General Relativity & Quantum Cosmology

In this talk  will develop Rafael D. Sorkin's heuristic that a partially ordered process of the birth of spacetime atoms in causal set quantum gravity can provide an objective physical correlate of our perception of time passing. I will argue that one cannot have an external, fully objective picture of the birth process because the order in which the spacetime atoms are born is a partial order. I propose that live experience in causal set theory is an internal ``view'' of the objective birth process in which events that are neural correlates of consciousness occur. In causal set theory, what ``breathes fire'' into a neural correlate of consciousness is that which breathes fire into the whole universe: the unceasing, partially ordered process of the birth of spacetime atoms.

Zoom link: https://pitp.zoom.us/j/95170823205?pwd=QW9YM3QrZU12Ti9HTUQ4TDlVNmN5Zz09

In this talk, I present a novel framework to do black-hole spectroscopy. This approach is based on a new technique so-called “quasinormal-mode filter”, which can be classified into two subsets: rational filter and full filter. On the theoretical level, I explain how to use the rational filter to understand the ringdown of numerical-relativity waveforms. On the observational level, I introduce a way to incorporate the filter into Bayesian analysis. The new Bayesian framework not only allows us to analyze the ringdown of a real gravitational-wave event without Markov chain Monte Carlo, but also yields a natural estimate of the ringdown start time. By applying our method to GW150914, we find strong and self-consistent evidence for the first overtone from multiple perspectives. On the other hand, the relationship between the full filter and the metric reconstruction is discussed. Its connection to a numerical-relativity technique “Cauchy-characteristic Matching” is provided. In the final part of the talk, I also briefly present our recent progress on fully relativistic 3D Cauchy-characteristic Matching.

Zoom link: https://pitp.zoom.us/j/95593408817?pwd=U2RXZDV2WUtwNDlaRDVBVTJjTEdBQT09

Though often not spelled out explicitly, dynamical reference frames appear ubiquitously in gauge theory and gravity. They appear, for example, when constructing dressed/relational observables, describing physics relative to the frame in a gauge-invariant way. In this talk, I will sketch a general framework for constructing such frames and associated relational observables. It unifies previous approaches and encompasses the transformations relating different frame choices. In gravitational theories, this gives rise to an arguably more physical reformulation of general covariance in terms of dynamical rather than fixed frames. I will then discuss an ensuing relational form of locality, including bulk microcausality and local subsystems associated with subregions, both of which can be defined gauge-invariantly relative to a dynamical frame. In the latter case, the frame incarnates as an edge mode field, linking with recent work on finite subregions. In particular, the corresponding boundary charges and symmetries can be understood in terms of reorientations of the frame. Notably, the resulting notion of a subsystem is frame-dependent, as are therefore correlations, thermal properties and specifically entropies. I will conclude with an outlook on the quantum realm and connections with recent developments on quantum reference frames. [Based on 2206.01193, 2205.00913, JHEP 172 (2022), PRL 128 170401.] 

Zoom link: https://pitp.zoom.us/j/97735460640?pwd=NThybFc3M3Z3cHhVRmRvczdrclhvZz09

There exists a solid framework to study gravitational waves in full, non-linear general relativity when the spacetime is asymptotically flat. An essential aspect of this framework is asymptotic symmetries. These symmetries are the BMS symmetries. The situation for cosmological spacetimes is different, however. Expanding spacetimes, whose expansion is decelerating such as matter- or radiation-dominated universes, share some similarities with the asymptotically flat case nonetheless. Not surprisingly, their asymptotic symmetries also share similarities with the BMS symmetries, but are not the same.

Zoom Link:  https://pitp.zoom.us/j/98168915236?pwd=ejFtaVpVZXAxT2NIdDhtMUNSbmRvQT09

In this talk I will present the recent results of my work with Edoardo d'Angelo, Nicolo Drago and Nicola Pinamonti. Using the methods of perturbative algebraic quantum field theory, we have formulated new flow equations on Lorentzian spacetimes that work for arbitrary states (not only the vacuum) and in Minkowski vaccum reduce to Wetterich equations. This has potential applications to the asymptotic safety approach to quantum gravity.

Zoom link: https://pitp.zoom.us/j/98616857260?pwd=ajZoS1lZc0cySlVmTVdVUnA5Ull6Zz09

We discuss how 3+1-dimensional physics including gravity can arise in the IKKT or IIB matrix model.  The model is related to string theory, but it also provides a direct and accessible definition of  3+1-dimensional physics on suitable backgrounds. This leads to a higher-spin extension of gravity on covariant quantum space-time with manifest volume-preserving diffeos, which is UV finite at one loop. 

Zoom Link: https://pitp.zoom.us/j/94588999721?pwd=VTdxQSt3MGhSMkt3UThCVjNRcXJBQT09

The Lense-Thirring spacetime describes a 4-dimensional slowly rotating approximate solution of vacuum Einstein equations valid to a linear order in rotation parameter. It is fully characterized by a single metric function of the corresponding static (Schwarzschild) solution. We shall discuss a generalization of the Lense-Thirring spacetimes to the case that is not necessarily fully characterized by a single (static) metric function. This generalization lets us study slowly rotating spacetimes in various higher curvature gravities as well as in the presence of non-trivial matter such as non-linear electrodynamics.  In particular, we construct slowly multiply-spinning solutions in Lovelock gravity and notably show that in four dimensions Einstein gravity is the only non-trivial theory amongst all up to quartic curvature gravities that
admits a Lense-Thirring solution characterized by a single metric function. We will also discuss a `magic square' version of our ansatz and show that it can be cast in the Painlevé-Gullstrand form (and thence is manifestly regular on the horizon) and admits a tower of exact rank-2 and higher rank Killing tensors that rapidly grows with the number of dimensions.

Zoom Link: TBD

Speaker Order: Ankit Aggarwal, University of Amsterdam Monireh Ahmadpour, University of Tehran Giovanni Canepa, Centre de Physique Théorique Roukaya Dekhil, Ludwig Maximilian University Arnaud Delfante, University of Mons Florian Ecker, Technische Universität Wien Gloria Odak, Centre de Physique Théorique

We begin by reviewing the role of coadjoint orbits in the representation theory of nilpotents groups and then, to connect with the recent applications in physics of coadjoint orbits "around the corner" of the mathematical framework developed by Kirillov, we review the classification of coadjoint orbits of the Virasoro group. This will allow us to connect with more recent developments, including e.g. the study of coadjoint orbits of BMS algebras.

The tree-level soft theorems were recently shown to arise from the conservation of infinite towers of charges extracted from the asymptotic Einstein equations. There is evidence this tower promotes the extended BMS algebra to an infinite higher-spin symmetry algebra. In this talk I will introduce towers of canonically conjugate memory and Goldstone operators, highlighting their role in parameterizing the gravitational phase space. I will discuss the conditions under which these towers provide a complete set of scattering states and demonstrate that they are the building blocks of both soft and hard charges. I will finally show that the tower of tree level soft symmetries can be used to extend the Dirac (Faddeev-Kulish) dressings to include the infinite towers of Goldstones and comment on their implications for the gravitational S-matrix.

Corner symmetries are those diffeomorphisms that become physical in codimension two, in that they support non-zero Noether charges. Recently we have shown how to extend phase space so that all such charges are integrable and give a representation of the corner symmetry algebra on this extended phase space. More recently we have studied the coadjoint orbits of what we now call the universal corner symmetry. One finds that certain complementary subalgebras, the extended corner symmetry and the asymptotic corner symmetry, can be associated with finite-distance and asymptotic corners, respectively. There is a simple geometric interpretation here in terms of an Atiyah Lie algebroid over a corner, whose structure group is the universal corner symmetry. The local geometry of a classical spacetime is encoded in related geometric structures.