^58 nucleons—collide at a quarter of the speed of light. These cosmic events offer a unique opportunity to probe the properties of matter under extreme conditions. In this talk, I will discuss our current understanding of the physics of these phenomena and of the way in which their dynamics is imprinted on their gravitational-wave and electromagnetic signals. I will present recent constraints on the properties of dense matter from neutron star merger observations, and I will highlight the potential of next-generation gravitational-wave experiments to provide precision tests of QCD in the nonperturbative regime. Finally, I will talk about theoretical challenges in this field and of our efforts to overcome them.

The properties of matter in the core of a neutron star (NS) are unknown due to the expected high densities and low temperatures. However, several studies have shown that some macroscopic properties of NSs, across a variety of equation of state (EOS) models, are tightly correlated with each other. In this talk, we explore how these correlations can be broadened throughout different stages of the NS’s life: after its birth as a proto-NS (shortly following a core-collapse supernova), during its midlife in a binary system, and just before its death as a hypermassive NS (produced shortly after the merger). In particular, we propose a new method for extracting information about the EOS by interpreting recently observed quasi-periodic oscillations in short gamma-ray bursts as oscillations of a hypermassive NS.

Gravitational-wave (GW) memory effects are the permanent changes in asymptotically flat spacetime that persist after the passage of a wave. Three types of memory effects have been well studied: displacement memory (permanent change in the relative positions of comoving test masses), spin memory, and center-of-mass memory (both related to permanent change in separation for test masses with initial relative velocities). The evolution equations in the asymptotic region of spacetime predict a hierarchy of persistent effects that have recently been called “higher memory effects”, with the well-known memory effects being the leading and subleading contributions. The higher memory effects provide new GW observables that are determined by a set of temporal moments of the gravitational radiation from an isolated source. These effects can deepen our understanding of asymptotically flat spacetimes and how information about source dynamics is encoded in gravitational waves. This talk explores three key aspects of higher memory effects in the context of gravity. First, we present the contribution from the moments of the News to the GW strain in terms of the source multipole moments. Second, within the post-Newtonian framework, we demonstrate how these moments contribute to the terms in the radiative multipole moments of the strain. Third, we discuss the prospects for detecting these effects up to the second order. Lastly, the talk will also explore analogous hierarchies of persistent effects in other gauge theories, including electromagnetism and classical Yang-Mills theory.

TBA

TBA