Conference Abstracts

Conference Abstracts

CPT Symmetric Universe
Latham Boyle
Perimeter Institute for Theoretical Physics

We propose that the state of the universe does not spontaneously violate CPT. Instead, the
universe before the Big Bang is the CPT reflection of the universe after the bang. Phrased another way, the universe before the bang and the universe after the bang may be re-interpreted as a universe/anti-universe pair, created from nothing. CPT selects a unique vacuum state for the QFT on such a spacetime, which leads to a new perspective on the cosmological baryon asymmetry, and a new explanation for the observed dark matter abundance. In particular, if we assume that the matter fields in the universe are described by the standard model of particle physics (including right-handed neutrinos), we predict that one of the heavy neutrinos is stable, and that its density automatically matches the observed dark matter density if its mass is 4.8×10 8 GeV. Among other predictions, we have: (i) that the three light neutrinos are majorana; (ii) that the lightest of these is exactly massless; and (iii) that there are no primordial long-wavelength gravitational waves. We mention connections to the strong CP problem and the arrow of time.

Simultaneous baldness and cosmic baldness and the Kottler spacetime
Valerio Faraoni
Bishop's University

The uniqueness of the Kottler/Schwarzschild-de Sitter solution (KSdS) of the vacuum Einstein equation with positive cosmological constant is discussed and certain putative alternatives in the literature are shown to either solve different equations or to be the KSdS solution in disguise. A simultaneous no-hair and cosmic no-hair theorem for the KSdS geometry in the presence of an imperfect fluid is proved.

Natural inflation from polymer quantization
Sanjeev Seahra
University of New Brunswick

We show how the semi-classical polymer quantization of chaotic inflation leads to a natural inflation-like scalar field potential with an approximate shift symmetry.  The particular polymer quantization scheme used is distinct from those appearing previously in the literature in that the operator for the field amplitude is ill-defined, rather than that of the field momentum.  Unlike the latter scenario, our model does not remove the big bang past attractor; i.e., there is no cosmological singularity avoidance.

Visualizing spacetime curvature
Robert Scott
Université de Bretagne Occidentale

A diagram is explored wherein spacetime is cut into approximately Minkowski pieces or ``wedges" and plotted in physical coordinates. The curvature of the boundaries of the wedges reveals the curvature of the spacetime. This wedge diagram provides a more general tool to visual spacetime curvature than the familiar embedding diagram, the latter being restricted to spatial curvature only. The wedge diagram is applied first to the Schwarzschild spacetime to demonstrate the physical significance of the curvature of spacetime. In particular it is shown how gravitational time dilation results from curvature of a radial-time slice of spacetime. Then this diagram is applied to the Friedmann-Robertston-Walker (FRW) spacetime revealing how expansion of the universe results from a qualitatively similar curvature of a radial-time slice, but with the roles of space and time reversed. The analogy between the two spacetimes suggests novel interpretations of the two spacetimes. We interpret the Schwarzschild spacetime as time expanding with increasing distance from the horizon. And we interpret the FRW spacetime as the universe expanding as the influence of the big bang peters out.
Tracing geodesics is an instructive exercise on the wedge diagrams, the turning of the boundaries between wedges providing a visual interpretation of the Christoffel symbols. We examine circular orbits in Schwarzschild to show how the dominant time curvature leads to turning of the wedge boundaries that balances the centripetal acceleration, resulting in acceleration-free circular spiral motion.

Black Hole Echology
Qingwen Wang
Perimeter Institute for Theoretical Physics

While recent detections of gravitational waves from the mergers of binary black holes match well with the predictions of General Relativity (GR), they cannot directly confirm the existence of event horizons. Exotic compact objects (ECOs) are motivated by quantum models of black holes, and can have exotic structure (or a “wall”) just outside the (would-be) horizon. ECOs produce similar ringdown waveforms to the GR black holes, but they are followed by delayed “echoes”. By solving linearized Einstein equations we can model these echoes and provide analytic templates that can be used to compare to observations. For concreteness, we consider GW150914 event, detected by the LIGO/Virgo collaboration, and study the model dependence of its echo properties. We find that echoes are reasonably approximated by complex gaussians, with amplitudes that decay as a power law in time, while their width in time (frequency) grows (shrinks) over subsequent echoes. We also show that trapped modes between a perfectly reflecting wall and angular momentum barrier in Kerr metric can exhibit superradiant instability over long times, as expected.


Modelling The Gravitational Collapse of Scalar Fields in Anti-de Sitter Spacetime
Brad Cownden
University of Manitoba

For phases like quark-gluon plasmas, the strong-coupling nature of the system means that perturbative approximations are invalid, and therefore conventional solution methods break down. However, using a duality first established by string theory, we are able to relate strongly coupled quantum field theories to weakly coupled gravitational systems (in one higher dimension). The most common use of this hidden relationship is to map between special quantum field theories — known as Conformal Field Theories (CFTs) — and general relativity in anti-de Sitter (AdS) space.

Motivated by the AdS/CFT correspondence, we examine the thermalization of a CFT through its dual description: the formation of a black hole in AdS. We numerically evolve the full Einstein equations in the presence of both massless and massive scalar fields for a variety of initial momentum profiles. The curvature of AdS is such that massless fields are able to travel to spatial infinity and back in finite time, and therefore these fields have multiple opportunities to collapse. Massive fields do not travel to infinity, but do undergo periodic motion that may lead to horizon formation at long times. 

The interplay between the initial conditions and the geometry of the space lead to a landscape of collapse behaviour that will be explored in this talk. Using the highest resolution available, we are able to extend our numerical results into amplitude regimes that are described by a perturbative theory. For certain initial profiles, the prediction of the perturbative theory — that AdS space is stable to black hole formation in this regime — is at odds with the numerical data. We will make comments on how this discrepancy may be resolved, and how the resolution could bring about significant improvements in modelling the formation of black holes from massless and massive scalar fields.

Exceptional H3 Cosmological Models.
Sepehr Rashidi
University of Waterloo

We consider inhomogeneous perfect fluid cosmological models with spacetimes that admit H3, Abelian G2 with one hypersurface-orthogonal Killing vector field. There exist interesting cosmological models that possess these properties that are well-behaved.

Hidden symmetries and test fields in rotating black hole spacetimes: part I
David Kubiznak
Perimeter Institute

Starting from the well known Laplace-Runge-Lenz vector of the Kepler problem, I will introduce dynamical (hidden) symmetries as genuine phase space symmetries that stand in contrast to the more familiar configuration space symmetries discussed by simplified versions of Noether's theorem. Proceeding to a relativistic description, I will demonstrate that such symmetries -- encoded in the so called Killing and Killing-Yano tensors -- play a crucial role in the study of rotating black holes described by the Kerr geometry and its higher-dimensional generalizations. In part II (presented by Pavel Krtous) it will be show that a single closed conformal Killing-Yano 2-form is enough to guarantee complete integrability of particle and light motion in general rotating black hole spacetimes in all dimensions. Recent developments on the separability of test fields in these spacetimes will also be discussed.

Hidden symmetries and test fields in rotating black hole spacetimes: part II
Pavel Krtouš
Institute of Theoretical Physics, Charles University

Kerr solution belongs into a class of geometries with a rich symmetry structure generated by the so-called principal tensor. As discussed in part I (presented by D. Kubizňák), these geometries (as well as their higher dimensional versions) possess a tower of Killing vectors and Killing tensors. These objects allow to define sufficient number of conserved quantities to show that the geodesic motion is complete integrable. Using an operator analogue of these conserved quantities, it turns out that basic field equations are separable. We review the older results for the scalar and Dirac fields and present a recent development for the electromagnetic and Proca fields.

Gradient Flow for Black Holes With Scalar Hair
Paul Mikula
University of Mantioba

Gradient flow is a system of partial differential equations that evolves a system along lines of steepest descent of the action. For the Einstein-Hilbert action, the gradient flow of the metric is a modified Ricci flow. Gradient flow equations will evolve any given initial field configuration towards one that is a solution to the equations of motion, this allows us to study stability of solutions as well as the behavior of a system far from equilibrium. We apply the gradient flow to a planar AdS black hole in 3+1 dimensions coupled to a Maxwell field and a charged scalar. This system is of particular interest because through the AdS/CFT correspondence, the boundary theory describes a superconductor. For black hole masses below a certain critical value the vacuum solution is unstable to the formation of scalar hair. The gradient flow will not only demonstrate this instability, but will move the system towards a hairy black hole solution that accounts for back-reactions of the matter fields on the metric.

No static soliton spacetimes in Einstein-Maxwell theory
Hari Kunduri
Memorial University

I will explain why any asymptotically flat (AF), static solution of the Einstein-Maxwell system (d>3) must have no magnetic field. This completes the classification of static non-extreme black holes in this setting. In particular, this implies that are no AF static spacetimes with non trivial topology in the domain of outer communication, with or without a black hole.

Harvesting entanglement from the black hole vacuum
Robie Hennigar
University of Waterloo

Two atoms coupled to a quantum field can become entangled, even if they remain spacelike separated for the duration of the interaction. This phenomenon has come to be known as entanglement harvesting, and it offers some insight into the entanglement properties of quantum fields. In this talk, I will discuss recent work considering the entanglement harvesting protocol for two qubits interacting with a conformally coupled scalar field on the BTZ black hole spacetime via an Unruh-DeWitt type model. I will discuss the implications of black hole properties on the entanglement extracted. In particular, I will discuss a no-go result for entanglement extraction near a black hole that originates from a combination of the Hawking effect and redshift suppression of non-local correlations between the qubits.

Horizons as Boundary Conditions: Part I
Ivan Booth
Memorial University

Numerical simulations of black hole mergers always show the location of the black hole horizons (either event or apparent) and by default one assumes that the shape and evolution of those horizons determines the physics of the interactions. However a little reflection shows that this is not so obviously true: by definition event horizons cannot send signals to infinity and by theorem apparent horizons always lie inside event horizons. Hence it is not the horizons themselves that interact but rather the ``near horizon’’ gravitational fields. What then does horizon geometry really tell us about black hole physics?

In this talk we will explain how horizons may be viewed as (partial) final boundary conditions which constrain the geometry and physics of the associated ``near horizon’’ spacetime. This will set the stage for the more concrete calculations discussed in Part II.

Horizons as Boundary Conditions Part II
Sharmila Gunasekaran
Memorial University

In this talk we will focus on the special case of spherical symmetry and consider how boundary conditions on the horizon restrict the geometry and physics of the rest of the spacetime. In particular we will consider black holes interacting with null dust, timelike dust and a massless scalar field. For the dust examples we will see that boundary conditions actually constrain more than the causal past of the horizon while the scalar field is better understood in terms of a characteristic initial value problem.


Aschenbach effect: The Orbital Velocity of Spinning Particles around the Rotating Blackholes
Jafar Khodagholizadeh
Faranghian University

The orbital velocity profile around the Kerr blackholes has a non-monotonic radial behaviour in the Locally Non-rotating Frames (LNRF)‎. ‎Using Mathisson-Papapetrou-Dixon equation for a massive spinning particle‎, ‎again this maximum-minimum feature has been shown by considering the linear spin approximation‎. ‎In addition to the blackholes spin‎, ‎the absolute value of particle's spin also plays an important role in Aschenbach effect‎. Therefore this effect can be used to constrain the particle's spin around the rotating blackholes

Rotational-tidal phasing of the binary neutron star waveform
Philippe Landry
University of Chicago

Tidal forces cause inspiralling binary neutron stars to deform, leaving a measurable imprint on the gravitational waves they emit. The induced stellar multipoles are an added source of gravitational radiation and modify the orbital dynamics, producing a slight acceleration of the coalescence which manifests as a phase shift in the waveform relative to point-particles. The dominant piece of this tidal phase comes from the mass quadrupoles, which contribute at fifth post-Newtonian order (5PN). Current quadrupoles and mass octupoles contribute at higher orders. For spinning neutron stars, additional multipole moments are induced by nonlinear couplings between spin and tides. In this talk, I show that at leading order these rotational-tidal deformations make a 6.5PN contribution to the tidal phase. Their effect on the waveform turns out to be larger than that of the mass octupoles, and nearly as large as that of the current quadrupoles, in systems with significant spin.

Universe as an Oscillator
Masooma Ali
University of New Brunswick

The canonical quantization program aims to formulate and solve the Wheeler DeWitt equation to calculate “wavefunctions” of the universe. In a similar vein the Euclidean (recently also Lorentzian) gravity program attempts to use a path integral approach to calculate these wave functions. But these calculations are hard and thus restricted to homogeneous cosmological models. In this talk I will discuss how some of the difficulties faced in these approaches can be mitigated by using a matter time gauge. Moreover, for an FLRW model with dust and a cosmological constant the dynamics maps exactly to an oscillator in dust time.

Simulating quantum gravity: a cosmological model
Syed Moeez Hassan
University of New Brunswick

Quantum Gravity is hard, and exact models are rare. In recent years, some focus has shifted to numerically studying quantum gravity. Some examples include Causal Dynamical Triangulations (CDT), Causal Sets, and Random Geometries to name a few. In this talk, I will present a new approach to numerically simulating quantum gravity in a cosmological model. Our starting point is a physical Hamiltonian obtained after a time gauge fixing. We calculate the wavefunction of the Universe, and the behavior of the mean volume of the Universe and its fluctuations.

Constructing Entanglement Wedges for Lifshitz Spacetimes with Lifshitz Gravity
Jonathan Cheyne
University of New Hampshire

Holographic relationships between entanglement entropy on the boundary of a spacetime and the area of minimal surfaces in the bulk provide an important entry in the bulk/boundary dictionary. While constructing the necessary causal and entanglement wedges is well understood in asymptotically AdS spacetimes, less is known about the equivalent constructions in spacetimes with different asymptotics. In particular, recent attempts to construct entanglement and causal wedges for asymptotically Lifshitz solutions in relativistic gravitational theories have proven problematic. We note a simple observation, that a Lifshitz bulk theory, specifically a covariant formulation of Ho\v{r}ava-Lifshitz gravity coupled to matter, has causal propagation defined by Lifshitz modes. We use these modes to construct causal and entanglement wedges and compute the geometric entanglement entropy for static spacetimes, which in such a construction matches the field theory prescription.

Gravitational Redshift in Kerr-Newman Geometry Using Gravity's Rainbow Theory
Anuj Kumar Dubey
INSPIRE, Chandrapur, Maharastra, India

Gravitational redshift is generally reported by most of the authors without considering the influence of the energy of the test particle using various spacetime geometries such as Schwarzschild, Reissner-Nordstrom, Kerr and Kerr-Newman geometries for static, charged static, rotating and charged rotating objects respectively. In the present work, the general expression for the energy dependent gravitational redshift is derived for charged rotating body using the Kerr-Newman geometry along with the energy dependent gravity’s rainbow function. It is found that the gravitational redshift is influenced by the energy of the photon. One may obtain greater correction in the value of gravitational redshift, using the high energy photons. Knowing the value of gravitational redshift from a high energy sources such as Gamma-ray Bursters (GRB), one may obtain the idea of upper bounds on the dimensionless rainbow function parameter (ξ).

Bounding MOTS area in the minimal supergravity
Aghil Alaee
University of Toronto

In this talk, we prove a class of geometric inequalities involving area, angular momentum, and charge of stable marginally outer trapped surfaces (MOTS) in 5-dimensional minimal supergravity which admit a torus action.  Analogous inequalities which also include contributions from a positive cosmological constant are also presented (This is a joint work with Marcus Khuri and Hari Kunduri).

Detecting violations of the principal of relativity with a quantum cavity
Jarod Kelly
University of New Brunswick

Several approaches to quantum gravity suggest dispersion relations governing plane wave propagation will be altered at high energy. Modified dispersion relations can be concretely realized in scalar field models where the equation of motion involves higher order spatial derivatives in a preferred frame. We consider such a model in two dimensions and in the presence of a finite cavity whose walls follow parallel inertial trajectories of speed v with respect to the preferred frame. We find the rest frame resonant frequencies of the cavity are functions of v, implying that the energy of field quanta measured by observers comoving with the cavity depends on the cavity’s state of motion. This violates the principle of relativity.

Charged Wormholes Supported by 2-Fluid Immiscible Matter Particle
Safiqul Islam
Harish-Chandra Research Institute, Allahabad, India

We provide a 2-fluid immiscible matter source that supplies fuel to construct wormhole spacetime. The exact wormhole solutions are found in the model having, besides real matter or ordinary matter, some quintessence matter along with charge distribution. We intend to derive a general metric of a charged wormhole under some density profiles of galaxies that is also consistent with the observational profile of rotation curve of galaxies. We have shown that the effective mass remains positive as well as the wormhole physics violate the null energy conditions. Some physical features are briefly discussed.

The loop quantum cosmology bounce as a Kasner transition
Edward Wilson-Ewing
University of New Brunswick

I will explain how the loop quantum cosmology bounce in the Bianchi type I space-time (vacuum or with a massless scalar field) can be viewed as a rapid transition between two classical solutions, with a simple transformation rule relating the Kasner exponents of the two epochs

Dilatonic Imprints on Exact Gravitational Wave Signatures
Fiona McCarthy
Perimeter Institute & University of Waterloo

The Moduli Space Approximation can be used to make analytical approximations of gravitational radiation in a slow-motion, strong-field regime. We can calculate approximations for the radiation released upon a binary collision of extremally charged dilatonic black holes for certain values of the dilatonic coupling constant and compare results with and without the dilatonic coupling and find departures from Einstein-Maxwell theory in the presence of a dilaton.

Noncommutative gravity with self-dual variables 
Marco de Cesare
University of New Brunswick

I will discuss a generalization of Palatini-Holst theory to a class of noncommutative spacetimes, where the noncommutative structure is achieved via a twist deformation. In our model, spacetime noncommutativity leads to a bimetric theory of gravity and to the introduction of further gravitational degrees of freedom, such as torsion and non-metricity; remarkably, it also requires the enlargement of the gauge group from the Lorentz group to GL(2,C), which includes local Weyl rescalings. The new gravitational degrees of freedom may play an important role in cosmology of the very early universe.
Choosing the value β = −i for the Barbero-Immirzi parameter, the action of the model turns out to depend only on the self-dual part of the connection, as in the standard commutative case. This particular choice makes the analysis of the dynamics much more tractable in the noncommutative case. A perturbative scheme is then adopted to study the effects of the noncommutative deformation on the dynamics.

Universal Horizon Preserving Diffeomorphisms & Black Hole Entropy
Matthew Roberson
University of New Hampshire

In relativistic gravity, requiring a spacetime surface be a Killing horizon breaks the general covariance of general relativity. The residual algebra of horizon preserving diffeomorphisms forms a Virasoro algebra near the horizon, the central charge of which yields the Bekenstein-Hawking entropy via the Cardy formula. This near horizon symmetry approach does not rely on any particular quantum gravity theory and hence provides an argument for why black hole entropy computations in various quantum gravity models all converge to a universal result. The exception to this rule may be Horava-Lifshitz gravity, where the ultraviolet theory is not relativistic and causal horizons are not Killing horizons but rather universal horizons. We investigate the near-horizon symmetry approach for universal horizons, compute the classical algebra of the horizon preserving diffeomorphisms, and show that it is compatible with a Lifshitz symmetry.

Kantowski-Sachs Models and Analysis
Joey Latta
Dalhousie University


The Penrose Inequality in AdS
Viqar Husain
University of New Brunswick


New spherically symmetric interior solutions with charge and anisotropic pressures
David Hobill
University of Calgary

Although a large number of solutions to the Einstein equations for spherically symmetric matter distributions are known, less than 10% of those solutions can be considered as having properties that make them physically realistic. We use one of those solutions (the Tolman VII solution) as a "seed" for constructing new solutions with charge and/or anisotropic pressures that maintain the property that they obey the conditions required for them to be considered as describing physically reasonable solutions to the Einstein-Maxwell equations. We then compare the physical differences between the new solutions and the Tolman VII solution.

Conformal Quasicrystals and Holography
Latham Boyle
Perimeter Institute for Theoretical Physics

Recent studies of holographic tensor network models defined on regular tessellations of hyperbolic space have not yet addressed the underlying discrete geometry of the boundary. We show that the boundary degrees of freedom naturally live on a novel structure, a conformal quasicrystal, that provides a discrete model of conformal geometry. We introduce and construct a class of one-dimensional conformal quasicrystals, and discuss a higher-dimensional example (related to the Penrose tiling). Our construction permits discretizations of conformal field theories that preserve an infinite discrete subgroup of the global conformal group at the cost of lattice periodicity. (Based on arXiv:1805.02665, with Ben Dickens and Felix Flicker.)

Unifying Fundamental Atoms? Classical/Quantum Consistency, Gauge Invariance and The W^±, Z and H Transition States 
Samir Abuzaid, MSc.,OSU,The Egyptian Philosophical Society

We show that accepting atomism, i.e. the postulate that every natural body is composed of identical fundamental atoms that are endowed with both of attraction and perpetual randomness, in current physics leads to two main results with respect to having a unified framework of physics.
The first is emergence of the classical world described by the theory of the general relativity GR, from the quantum world. Emergent domains, in general can't have a unified formulation with the underlying one, instead they share a consistency relation. An example of such a case is emergence of thermodynamics from classical Newtonian mechanics. Through the postulated fundamental atoms, mathematical consistency relations between the the classical and the quantum realms are constructed on the basis of modeling classical bodies as a continuum of quantum particles. 
The second is that the three fundamental forces at the quantum level reduce to the forces of attraction between the postulated fundamental atoms, which leads to unification of the four fundamental forces. In this view, by definition the elementary force carriers (the elementary bosons) are composed of the postulated fundamental atoms and possess a structure. Such a structure associated with the random nature of the fundamental atoms makes possible to model interaction of such force carriers through the theory of random walk RW. This leads to extending the use of the theory of RW from modeling potential theory to modeling gauge theory. 
These two main results are proved experimentally on both of the classical and the quantum levels. On the classical level these consistency relations are proved through comparison to the Post-Newtonian approximation; calculation of the periastron precession of binary pulsar B 1913+16; and bending of light rays grazing the surface of the Sun. On the subatomic level these consistency relations are proved through calculation of the relative values of the four basic forces; describing the physical mechanisms of the process of symmetry breaking; and calculating the masses of the 'elementary' bosons including the mass of the Higgs particle. 
In the attached material, we present detailed probabilistic calculations that give rise to the constructed consistency relations, on one hand, and to the mechanisms that give rise to the EW theory as well as to gauge theory, on the other.

Both the meeting and workshop are financially supported by AARMS, the Perimeter Insititute and StFX.