# Program

###### Please note that all times mentioned in this webpage are on Eastern Daylight Time (UTC/GMT -4 hours).

Tuesday, May 4, 2021 – Session I

Tuesday, May 4, 2021 – Session II

Wednesday, May 5, 2021 - Session I

Wednesday, May 5, 2021 - Session II

Thursday, May 6, 2021 - Session I

Thursday, May 6, 2021 - Session II

## Tuesday, May 4, 2021 – Session I

#### Chair: Finnian Gray

**1:00 pm – 1:30 pm**

**Speaker:** Jose Pinedo Soto (University of Alberta)

**Title:** Ultrarelativstic spinning objects in non-local infinite-derivative gravity

**Abstract:** One of the chronic problems in physics is the appearance of singularities and the General Theory of Relativity (GR) is no exception. Even though this is one of the most successful theories we have to describe nature, we find that this issue neglects all of its predictivity at small scales.

This talk will present a non-local yet classical and Lorentz-invariant modification of General Relativity at a small length scale \ell. This is a short-range modification that resolves the singularity problem at the linear level.

I will derive the solutions for a massive spinning pencil-like object boosted at the speed of light under a suitable limit known as the Penrose limit for the non-local gravity scenario. This type of linear solution is especially interesting because on GR they describe an exact solution to the non-linear field equations, however, unlike the classical result the singularity vanishes for these new non-local solutions.

**1:30 pm – 2:00 pm**

**Speaker:** Tore Deniz Boybeyi (Middle East Technical University)

**Title:** More on Fierz-Pauli Massive Gravity

**Abstract:** A brief status quo on modern massive gravitation theories will be presented. Then, we argue that their weak field limit reduces to the already well-known Fierz-Pauli (FP) theory. Extensively studied pathologies in the zero mass limit of FP will be mentioned. Finally, novel discontinues of the FP and their relevance to the large-scale structure of the universe will be presented.

**2:00 pm – 2:30 pm**

**Speaker:** Kam To Billy Chan (Memorial University of Newfoundland)

**Title:** The many marginally outer trapped surfaces of Schwarzschild spacetime

**Abstract:** Despite the constant stream of black hole merger observations, black hole mergers are not fully understood. The details of how the two apparent horizons end up as one horizon is unclear due to the non-linear nature of the merger process. Recent numerical work has shown that there is a merger of self-intersecting Marginally Outer-Trapped Surfaces (MOTS) during the black hole merger. Following papers have investigated further into MOTS in a simpler and static scenario, that of a Schwarzschild black hole. Such cases require less machinery and are solved with everyday computers. Those numerical calculations show an infinite number of self-intersecting MOTS hidden within the apparent horizon. In this talk, I will discuss the current understanding of black hole mergers as has been numerically shown and my work investigating Schwarzschild MOTS in maximally-extended Kruskal-Szekeres coordinates.

**2:30 pm – 3:00 pm**

**Speaker:** Liam Bussey (Memorial University of Newfoundland)

**Title:** Eigenvalues of the MOTS stability operator for slowly rotating Kerr black holes

**Abstract:** We study the eigenvalues of the MOTS stability operator for the Kerr black hole with angular momentum per unit mass |a|<<M. We prove that each eigenvalue depends analytically on a (in a neighbourhood of a=0), and compute its first nonvanishing derivative. Recalling that a=0 corresponds to the Schwarzschild solution, where each eigenvalue has multiplicity 2l+1, we find that this degeneracy is completely broken for nonzero a. In particular, for 0<|a|<<M we obtain a cluster consisting of l distinct complex conjugate pairs and one real eigenvalue. As a special case of our results, we get a simple formula for the variation of the principal eigenvalue. For perturbations that preserve the total area or mass of the black hole, we find that the principal eigenvalue has a local maximum at a=0. However, there are other perturbations for which the principal eigenvalue has a local minimum at a=0.

This is based on a joint work with Dr. Graham Cox and Dr. Hari Kunduri.

**3:00 pm – 3:30 pm**

**Speaker: **Jamie Griffiths (University of Calgary)

**Title:** Subtleties in the Near Horizon Geometry of Spherically Symmetric, Extremal Black Holes in 4D Einstein-Maxwell Theory

**Abstract:** The near horizon geometry (NHG) of an extremal Reissner-Nordstrom black hole is known to be AdS_2 x S^2, but the method for obtaining the NHG is still unclear. The limit to extremality is taken when zooming in on a coordinate patch between the horizons of a non-extremal RN black hole. This process shows that the volume between the two horizons remains finite, despite the horizons coinciding. In this talk, I will present a novel approach to this problem that illustrates two points. First, in the extremal limit, the patch between the horizons "bubbles out" and becomes a fully disconnected spacetime. Second, there is a true near horizon geometry, also AdS_2 x S^2, to the degenerate horizon of the extremal black hole. Both of these points have important implications for extremal black hole entropy, which I hope to be able to discuss by the time of this talk.

## Tuesday, May 4, 2021 – Session II

#### Chair: Erickson Tjoa

**4:00 pm – 4:30 pm**

**Speaker:** Luciano Combi (National University of La Plata)

**Title:** GRMHD simulations of binary neutron star mergers with weak interactions

**Abstract:** Neutron star mergers are prime sources of gravitational and electromagnetic radiation, as well as rich sites of nucleosynthesis. This was demonstrated in the first multi-messenger detection GW1710817 by LIGO and Virgo, where a kilonova and a short gamma ray burst were observed along with gravitational waves. The physics involved in the merger is complex and highly non-linear. In order to make accurate predictions and prepare future observations, we have to rely on numerical simulations that include many important ingredients. In this presentation, we show preliminary results of general relativistic simulations of binary neutron stars in which we include weak interactions, magnetic fields, and realistic nuclear equations of state (EOS). In particular, we investigate equal-mass binaries with different EOS and we characterize the dynamical ejecta and its composition. In the post-merger phase, we analyze the relevance of the magnetic field for angular momentum transport of the accretion disk, along with the role of neutrino emission/absorption for the launching of winds and their composition. Finally, we comment on how these results shed light on the interpretation of current and future observations.

**4:30 pm – 5:00 pm**

**Speaker:** Ramiro Cayuso (University of Waterloo/Perimeter Institute of Theoretical Physics)

**Title:** Numerical simulations on Effective Field theory motivated extensions to General Relativity

**Abstract:** Exploring and understanding departures from General Relativity has long been an important goal of theoretical physics. With the increasing availability of observational data intense efforts have been placed into observing deviations from Einstein's gravity.

A wide range of alternative theories have been proposed, some of whose predictions have been put under tests in different relevant regimes. Most of these predictions have been obtained in the linearized regime, whilst only recently some effort has been put into obtaining predictions in the very important non-linear regime .e.g compact binary coalescence.

In this talk I will present what formal and practical challenges appear when performing numerical simulations in a particular Effective Field Theory motivated extension to General Relativity, and how the implementation of some methods than can help us get accurate predictions in the non-linear regime.

**5:00 pm – 5:30 pm**

**Speaker:** Zhenwei Lyu (University of Guelph)

**Title:** Constraints on Axions from GW170817

**Abstract:** Constraints on axions from GW170817 observed by LIGO and Virgo collaboration, using axion charged binary neutron star waveform model.

**5:30 pm – 6:00 pm**

**Speaker:** Aman Agarwal (University of Guelph/Perimeter Institute of Theoretical Physics)

**Title:** Illuminating the pair-instability supernovae mass gap

**Abstract:** The collapse of a rotating massive star forming a black hole remnant system may lead to r-process material outflow whose astrophysical origin remains a mystery. LIGO has recently reported binary black hole mergers with very massive black holes (> 50 Msun) that reside in the pair-instability supernova (PISN) mass gap. We speculate similar black holes may be formed by these rapidly spinning stars above the mass gap, which as collapsars evolve into the mass gap by losing large amounts (>~10 Msun) of material to massive accretion disk outflows. We estimate the amount of r-process material synthesized in such events and argue that such extreme versions of collapsars, though much rarer, might produce a similar amount of r-process nuclei compared to their ordinary counterparts below the PISN mass gap. The radioactive decay of the nuclei present in such outflows may result in extreme versions of kilonovae, the electromagnetic radiations released due to such decays. If such kilonovae exist, they maybe targeted by future follow-up observations of long gamma-ray bursts or in blind surveys with the Vera Rubin Observatory or the Nancy Grace Roman Space Telescope.

## Wednesday, May 5, 2021 – Session I

#### Chair: Ramiro Cayuso

**1:00 pm – 1:30 pm**

**Speaker:** Dalila Pirvu (University of Waterloo)

**Title:** Bubble Clustering in Vacuum Decay

**Abstract:** False vacuum decay in quantum mechanical first order phase transitions is a phenomenon with wide implications in cosmology, and presents interesting theoretical challenges. In the standard approach, it is assumed that false vacuum decay proceeds through the formation of bubbles that nucleate at random positions in spacetime and subsequently expand. In this paper we investigate the presence of correlations between bubble nucleation sites using a recently proposed semi-classical stochastic description of vacuum decay. This procedure samples vacuum fluctuations, which are then evolved using lattice simulations. We compute the two-point function for bubble nucleation sites from an ensemble of simulations, demonstrating that nucleation sites cluster in a way that is qualitatively similar to peaks in random Gaussian fields. We comment on the implications for first order phase transitions during and after an inflationary era.

**1:30 pm – 2:00 pm**

**Speaker:** Fiona McCarthy (Perimeter Institute of Theoretical Physics/University of Waterloo)

**Title:** Baryonic Feedback Effects in CMB lensing: parameter biases and mitigation

**Abstract:** A key science goal of upcoming cosmic microwave background (CMB) surveys is to measure fundamental physical parameters such as the neutrino mass neutrinos and the number of relativistic particles in the early Universe. To make accurate measurements, our models of the primary CMB and the matter distribution that lenses it must be adequate. Incorrect modelling of the mass distribution responsible for the lensing of the CMB - in particular late Universe effects such as AGN (active galactic nuclei) feedback - can mimic the signals we are searching for. I will discuss the biases we expect from the level of the current modelling uncertainty, and several mitigation techniques to avoid these biases without increasing statistical error.

**2:00 pm – 2:30 pm**

**Speaker:** Mustafa Saeed (University of New Brunswick)

**Title:** Cosmological perturbations with a matter clock

**Abstract:** Cosmology presupposes that on scales of $10^{8}$ light years the universe is the same at every point and in every direction. This is observationally supported by the cosmic microwave background (CMB) which has a temperature of 2.7 Kelvin in all directions. However, there exist small perturbations on this symmetric background - for example the CMB has perturbations of 0.001 Kelvin. A study of these fluctuations is cosmological perturbation theory. In this talk, I will review the standard theory of cosmological perturbations, explain our framework which is canonical in nature and then generalize our treatment to include a matter clock.

**2:30 pm – 3:00 pm**

**Speaker:** Maxence Corman (University of Waterloo/Perimeter Institute of Theoretical Physics)

**Title:** Nonlinear dynamics of Flux Compactification

**Abstract:** One proposed framework for the accelerated expansion of the Universe is string/M-theory, where extra dimensions are required for consistency. One of the dominant mechanism to hide extra dimensions is the spatial compactification on small length scales.

We focus on a simple model of flux compactification: Einstein-Maxwell theory in D-dimensions with a positive cosmological constant and a q-form field strength such that the extra dimensions are stably compactified on a sphere.

This model not only allows highly symmetric solutions, where the sphere is stabilized against collapse by a uniform field strength, but also warped solutions where the compact space is inhomogeneous.

We perform a fully non-linear evolution of these stationary or homogeneous solutions.

We find a regime where the perturbatively unstable homogeneous solutions evolve to the stable stationary warped solutions. Outside this range of the parameter space we find that unstable solutions evolve towards a singular state, overshooting stable solutions in some cases. We discuss cosmological implications for the effective four-dimensional universe.

## Wednesday, May 5, 2021 – Session II

#### Chair: Nils Siemonsen

**4:00 pm – 4:30 pm**

**Speaker:** Samantha Hergott (York University)

**Title:** Black hole singularity resolution via the modified Raychaudhuri equation in Loop Quantum Gravity

**Abstract:** By deriving Loop Quantum Gravity corrections to the Raychaudhuri equation in the interior of a Schwarzschild black hole and near the classical singularity, we show that such LQG effects result in defocusing of the geodesics close to the singularity and lead to its resolution. This is due to the appearance of repulsive terms in the effective Raychaudhuri equation which prevent the formation of conjugate points and regulate the curvature invariants.

**4:30 pm – 5:00 pm**

**Speaker:** Keagan Blanchette (York University)

**Title:** An investigation of the quantum gravity corrections to the Raychaudhuri equation via the generalized uncertainty principle

**Abstract:** We compute the quantum gravitational corrections to the Raychaudhuri equation for the interior of a Schwarzchild spacetime, arising from deforming the algebra of this system. Such a deformation is inspired by the Generalized Uncertainty Principle. The correction terms to the Raychaudhuri equation prevent the formation of singularity in this spacetime and effectively resolve it. We consider various scenarios in which such a resolution takes place in this framework and also compare our results with similar recent results obtained in Loop Quantum Gravity.

**5:00 pm – 5:30 pm**

**Speaker:** Erickson Tjoa (University of Waterloo)

**Title:** Harvesting correlations in Schwarzschild and collapsing shell spacetimes

**Abstract:** We study the harvesting of correlations by two Unruh-DeWitt static detectors from the vacuum state of a massless scalar field in a background Vaidya spacetime consisting of a collapsing null shell that forms a Schwarzschild black hole (hereafter Vaidya spacetime for brevity), and we compare the results with those associated with the three preferred vacua (Boulware, Unruh, Hartle-Hawking-Israel vacua) of the eternal Schwarzschild black hole spacetime. To do this we make use of the explicit Wightman functions for a massless scalar field available in (1+1)-dimensional models of the collapsing spacetime and Schwarzschild spacetimes, and the detectors couple to the proper time derivative of the field. First we find that, with respect to the harvesting protocol, the Unruh vacuum agrees very well with the Vaidya vacuum near the horizon even for finite-time interactions. Second, all four vacua have different capacities for creating correlations between the detectors, with the Vaidya vacuum interpolating between the Unruh vacuum near the horizon and the Boulware vacuum far from the horizon. Third, we show that the black hole horizon inhibits \textit{any} correlations, not just entanglement. Finally, we show that the efficiency of the harvesting protocol depend strongly on the signalling ability of the detectors, which is highly non-trivial in presence of curvature. We provide an asymptotic analysis of the Vaidya vacuum to clarify the relationship between the Boulware/Unruh interpolation and the near/far from horizon and early/late-time limits. We demonstrate a straightforward implementation of numerical contour integration to perform all the calculations.

**5:30 pm – 6:00 pm**

**Speaker:** Greg Kaplanek (McMaster University)

**Title:** Hot Qubits on the Horizon

**Abstract:** Perturbation theory for gravitating quantum systems tends to fail at very late times (a type of perturbative breakdown known as secular growth). We argue that gravity is best treated as a medium/environment in such situations, where reliable late-time predictions can be made using tools borrowed from quantum optics. To show how this works, we study the explicit example of a qubit hovering just outside the event horizon of a Schwarzschild black hole (coupled to a real scalar field) and reliably extract the late-time behaviour for the qubit state. At very late times, the so-called Unruh-DeWitt detector is shown to asymptote to a thermal state at the Hawking temperature. (based on the article [arXiv:2007.05984])

**6:00 pm – 6:30 pm**

**Speaker:** Finnian Gray (Perimeter Institute of Theoretical Physics/University of Waterloo)

**Title:** Quantum imprints of gravitational shockwaves

**Abstract:** Gravitational shockwaves are simple exact plane wave solutions to the Einstein field equations which provide a testing ground for theoretical aspects of gravitational waves, in particular the gravitational memory effect. This effect is intimately connected to supertranslations which preserve asymptotically flat spacetimes and are encoded in the BMS group. Recently it has been claimed in the literature that shockwaves leave no imprint on the vacuum states of a massless scalar field and, in particular, do not change the thermal spectrum of Unruh radiation for an accelerating observer.

The aim of this work is to clarify their imprints on a quantum field via an Unruh--DeWitt detector. Namely, in the context of this scenario we will see that it is possible for a single detector to observe a shock wave and that the entanglement harvesting protocol can extract information of gravitational shockwaves from the vacuum sate of a massless scalar.

## Thursday, May 6, 2021 – Session I

#### Chair: Greg Kaplanek

**1:00 pm – 1:30 pm**

**Speaker:** Yilber Fabian Bautista (Perimeter Institute of Theoretical Physics/York University)

**Title:** From Scattering in Kerr Backgrounds toHigher-Spin Amplitudes

**Abstract:** The scattering of massless waves of helicity $|h|=0,\frac{1}{2},1,2$ in Schwarzschild and Kerr backgrounds is revisited in the long-wavelenght regime. The Newman-Penrose scattering amplitudes arising from the Black Hole Perturbation Theory (BHPT) framework are found in agreement with the classical limit of QFT amplitudes at finite values of the scattering angle and arbitrary spin orientation. The latter amplitudes are obtained from on-shell methods and describe the $2\to 2$ scattering of a massless particle of helicity $|h|$ with a massive particle of arbitrary spin $S$, where $S=0$ corresponds to the Schwarzschild case. In the particular limit of small scattering angle, classical observables such as the scattering cross-section, impact parameter and time delay are studied for all helicity combinations at all orders in spin, matching previous eikonal results. The effect of the black hole spin in the polarization of the waves is found in agreement with previous analysis. Finally, unitarity constraints based on partial amplitudes and positive time delay are also discussed.

**1:30 pm – 2:00 pm**

**Speaker:** Nils Siemonsen (Perimeter Institute of Theoretical Physics)

**Title:** Non-linear evolution of the ergoregion instability in boson stars

**Abstract:** Rotating horizonless ultra-compact objects can exhibit an ergoregion; a region of spacetime in which all observers are forced to co-rotate. In these ergoregions, an asymptotically timelike Killing field turns spacelike, indicating that energies, measured by an observer at infinity, can be negative. Consequently, the energy of massless test fields is no longer bounded from below and a cascade into lower and lower energy states ensues - the ergoregion instability emerges. We study the non-linear development of the instability in highly compact scalar boson stars by evolving the coupled system of Einstein-Klein-Gordon equations numerically in axisymmetry. We characterize the linear instability phase, the final saturated state of the system, and radiated gravitational and scalar waves.

**2:00 pm – 2:30 pm**

**Speaker:** Fred Tomlinson (University of Edinburgh)

**Title:** New existence results for 5d vacuum black holes

**Abstract:** The classification of equilibrium black holes is of fundamental interest in general relativity. The 4d vacuum case is well understood - all stationary, axisymmetric and asymptotically flat solutions are given by the Kerr metric. In 5d the picture is much less clear; whilst there is a uniqueness result for stationary, biaxisymmetric and asymptotically flat solutions, the corresponding existence problem is still open. In this talk I will discuss some recent results which give new tools for answering this problem. Using the integrability of this sector of gravity one can translate the problem into that of determining whether a particular system of polynomial equations has any solutions. These equations can be studied in the case of the simplest black lens - a black hole with lens space topology. A combination of analytic and numerical results show that there are no solutions to these equations, demonstrating that these spacetimes must be singular. This represents an important step towards the full classification in 5d.

**2:30 pm – 3:00 pm**

**Speaker:** Turkuler Durgut (Memorial University of Newfoundland)

**Title:** Asymptotically Anti-de Sitter Gravitational Solitons

**Abstract:** In this talk, I will consider the stability of asymptotically anti-de Sitter gravitational solitons. These are globally stationary, asymptotically (globally) AdS spacetimes with positive energy but without horizons. I will introduce my ongoing project investigating solutions of the linear wave equation in this class of backgrounds. I will provide analytical expressions for the behavior of the scalar field near the soliton bubble and at spatial infinity. The special BPS (supersymmetric) case will then be examined as an example of a solution where stable trapping occurs. This project is joint work with Dr. Hari K. Kunduri and Dr. Robie A. Hennigar.

## Thursday, May 6, 2021 – Session II

#### Chair: Fiona McCarthy

**4:00 pm – 4:30 pm**

**Speaker:** Xinyu Li (Canadian Institute for Theoretical Astrophysics/Perimeter Institute of Theoretical Physics)

**Title:** Neutrino Fast Flavor Conversions in Neutron-star Post-Merger Accretion Disks

**Abstract:** A compact accretion disk may be formed in the merger of two neutron stars or of a neutron star and a stellar-mass black hole. Outflows from such accretion disks have been identified as a major site of rapid neutron-capture (r-process) nucleosynthesis and as the source of 'red' kilonova emission following the first observed neutron-star merger GW170817. We present long-term general-relativistic radiation magnetohydrodynamic simulations of a typical post-merger accretion disk at initial accretion rates of 1Msun/s over 400ms post-merger. We include neutrino radiation transport that accounts for effects of neutrino fast flavor conversions dynamically. We find ubiquitous flavor oscillations that result in a significantly more neutron-rich outflow, providing lanthanide and 3rd-peak r-process abundances similar to solar abundances. This provides strong evidence that post-merger accretion disks are a major production site of heavy r-process elements. A similar flavor effect may allow for increased lanthanide production in collapsars. The formalism presented here may also be used in simulations of core-collapse supernovae to explore whether fast conversions strengthen or weaken the explosion.

**4:30 pm – 5:00 pm**

**Speaker:** Andrea Giusti (ETH Zurich)

**Title:** Thermodynamics of scalar-tensor gravity

**Abstract:** In the 90s it was shown that the Einstein equation could be understood as an equation of state, general relativity as the equilibrium state of gravity, and f(R) gravity as a non-equilibrium one. In this presentation I discuss how the application of Eckart's first order thermodynamics to the effective dissipative fluid describing scalar-tensor gravity leads to the formulation of a thermodynamics for the space of theories of gravity. Surprisingly, within this picture one obtains simple expressions for the effective heat flux, "temperature of gravity", shear and bulk viscosity, and entropy density, plus a generalized Fourier law in a consistent Eckart thermodynamical picture. Well-defined notions of temperature and approach to equilibrium, missing in the current thermodynamics of spacetime scenarios, naturally emerge.

**5:00 pm – 5:30 pm**

**Speaker:** Juan Margalef Bentabol (Penn State University)

**Title:** Covariant phase space of gravity with boundaries: metric vs tetrad formulation

**Abstract:** We use covariant phase space methods to study the metric and tetrad formulations of General Relativity in a manifold with boundary and compare the results obtained in both approaches. Proving their equivalence has been a long-lasting problem that we solve here by using the cohomological approach provided by the relative bicomplex framework. This allows us to prove the equivalence without having to make any ad hoc choices. We also compute several relevant charges in both schemes and show that, as expected, they are equivalent.

**5:30 pm – 6:00 pm**

**Speaker:** Sharmila Gunasekaran (University of Alberta)

**Title:** Slow decay of waves in gravitational solitons

**Abstract:** Gravitational solitons are globally stationary, horizonless asymptotically flat spacetimes with positive energy. They have generated attention in theoretical high-energy physics as possible classical microstate geometries of black holes. A natural question to consider is whether they are stable. In this talk, I will address stability at the simplest level by investigating solutions to the linear wave equation in a particular soliton spacetime. I will describe a methodology, introduced by Holzegel-Smulevici to prove that massless scalar waves in a particular family of soliton spacetimes cannot decay faster than inverse logarithmically in time. The proof involves the construction of quasimodes which are approximate solutions to the wave equation. This slow decay can be attributed to the stable trapping of null geodesics and is suggestive of instability at the nonlinear level. This is joint work with Hari Kunduri.