Quantum Gravity

I will describe a holographic model for the three dimensional de Sitter static patch where the boundary theory is the so called $T\bar{T}+\Lambda_2$ deformation of the conformal field theory dual to AdS_3 quantum gravity. This identification allows us to obtain the cosmic horizon entropy from a microstate count, and the microstates themselves are a dressed version of those that account for the entropy of certain black holes in AdS space. I will also show how the effect of this dressing at the cosmic horizon is to replace the spacetime dependence of the fields of the undeformed holographic CFT with dependence on the indices of large matrices.

Zoom link:  https://pitp.zoom.us/j/95396921570?pwd=NGFoOGlGY1ZDU2pnNFRwWit3b2w0Zz09

One fruitful strategy of tackling quantum gravity is to adapt quantum field theory to the situation where spacetime geometry is dynamical, and to implement diffeomorphism symmetry in a way that is compatible with regularization and renormalization. It has taken a while to address the underlying technical and conceptual challenges and to chart a quantum field-theoretic path toward a theory of quantum gravity that is unitary, essentially unique and can produce "numbers" beyond perturbation theory. In this context, the formulation of Causal Dynamical Triangulations (CDT) is a quantum-gravitational analogue of what lattice QCD is to nonabelian gauge theory. Its nonperturbative toolbox builds on the mathematical principles of “random geometry” and allows us to shift emphasis from formal considerations to extracting quantitative results on the spectra of invariant quantum observables at or near the Planck scale. A breakthrough result of CDT quantum gravity in four dimensions is the emergence, from first principles, of a nonperturbative vacuum state with properties of a de Sitter universe. I will summarize these findings, highlight the nonlocal character of observables in quantum gravity and describe the interesting physics questions that are being tackled using the new notion of quantum Ricci curvature.

Zoom Link: https://pitp.zoom.us/j/92791576774?pwd=VEg3MEdKOWsxOEhXOHVIQUhPcUt0UT09

In this talk, I will show that supergravity on asymptotically flat spaces possesses a (nonlinear) asymptotic symmetry algebra, containing an infinite number of fermionic generators. Starting from the Hamiltonian action for supergravity with suitable boundary conditions on the graviton and gravitino fields, I will derive a graded extension of the BMS_4 algebra at spatial infinity, denoted by SBMS_4. These boundary conditions are not only invariant under the SBMS_4 algebra, but lead to a fully consistent canonical description of the supersymmetries, which have well-defined Hamiltonian generators. One finds, in particular, that the graded brackets between the fermionic generators yield BMS supertranslations, of which they provide therefore “square roots”.  I will comment on some key aspects of extending the asymptotic analysis at spatial infinity to fermions and on the structure of the SBMS_4 algebra in terms of Lorentz representations.

Zoom link: https://pitp.zoom.us/j/95951230095?pwd=eHIwUXB5SUkvd0IvZnVUN3JJMFE1QT09

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

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

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.