Welcome and Opening Remarks

• Niayesh Afshordi (University of Waterloo & Perimeter Institute)

Room: PI/4-405 - Bob Room

Quantum Gravity and its connection to observations

• Astrid Eichhorn (University of Southern Denmark)

Room: PI/4-405 - Bob Room To make progress in developing a quantum theory of gravity, we need to connect candidate theories to observations. I will review ideas on connecting quantum gravity to observations in particle physics, to searches for dark matter and to observations of black holes, in particular with the (next-generation) Event Horizon Telescope.

Imaging Supermassive Black Holes and mapping spacetime

• Roman Gold (University of Southern Denmark)

Room: PI/4-405 - Bob Room I will give an overview of how the Event Horizon Telescope achieves its horizon scale science and what's to come. I will also review selected recent results from the Event Horizon Telescope both on Sgr A* and refined analysis of M87*. A focus will be on analysis aspects that are relevant for any theory / model building along with a few examples. The presentation aims to provide key conceptual aspects relevant to gravity experts who are new to VLBI.

The Spacetime of Acceleration

• Ruth Gregory (King's College, London)

Room: PI/4-405 - Bob Room I will review recent work on accelerating spacetimes in 3D, explaining how acceleration manifests for point particles and BTZ black holes, as well as a novel BTZ-like solution in a disconnect region of parameter space. I will also discuss some holographic aspects of the solutions.

What is the simplicity of the early universe trying to tell us?

• Latham Boyle (Perimeter Institute)

Room: PI/4-405 - Bob Room "After reviewing some key hints and puzzles from the early universe, I will introduce recent joint work with Neil Turok suggesting a rigid and predictive new approach to addressing them. Our universe seems to be dominated by radiation at early times, and positive vacuum energy at late times. Taking the symmetry and analyticity properties of such a spacetime seriously leads to a new formula for the gravitational entropy of our universe, and a picture in which the Big Bang may be regarded as a kind of mirror. I will explain how this line of thought suggests new explanations for a number of observed properties of the universe, including: its homogeneity, isotropy and flatness; the arrow of time (i.e. the fact that entropy increases *away* from the bang); several properties of the primordial perturbations; the nature of dark matter (which, in this picture, is a right-handed neutrino, radiated from the early universe like Hawking radiation from a black hole); the origin of the primordial perturbations; and even the existence of three generations of standard model fermions. I will discuss some observational predictions that will be tested in the coming decade, and some key open questions."

Are we considering enough? Inclusivity in Quantum Gravity and Cosmology

• Jarita Holbrook (University of Edinburgh)

Room: PI/4-405 - Bob Room

Panel Discussion I: Where and when will Quantum Gravity crack? What are we missing? Room: PI/4-405 - Bob Room

Emergent Metric Space-Time from the BFSS Matrix Model

• Robert Brandenberger (McGill University)

Room: PI/4-405 - Bob Room Effective field theory analysis are inadequate to describe the evolution of the very early universe. I will describe an approach to obtain emergent time, emergent space, an emergent metric and an emergent early universe cosmology starting from the BFSS matrix model, a proposed non-perturbative definition of superstring theory. In this approach there is no cosmological constant problem.

Where effective field theory fails: Windows into quantum gravity?

• Adam Koberinski (University of Pittsburgh)

Room: PI/4-405 - Bob Room Effective field theory is a computationally powerful and flexible theoretical framework, finding application in many areas of physics. In particle physics, Weinberg’s folk theorem also promises that any theory that reproduces the predictions of the Standard Model will, at low energies, look like an effective field theory. In one sense the power of this framework should be a cause for pride in the progress of physics and discovery of real structural features of the world. But given the inability to fully unite quantum theory and gravity into a consistent theoretical picture, there is also cause for pessimism: indirect tests of candidate theories of quantum gravity will ultimately reduce to something like an effective field theory, undermining efforts to find low-energy windows into new physics. One way around this pessimism is to look at where and why effective field theory breaks down in current physical theories. I will point to familiar breakdowns (the cosmological constant, inflation, the hierarchy problem), offering a take on what these breakdowns tell us about the shape of physics beyond the Standard Model. Cracks in the wall of effective field theory allow for a dim glimpse of what might lie beyond.

Experimental Cosmology

• Matthew Johnson (Perimeter Institute & York University)

Room: PI/4-405 - Bob Room

Final but Incomplete?

• Richard Dawid (Stockholm University)

Room: PI/4-405 - Bob Room String theory has not come close to a complete formulation after half a century of intense research. On the other hand, a number of features of the theory suggest that the theory in its complete form may be a final theory. The combination of conceptual incompleteness and allusions to finality seems difficult to grasp. Two main points are made in this talk. First, it is pointed out that finality claims in the context of string theory are motivated in a fundamentally different way than traditional claims of finality one finds in earlier physics. Second, it is argued that finality and chronic conceptual incompleteness may be related to each other in a string theory context in an interesting way. The talk ends with discussing possible implications of this situation for the long-term prospects of theory building in fundamental physics.

The role of singularities in the search for quantum gravity

• Karen Crowther (University of Oslo)

Room: PI/4-405 - Bob Room Singularities in general relativity and quantum field theory are often taken not only to motivate the search for a more-fundamental theory (quantum gravity), but also to characterise this new theory and shape expectations of what it is to achieve. In this talk, I will explore how different types of singularities play a role in the search for quantum gravity, and how different `attitudes' towards singularities can lead to different scenarios for the new theory. [Based on joint work with Sebastian DeHaro].

Conversations & controversies in the early universe

• Phil Halper (Independent)

Room: PI/4-405 - Bob Room For the last ten years I have been documenting various scenarios for the early universe in a YouTube series called ‘Before the Big Bang”. Having interviewed many of the leader figures of the field including Stephen Hawking, Roger Penrose, Alan Guth (and hosted debates between them), this will be a broad survey of inflation, it’s suggested prequels and alternatives. I shall highlight the strengths and weaknesses of the various proposals and give an inside track of the claims and counter claims in attempts to move beyond the standard Big Bang model.

Panel Discussion II: Do we need Quantum Gravity to crack the Big Bang? What about inflation? fact or fiction? Room: PI/4-405 - Bob Room

Universality of black hole thermodynamics

• Samir Mathur (Ohio State University)

Room: PI/4-405 - Bob Room

Causality at the End-of-World

• Beni Yoshida (Perimeter Institute)

Room: PI/4-405 - Bob Room The success of the AdS/CFT correspondence motivates a holographic approach for spacetime beyond AdS, including our own universe. One possible method involves using an asymptotically-AdS holography and introducing a finite radial cutoff by inserting an End-of-World (EoW) brane. However, previous studies have shown that this leads to nonlocal effects on the boundary and violates entanglement sub-additivity. In this work, we address these issues by examining a two-particle scattering process through the lens of holographic quantum tasks. Our findings suggest that connectedness of entanglement wedge does indeed require nonlocal domain of dependence, but that violation of sub-additivity can be avoided. We also discuss an important question that arises from our results, namely whether the non-locality on the EoW brane is real or apparent. We argue that it is the latter. This talk is based on ongoing work with Takato Mori.

Do First-Class Constraints Generate Gauge Transformations? A Geometric Perspective.

• Clara Bradley (University of California, Irvine)

Room: PI/4-405 - Bob Room The constrained Hamiltonian formalism is the basis for canonical quantization techniques. However, there are disagreements surrounding the notion of a gauge transformation in such a formalism. The standard definition of a gauge transformation in the constrained Hamiltonian formalism traces back to Dirac: a gauge transformation is a transformation generated by an arbitrary combination of first-class constraints. On the basis of this definition, Dirac argued that one should extend the form of the Hamiltonian in order to include all of the gauge freedom. However, Pitts (2014) argues that in some cases, a first-class constraint does not generate a gauge transformation, but rather "a bad physical change". Similarly, Pons (2005) argues that Dirac's analysis of gauge transformations is "incomplete" and does not provide an account of the symmetries between solutions. Both authors conclude that extending the Hamiltonian in the way suggested by Dirac is unmotivated. If correct, these arguments could have implications for other issues in the foundations of the constrained Hamiltonian formalism, including the Problem of Time. In this talk, I use a geometric formulation of the constrained Hamiltonian formalism to show that one can motivate the extension to the Hamiltonian independently from consideration of the gauge transformations, and I argue that this supports the standard definition of a gauge transformation without falling prey to the criticisms of Pitts (2014) and Pons (2005). Therefore, in order to maintain that first-class constraints do not generate gauge transformations, one must reject the claim that the constrained Hamiltonian formalism is fully described by the geometric picture; I suggest two avenues for doing so.

Quantum Superpositions of Black Holes

• Robert Mann (University of Waterloo)

Room: PI/4-405 - Bob Room If relativistic gravitation has a quantum description, it must be meaningful to consider a spacetime metric in a genuine quantum superposition. Here I present a new operational framework for studying “superpositions of spacetimes” via model particle detectors. After presenting the general approach, I show how it can be applied to describe a spacetime generated by a BTZ black hole in a superposition of masses and how such detectors would respond. The detector exhibits signatures of quantum-gravitational effects reminiscent of Bekenstein’s seminal conjecture concerning the quantized mass spectrum of black holes in quantum gravity. I provide further remarks in distinguishing spacetime superpositions that are genuinely quantum-gravitational, notably with reference to recent proposals to test gravitationally-induced entanglement, and those in which a putative superposition can be re-expressed in terms of dynamics on a single, fixed spacetime background.

In Search of Lost Spacetime

• Chris Smeenk (Western University)

Room: PI/1-100 - Theatre The classical spacetime manifold of general relativity disappears in quantum gravity, with different research programs suggesting a variety of alternatives in its place. As an illustration of how philosophers might contribute to an interdisciplinary project in quantum gravity, I will give an overview of recent philosophical debates regarding how classical spacetime "emerges." I will criticize some philosophers as granting too much weight to the intuition that a coherent physical theory must describe objects as located in space and time. I will further argue, based in part on historical episodes, that an account of emergence needs to recover the structural features of classical GR responsible for its empirical success. This is more demanding than it might at first appear, although the details of recovery will differ significantly among different approaches to quantum gravity.

Holographic Cosmology

• Paul McFadden (Newcastle University)

Room: PI/4-405 - Bob Room Holography has profoundly transformed our understanding of quantum gravity in spacetimes with asymptotic negative curvature. Its implications for cosmology are equally profound, suggesting that time is emergent and that our universe has a dual description in terms of a three-dimensional quantum field theory. This talk will outline key features of holographic cosmology, from the perspective it offers for the cosmic singularity to the strategies it presents for computing cosmological observables. Recent results for the de Sitter wavefunction will be discussed and their interpretation in the language of three-dimensional conformal field theory.

Panel Discussion III: Holography: fact or fiction? Room: PI/4-405 - Bob Room

Echoes and Entropy of Quantum Black Holes

• Naritaka Oshita (Kyoto University)

Room: PI/4-405 - Bob Room It has been proposed that quantum-gravitational effects may change the near-horizon structure of black holes, e.g. firewalls or ultra-compact objects mimicking black holes. Also, a Lorentz-violating theory as a candidate of quantum gravity, e.g. the Horava-Lifshitz theory, changes the causal structure of black holes due to the superluminal propagation of excited modes. The late-time part of the gravitational wave ringdown from a black hole is significantly affected by those effects, and the emission of gravitational wave echoes may be induced. The black hole quasi-normal (QN) modes are affected by the change of the horizon structure, which results in the drastic modification of the late-time signal of the gravitational wave. In this talk, I will discuss how the gravitational wave echo can be modeled and how the echo model is reasonable from an entropic point of view by counting QN modes to estimate the black hole entropy.

Looking for Echoes from Quantum Black Holes

• Jahed Abedi (University of Stavanger)

Room: PI/4-405 - Bob Room I present an unprecedented template-based search for stimulated emission of Hawking radiation (or Boltzmann echoes) by combining the gravitational wave data from 65 binary black hole merger events observed by the LIGO/Virgo collaboration. With a careful Bayesian inference approach, I found no statistically significant evidence for this signal in either of the 3 Gravitational Wave Transient Catalogs GWTC-1, GWTC-2 and GWTC-3. However, the data cannot yet conclusively rule out the presence of Boltzmann echoes either, with the Bayesian evidence ranging within 0.3-1.6 for most events, and a common (non-vanishing) echo amplitude for all mergers being disfavoured at only 2:5 odds. The only exception is GW190521, the most massive and confidently detected event ever observed, which shows a positive evidence of 9.2 for stimulated Hawking radiation. An optimal combination of posteriors yields an upper limit of A<0.42 (at 90% confidence level) for a universal echo amplitude, whereas A∼1 was predicted in the canonical model. The next generation of gravitational wave detectors such as LISA, Einstein Telescope, and Cosmic Explorer can draw a definitive conclusion on the quantum nature of black hole horizons.

The H0 tension and sample variance

• Will Percival (University of Waterloo & Perimeter Institute)

Room: PI/4-405 - Bob Room Local measurements of the expansion rate of the local universe differ from predictions of simple models fitted to large-scale cosmological measurements, at a statistically significant level. Sample variance (often called cosmic variance) is a key component of errors placed on measurements made from a small data set. For the Hubble constant, which parametrises the expansion rate, the size of the patch of the Universe covered by recent supernovae observations has a radius of 300Mpc. The smaller the patch, the larger the patch-to-patch fluctuations and the larger the error on the measured value of H0 from sample variance. Using the H0 measurement from supernovae as an example, I will consider a number of different ways to estimate sample variance using techniques developed for multiple uses, and show that they all approximately agree. The sample variance error on H0 measurements from the recent Pantheon supernovae sample is +/-1 kms^-1Mpc^-1, insufficient to explain the Hubble tension in a standard Lambda-CDM universe. This will demonstrate methods for comparing variations in expansion rate in the universe and what we mean by saying the universe is expanding (on average), or that galaxies move apart with particular velocities.

The S8 tension in cosmology

• Mike Hudson (University of Waterloo )

Room: PI/4-405 - Bob Room In addition to the now-well known “Hubble tension”, in recent years a second tension has emerged: the $S_8$ tension. This is a measure of the homogeneity of the Universe. Specifically, $S_8$ is defined as $(\Omega_{\mathrm matter}/0.3)^{0.5} \sigma_8$ where $\sigma_8$ is the standard deviation of the density fluctuation in an 8 $h^{-1}$ Mpc radius sphere. As with the Hubble tension, there is disagreement, at greater than 4 $\sigma$ significance between what is predicted by extrapolating the fluctuations in the Cosmic Microwave Background forward to the present day, and what is measured by multiple probes of the inhomogeneity in the nearby Universe. I will discuss the diverse lines of evidence for the tension, showing it is not restricted to one probe, but is seen in weak gravitational lensing, peculiar velocities and redshift-space distortions and cluster abundances. I will conclude by discussing prospects for future measurements.

Nonlocality in the causal set approach to quantum gravity

• Fay Dowker (Imperial College London)

Room: PI/4-405 - Bob Room The nonlocality of causal sets gives us hope of solving the cosmological constant puzzle (``why is the universe so smooth, big and old if there is only one scale---the discreteness scale---in the theory?’’) On the other hand locality, , GR and local QFT, must be recovered from quantum gravity in the continuum approximation at large scales, which is a challenge. If we are lucky though (like Goldilocks, the universe gets the nonlocality ``just right’’) nonlocality may be a rich source of phenomenology. Yasaman Yazdi’s talk will be on cosmological models based on the nonlocality of causal sets. I will give a couple of examples of more astrophysical phenomenological models based on simple assumptions---randomness due to spacetime uncertainty and Lorentz invariance---and pose a research question: is there a model of ``quantum swerves’’?

Everpresent Λ Cosmology

• Yasaman Yazdi (Imperial College London)

Room: PI/4-405 - Bob Room I will discuss a number of theoretical, observational, and conceptual aspects of the Everpresent Λ cosmological model arising from fundamental principles in causal set theory and unimodular gravity. In this framework the value of the cosmological constant (Λ) fluctuates, in magnitude and in sign, over cosmic history. At each epoch, Λ stays statistically close to the inverse square root of the spacetime volume. Since the latter is of the order of H^2 today, this provides a way out of the cosmological constant puzzle without fine tuning. I will review the theoretical background of this idea. I will also describe a phenomenological implementation of this model, and discuss recent results on the statistics of its simulations and observational tests of it.

Panel Discussion IV: UV/IR coupling in Quantum Gravity: Feature or Bug?[ Room: PI/4-405 - Bob Room

Physics with non-perturbative quantum gravity: the end of the Hawking evaporation

• Carlo Rovelli (Centre de Physique Théorique (CPT))

Room: PI/4-405 - Bob Room Quantum gravitational phenomena dominate the physics of a black hole in the past of a high curvature spacelike surface. Because of the backreaction of the evaporation, this surface crosses the horizon. I describe a recent line of investigation on the possible evolution compatible with nonperturbative quantum gravity, and in particular I illustrate what can be predicted using loop quantum gravity.

Primordial fluctuations from quantum gravity

• Francesca Vidotto (Western University)

Room: PI/4-405 - Bob Room In modern cosmology there is an agreement that the seeds of structure formations resides in the quantum fluctuation of the geometry in the early universe, but there is no agreement about how these could be derived from a quantum theory of gravity. In this talk I present a proposal based on the covariant formulation of Loop Quantum Gravity. I describe how to define a wavefunction of the universe in this context, and a how we can study fluctuations and correlations between spacial regions. The results obtained so far has been made possible by recent progress in numerical computations. I discuss the current state of this research program and the possible implications for modeling the early universe.

Analogical reasoning in quantum gravity

• Doreen Fraser (University of Waterloo)

Room: PI/4-405 - Bob Room Analogical reasoning has a long and distinguished history as a method for making discoveries in physics. I will discuss novel uses of formal analogies in twentieth century particle physics and condensed matter physics. I will then offer some reflections on how methodological lessons from these cases could inform the use of analogies in discoveries related to quantum gravity.

The emergence of spacetime is governed by a quantum Mach's principle

• Lee Smolin (Perimeter Institute)

Room: PI/4-405 - Bob Room "I describe a candidate for a fundamental physical theory called the causal theory of views. This describes a world constructed by a continual creation of events; where an event is a transition at which a small portion of the possible becomes actual. I first recall older results which includes the emergence of space and, with space, a non-relativistic N-body quantum dynamics. I next describe recent progress on this model including, in a different limit, a formulation of a cut off quantum field theory, which we describe in terms of an S-Matrix formulation of amplitudes. The dynamics is specified by an action principle consisting of a kinetic energy and potential energy term. The former are based on measures of how quickly components of causal change do so with respect to averaged notions. The potential energy terms measure how much local moves alter an observer's ""view"" of the universe, as seen from their perspective. These results show that quantum dynamics is restored in an N to infinity limit. Measurable non-linear corrections to quantum dynamics emerge to higher order in 1/sqrt{N}. "

Wrap Up and Discussion: An inclusive roadmap for cracking Quantum Gravity

• Steve Weinstein (University of Waterloo)

Room: PI/4-405 - Bob Room