General Relativity & Quantum Cosmology

In my talk, I'll review the key idea underlying asymptotically safe quantum gravity, before giving an overview of the current status and open questions. I'll then spotlight three challenges. The first challenge is how to test quantum gravity and I'll advocate that by linking quantum gravity to particle physics and cosmology, we can probe quantum gravity at all scales. The second challenge is how to connect to other ideas about quantum spacetime and I'll discuss effective asymptotic safety as a way to link to, e.g., string theory. The third challenge is the status of global symmetries and I'll discuss what we know about the preservation of global symmetries in asymptotic safety and what this implies for a broader perspective on the question. --- The presenter will be joining via Zoom for this talk.

The Celestial Holography program encompasses recent efforts to understand the flat space hologram in terms of a CFT living on the celestial sphere. A key development instigating these efforts came from understanding how soft limits of scattering encode infinite dimensional symmetry enhancements corresponding to the asymptotic symmetry group of the bulk spacetime. Historically, the construction of the bulk-boundary dual pair has followed bottom up approach matching symmetries on both sides. Recently, however, there has been exciting progress in formulating top down descriptions using insights from twisted holography. We review salient aspects of the celestial construction, the status of the dictionary, and active research directions.

At first sight, it seems that almost any QFT, seen as a low-energy effective field theory (EFT), can be coupled to gravity -- simply add an Einstein-Hilbert term, change all derivatives by covariant ones, and you're done. However, a growing body of evidence coming from String Theory compactifications, general properties of black hole evaporation, and rigorous results in holography, suggests that the opposite is true. We can work out the hidden constraints that quantum gravity consistency imposes in the matter sector systematically, an approach that receives the colourful name of "The Swampland Program". I will briefly review the program, its motivation, current results, and long-term goals.

I will review implications of the AdS/CFT correspondence for quantum gravity, paying particular attention to holographic RG flows and the processing of information. Deformed hyperbolic geometries correspond to boundary field theory RG flows. Conversely, as I will exemplify using the recent approach of discrete holography, boundary RG flows may be used to reconstruct the bulk spacetime, for instance via tensor networks. I will discuss recent approaches towards including gravity dynamics into this reconstruction process. --- The presenter will be joining via Zoom for this talk.

I will briefly review the key concepts underlying the Effective Field Theory of General Relativity, and give a couple of examples of how it works. Then I will describe seven lessons which can be extracted from the theory. Finally I discuss some of the limitations of the EFT framework.

 

Motivated by the static patch of de Sitter space, we discuss timelike surfaces in general relativity and the initial boundary value problem. We consider the Dirichlet problem and a conformal version thereof. Time permitting we discuss some ideas in lower dimensions.

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Zoom link https://pitp.zoom.us/j/98357788662?pwd=b3lHS3Q3T2xkNm53SkR3QVVvTGJMdz09

In general relativity, the remnant black hole of a binary black hole merger emits “ringdown” radiation: a superposition of quasinormal modes with characteristic complex frequencies. The detection of more than one of these frequencies would serve as smoking-gun evidence of a black hole and would enable a test of the no-hair theorem. In this talk, I will discuss strategies for identifying quasinormal modes within simulated ringdown waveforms both in linear perturbation theory and full general relativity. I will show that a rich spectrum of modes could exist in the ringdown, including overtones, retrograde modes and nonlinear modes, and that interesting quasi-universal relationships exist between some of their amplitudes. I will also discuss the spectral instability of quasinormal modes and its potential imprints on the ringdown waveform. These results could guide the analysis of real ringdown signals detected by gravitational-wave detectors.

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Zoom link https://pitp.zoom.us/j/92002625163?pwd=bHNDVVZKYnVNTENLNXJBLzVNKzVSQT09

Building on the fact that quantum uncertainty is a dynamical degree of freedom in itself in quantum mechanics, I will start with the remark that its evolution can provide a notion of intrinsic clock. To illustrate the non-triviality of this idea, we’ll check the dynamics of the uncertainty if gravitizing quantum mechanics into a non-linear Schrödinger equation, and will see that it (surprisingly?) follows the same trajectories as planetary orbits. A third thread will come from the black hole mini-superspace in general relativity. We will see that, assuming that the quantization preserves all the symmetries of the theory, universal quantum gravity corrections can also be written as a non-linear Schrödinger equation, leading to a non-trivial evolution of the “classicality” of black hole wave-packets along the radial coordinate.

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Zoom link: https://pitp.zoom.us/j/95808416532?pwd=cHJnVXpzdDd3eXdFbUlMamFsMFV3UT09

Anticipating the launch of several next-generation gravitational wave (GW) detectors in the 2030s, we will be able to more precisely measure spacetime ripples from binary black hole (BH) mergers in a larger parameter space. The forthcoming data will require us to develop more accurate predictions of GWs not only in General Relativity (GR) but also in theories beyond GR and diverse astrophysical environments. Black hole perturbation theory is a cornerstone for making these predictions. In recent years, there have been extensive studies of perturbations of BHs in theories beyond GR, but only for non-rotating or slowly rotating BHs. In this talk, I will present a new formalism, based on Teukolsky's seminal work in the 1970s, to study perturbations of BHs with arbitrary spin in beyond-GR theories and in more complicated astrophysical environments. I will first discuss how to derive a modified Teukolsky equation for BHs deforming perturbatively from their counterparts in GR due to beyond-GR or environmental effects and the necessary techniques to evaluate this equation. Subsequently, I will discuss some applications of this formalism. Specifically, I will prescribe utilizing this formalism to investigate the isospectrality breaking of quasinormal modes (QNMs) in beyond-GR theories, compute the QNM frequency shifts in some specific theories, and efficiently extract these shifts from observation data. Furthermore, I will also show how to apply this formalism to study extreme mass-ratio inspirals beyond GR.

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Zoom link: https://pitp.zoom.us/j/99282316326?pwd=REtBSFUxdlgxUGVwZFFvVEVBVnFTUT09