Sylvain Carrozza, Perimeter Institute
Random tensors, melonic theories and quantum gravity
I will present a brief review of large-N tensor models and their applications in quantum gravity. On the one hand, they provide a general platform to investigate random geometry in an arbitrary number of dimensions, in analogy with the matrix models approach to two-dimensional quantum gravity. Previously known universality classes of random geometries have been identified in this context, with continuous random trees acting as strong attractors. On the other hand, the same combinatorial structure supports a generic family of large-N quantum theories, collectively known as melonic theories. Being largely solvable, they have opened a new window into strongly-coupled quantum theory, and via holography, into quantum gravity. Prime examples are provided by the SYK model and generalizations, which capture essential features of Jackiw-Teitelboim gravity.
Clifford Cheung, California Institute of Technology
From Gluon Scattering to Black Hole Orbits
The study of scattering amplitudes has uncovered extraordinary dualities linking real-world particles such as gravitons, gluons, and pions. We discuss how these developments have been amalgamated with classic tools from effective field theory to derive new results relevant to the search for gravitational waves at LIGO. This approach has produced now state-of-the-art results on conservative orbital dynamics of binary black holes in the post-Minkowskian expansion. We also comment on recent work extending this framework to include tidal effects and spin.
Steve Giddings, University of California, Santa Barbara
Mathematical structure of quantum gravity
A quantum theory of gravity is expected to be described by a Hilbert space endowed with additional mathematical structure appropriate for describing gravitational physics. I discuss aspects of this structure that can be inferred perturbatively, along with connections to arguments for holography and nonperturbative questions.
Daniel Harlow, Massachusetts Institute of Technology
Lessons for quantum gravity from quantum information theory
Gravity is unique among the other forces in that within general relativity we are able to do calculations which, when properly interpreted, give us information about non-perturbative quantum gravity. A classic example is Bekenstein and Hawking's calculation of the entropy of a black hole, and a more recent example is the calculation of the ``Page curve'' for certain evaporating black holes. A common feature of both of these calculations is that they compute entropies without using von Neumann's formula S=-Tr(\rho \log \rho). In this strange situation where we are able to compute entropies without understanding the details of the states for which they are the entropy, quantum information theory is a powerful tool that lets us extract information about those states. In this talk I'll review aspects of these developments, emphasizing in particular the role of quantum extremal surfaces and quantum error correction.
Ted Jacobson, University of Maryland
Reflections on quantum gravity in 2020
Renate Loll, Radboud University Nijmegen
The Remarkable Roundness of the Quantum Universe
It has taken several decades of exploring statistical models of quantum gravity (aka nonperturbative gravitational path integrals) to understand how diffeomorphism-invariance, unitarity and the presence of a causal structure can be simultaneously accounted for in a lattice gravity framework. Causal Dynamical Triangulations (CDT) incorporates all of these features and provides a toolbox for extracting quantitative results from a first-principles quantum formulation, with very few free parameters. Recently, we have introduced the "quantum Ricci curvature", an observable that --somewhat remarkably-- remains meaningful in a maximally nonclassical, Planckian regime. Measuring this curvature in fully-fledged 4D quantum gravity, we have discovered exciting evidence that the quantum universe dynamically generated in CDT is compatible with a constantly curved de Sitter space.
Henry Maxfield, University of California, Santa Barbara
Black hole information, spacetime wormholes, and baby universes
Recent discoveries suggest that semiclassical gravity is more consistent with unitarity than previously believed. I will argue that it makes predictions for the measurements of asymptotic observers that are in complete accord with the idea that black holes are ordinary quantum systems, with states counted by the Bekenstein-Hawking formula. The argument uses the semiclassical gravitational path integral, incorporating newly discovered `spacetime wormhole' topologies. These new ideas revive an old paradigm, relating the information problem to the physics of baby universes.
Hermann Nicolai, Albert Einstein Institute
Approaches to Quantum Gravity: Key Achievements and Open Issues
This talk will provide an overview of current approaches to quantum gravity, with their respective merits and open problems (`comparative quantum gravity'). To this aim I will focus on some key issues that must be addressed by all approaches
Monica Pate, Harvard University
Soft modes in quantum gravity
I will review advances for gravity in asymptotically flat spacetimes arising from investigations into their structure in the infrared. The recently-discovered infinite-dimensional symmetries of the scattering problem is the central result underlying much of the progress. Key examples include symmetry-based explanations for the previously-observed universal nature of infrared phenomena including soft theorems and memory effects. Moreover, the appearance of a Virasoro symmetry among the symmetries of four-dimensional gravity has led to a proposal for holography in which the scattering amplitudes in quantum gravity are dual to correlation functions of a two-dimensional conformal theory. The other infinite-dimensional symmetry groups place additional non-trivial constraints on the dual theory.
Alejandro Perez, Centre de Physique Theorique de Luminy
Quantum gravity from the loop perspective
I will summarise the main achievements of loop quantum gravity and provide my view on the issues that I consider of central importance for present and future efforts.
Mairi Sakellariadou, King's College London
Quantum gravity signals in cosmology and gravitational waves
I will highlight cosmological consequences of models inspired from string theory or non-perturbative approaches to QG. In particular, I will address the initial singularity, inflation and the late-time accelerated expansion. I will then briefly discuss how recent gravitational waves data can provide a test for some QG models.
Frank Saueressig, Radboud University Nijmegen
Asymptotically Safe Amplitudes from the Quantum Effective Action
This talk will feature a brief introduction to the gravitational asymptotic safety program before reviewing the current status of the field. Motivated by recent developments, I will introduce the form factor formulation of the quantum effective action and explain how various quantum gravity programs can be embedded into this framework. Finally, I will discuss a novel gravity-matter model whose scattering amplitudes exhibit all features expected from Asymptotic Safety.
Sumati Surya, Raman Research Institute
The Quantum Dynamics of Causal Sets: Directions and Challenges
I will begin with a short review of causal set theory (CST) focusing on the features that distinguish it from other approaches to quantum gravity. Most striking is a characteristic non-locality due to the Lorentz non-violating Poisson-discreteness in the continuum approximation. The discrete causal structure is rich enough, however, to extract local continuum geometric information, with geometric and topological observables corresponding to order-invariants in the causal set.
These observables provide the necessary equipment in our search for a suitable quantum dynamics of causal sets. I will focus the rest of the talk on recent progress on this journey and the choices and challenges that lie ahead.
Aron Wall, University of Cambridge
Progress in horizon thermodynamics
I will review some developments in horizon thermodynamics from the past few years, highlighting especially the distinct notions of entropy that seem to apply to dynamically evolving black holes, and their extension from classical to semiclassical gravity.
Silke Weinfurtner, Nottingham University
Towards quantum simulators for fundamental physics
Analogue gravity summarises an effort to mimic physical processes that occur in the interplay between general relativity and field theory in a controlled laboratory environment. The aim is to provide insights in phenomena that would otherwise elude observation: when gravitational interactions are strong, when quantum effects are important, and/or on length scales that stretch far beyond the observable Universe. The most promising analogue gravity systems up-to-date are fluids, superfluids, superconducting circuits, ultra-cold atoms and optical systems. While deepening our understanding of the laboratory systems at hand, the long term vision of analogue gravity studies is to advance fundamental physics through interdisciplinary research, by establishing and nurturing a new culture of collaboration between the various communities involved. I will discuss recent efforts to explore the quantum origin of the Universe, accelerated observer radiation, and rotating black hole physics in the laboratory.