# PI Day

Conference Date:
Thursday, June 1, 2017 (All day)
Pirsa Collection:
Scientific Areas:
Other

June 1 is PI day, when we get the PI community together to hear what each other is doing.  This time our graduate students will showcase their research. The day will consist of talks given by students in all area of physics at PI. All PI Residents are encouraged to attend and participate in discussions throughout the day.

Anyone attending the event will be included in coffee breaks.

Please RSVP to Jamie Cooper (jcooper@pitp.ca) by May 24th if you wish to attend.

• Matt Beach, University of Waterloo
• Todd Sierens, Perimeter Institute
• Laura Sberna, Perimeter Institute
• Andres Schlief, McMaster University
• Lakshya Bhardwaj, Perimeter Institute
• Florian Hopfmueller, Perimeter Institute
• Adrián Franco Rubio, Perimeter Institute
• Job Feldbrugge, Perimeter Institute
• Vasudev Shyam, Perimeter Institute
• Yasaman Yazdi, University of Waterloo
• Miroslav Rapcak, Perimeter Institute
• Elizabeth Gould, Perimeter Institute
• Jonah Miller, Perimeter Institute
• Natacha Altamirano, University of Waterloo
 Time Event Location 9:25 – 9:30am Welcome and Opening Remarks Skyroom 9:30 – 9:50am Matt Beach, University of WaterlooRelative entropy with a twist Skyroom 9:50 – 10:10am Todd Sierens, Perimeter InstituteQuantum critical responses via holographic models and conformal perturbation theory Skyroom 10:10 – 10:30am Laura Sberna, Perimeter InstituteQuantum tunneling with a Lorentzian path integral Skyroom 10:30 – 11:00am Coffee Break Bistro – 1st Floor 11:00 – 11:20am Andres Schlief, McMaster UniversityRise of nonperturbative effects below the upper critical dimension Skyroom 11:20 – 11:40am Lakshya Bhardwaj, Perimeter InstituteTopological field theories and gapped phases of matter Skyroom 11:40 – 12:00pm Florian Hopfmueller, Perimeter InstituteGravity degrees of freedom on a null surface Skyroom 12:00 – 1:30pm Break 1:30 – 1:50pm Adrián Franco Rubio, Perimeter InstituteEntanglement structure and UV regularization in cMERA Skyroom 1:50 – 2:10pm Job Feldbrugge, Perimeter InstituteLorentzian quantum cosmology Skyroom 2:10 – 2:30pm Vasudev Shyam, Perimeter InstituteThe holographic dual to general covariance Skyroom 2:30- 3:00pm Group Photo and Coffee Break Perimeter HillBistro – 1st Floor 3:00 – 3:20pm Yasaman Yazdi, University of WaterlooEntanglement entropy of scalar fields in causal set theory Skyroom 3:20 – 3:40pm Miroslav Rapcak, Perimeter InstituteDualities for chiral algebras from S-duality Skyroom 3:40 – 4:00pm Elizabeth Gould, Perimeter InstituteRethinking time at the Big Bang Skyroom 4:00 – 4:30pm Coffee Break Bistro – 1st Floor 4:30 – 4:50pm Jonah Miller, Perimeter InstituteThe thick sandwich problem in (2+1)-dimensional causal dynamical triangulations Skyroom 4:50 – 5:10pm Natacha Altamirano, University of WaterlooWeak measurements, decoherence and cosmology Skyroom 5:10 – 5:15pm Closing Remarks Skyroom

Natacha Altamirano, University of Waterloo

Weak measurements, decoherence and cosmology

In this work we consider a recent proposal in which gravitational interactions are mediated via  the exchange of classical information and apply it to a quantized Friedman-Robertson-Walker (FRW) universe with the assumption that any test particles must feel a classical metric. We show that such a model results in decoherence in the FRW state that manifests itself as a dark energy fluid that fills the spacetime. Motivated by quantum-classical interactions this model is yet another example of theories with violation of energy-momentum conservation whose signature could have significant consequences for the observable universe.

Matt Beach, University of Waterloo

Relative entropy with a twist

Quantum relative entropy is a measure of the indistinguishability of two quantum states in the same Hilbert space. I will discuss the relative entropy between a state with periodic boundary conditions and one with twisted boundary conditions for a free 1+1 CFT with c=1. I will also highlight the unresolved discrepancy between analytic and numeric results.

Lakshya Bhardwaj, Perimeter Institute

Topological field theories and gapped phases of matter

I will introduce the idea that topological field theories describe the low-energy properties of gapped local quantum systems. This idea has proved fruitful in recent studies of gapped phases of matter.

Job Feldbrugge, Perimeter Institute

Lorentzian quantum cosmology

Using Picard-Lefschetz theory we show that the Lorentzian path integral forms a good starting point for quantum cosmology which avoids the conformal factor problem present in Euclidean gravity. We study the Lorentzian path integral for a homogeneous and isotropic model with a positive cosmological constant. Applied to the “no-boundary” proposal, we show that this leads to the inverse of the result obtained by Hartle and Hawking. Including a inflation field, the Lorentzian path integral prefers to start at the 'top of the hill' leading to good initial conditions for slow roll inflation. However, when including gravitons the fluctuations seem to be unstable.

Elizabeth Gould, Perimeter Institute

Rethinking time at the Big Bang

I will discuss two ways in which revising the notion of time at the Big Bang will lead to testable predictions. I will then contrast these predictions against standard ΛCDM scenario, and cosmological observations. The first model, Holographic Cosmology, is based on a 3d quantum field theory without time, suggesting the possibility of nonperturbative effects on large angles (l<30) in the CMB sky. The second model, Periodic Time Cosmology, relates (past and future) cosmic expansion history to the spectrum of cosmic perturbations by demanding consistency with an exactly periodic notion of time. Comparing this model to observations leads to surprising implications for dark energy and/or neutrino masses in cosmology.

Florian Hopfmueller, Perimeter Institute

Gravity degrees of freedom on a null surface

A canonical analysis for general relativity is performed on a null surface without fixing the diffeomorphism gauge, and the canonical pairs of configuration and momentum variables are derived. Next to the well-known spin-2 pair, also spin-1 and spin-0 pairs are identified. The boundary action for a null boundary segment of spacetime is obtained, including terms on codimension two corners.
FH, Laurent Freidel arXiv:1611.03096, Phys. Rev. D 95, 104006 (2017)

Jonah Miller, Perimeter Institute

The thick sandwich problem in (2+1)-dimensional causal dynamical triangulations

Causal dynamical triangulations (CDT) is a sum-over-histories approach to quantum gravity which leverages the techniques developed in lattice quantum field theory. In this talk, I discuss the thick sandwich problem in CDT: Given initial and final spacelike hypersurfaces, each with a fixed geometry, what is the transition amplitude for one transitioning into the other? And what geometries dominate the associated path integral? I discuss preliminary studies performed in this direction. I also highlight open problems and interesting directions for future research.

Miroslav Rapcak, Perimeter Institute

Dualities for chiral algebras from S-duality

During past few decades, string theory has been used as a source of conjectural dualities in various areas of physics and mathematics. We have extended these applications of string dualities to the study of chiral algebras in 2d CFT.  In this talk, I will sketch how to use S-duality of D3-D5-NS5 systems to shed some light on already known dual constructions of chiral algebras and generate huge amount of new dualities.

Entanglement structure and UV regularization in cMERA

We give an introduction to cMERA, a continuous tensor networks ansatz for ground states of QFTs. We also explore a particular feature of it: an intrinsic length scale that acts as an ultraviolet cutoff. We provide evidence for the existence of this cutoff based on the entanglement structure of a particular family of cMERA states, namely Gaussian states optimized for free bosonic and fermionic CFTs. Our findings reflect that short distance entanglement is not fully present in the ansatz states, thus hinting at ultraviolet regularization.

Laura Sberna, Perimeter Institute

Quantum tunneling with a Lorentzian path integral

We describe the tunneling of a quantum mechanical particle with a Lorentzian (real-time) path integral. The analysis is made concrete by application to the inverted harmonic oscillator potential, where the path integral is known exactly. We apply Picard-Lefschetz theory to the time integral of the Feynmann propagator at fixed energy, and show that the Euclidean integration contour is obtained as a Lefschetz thimble, or a sum of them, in a suitable limit. Picard-Lefschetz theory is used to make the integral manifestly convergent and is also essential for the saddle point or semi-classical approximation. The very simple example of the inverted harmonic oscillator presents many interesting mathematical features, such as the Stokes phenomenon and multiple relevant complex saddles. We also attempt to construct a more realistic picture of the tunneling process, by allowing for spreading in energy and duration.

Andres Schlief, McMaster University

Rise of nonperturbative effects below the upper critical dimension

Extracting low energy universal data of quantum critical systems is a task whose difficulty increases with decreasing dimension. The increasing strength of quantum fluctuations can be tamed by using renormalization group (RG) schemes based on dimensional regularization close to the upper critical dimension of the system.  By presenting a non-perturbative approach that allows the reliable extraction of the low energy universal data for the antiferromagnetic quantum critical metal in -spatial dimensions, I will exemplify how an emergent non-commutativity between the low-energy limit and the dimensional limit preempts RG schemes based on dimensional regularization to access the correct low-energy universal data in integer dimensions.

Vasudev Shyam, Perimeter Institute

The holographic dual to general covariance

One of the defining features of holography is the geometerization of the renormalization group scale. This means that when a quantum field theory is holographically dual to a bulk gravity theory, then the direction normal to the boundary in the bulk (the radial' direction) is to be interpreted as the energy scale of the dual quantum field theory. So this direction can be seen to be emergent', and the evolution of bulk fields along this direction is equated with the renormalization group flow of sources or couplings of boundary operators. Given that gravitational theories are generally covariant, this emergent direction must be treated on equal footing as those of the space on which the boundary field theory lives. I will describe the precise integrability condition the renormalization group flow need satisfy which encodes this peculiar response of the quantum field theory under coarse graining so as to respect this property of covariance. In other words, this condition is dual' to general covariance itself.

Todd Sierens, Perimeter Institute

Quantum critical responses via holographic models and conformal perturbation theory

We investigate response functions near quantum critical points, allowing for finite temperature and a mild deformation by a relevant scalar. When the quantum critical point is described by a conformal field theory, we use conformal perturbation theory and holography to determine the two leading corrections to the scalar two-point function and to the conductivity. We build a bridge between the couplings fixed by conformal symmetry with the interaction couplings in the gravity theory. We construct a minimal holographic model that allows us to numerically obtain the response functions at all frequencies, independently confirming the corrections to the high-frequency response functions. In addition to probing the physics of the ultraviolet, the holographic model probes the physics of the infrared giving us qualitative insight into new physics scalings.

Yasaman Yazdi, University of Waterloo

Entanglement entropy of scalar fields in causal set theory

Entanglement entropy is now widely accepted as having deep connections with quantum gravity. It is therefore desirable to understand it in the context of causal sets, especially since they provide in a natural and covariant manner the UV cutoff needed to render entanglement entropy finite. Defining entropy in a causal set is not straightforward because the usual hypersurface data on which definitions of entanglement typically rely is not available. Instead, we appeal to a more global expression which, for a gaussian scalar field, expresses the entropy of a spacetime region in terms of the field’s correlation function within that region. In this talk I will present results from evaluating this entropy for causal sets sprinkled into a 1 + 1-dimensional causal diamond in flat spacetime, and specifically for a smaller causal diamond within a larger concentric one. In the first instance we find an entropy that obeys a (spacetime) volume law instead of the expected (spatial) area law. We find, however, that one can obtain the expected area law by following a prescription for truncating the eigenvalues of a certain “Pauli-Jordan” operator and the projections of their eigenfunctions on the Wightman function that enters into the entropy formula.

## Weak measurements, decoherence and cosmology

Thursday Jun 01, 2017
Speaker(s):

In this work we consider a recent proposal in which gravitational interactions are mediated via the exchange of classical information and apply it to a quantized Friedman-Robertson-Walker (FRW) universe with the assumption that any test particles must feel a classical metric. We show that such a model results in decoherence in the FRW state that manifests itself as a dark energy fluid that fills the spacetime.

Collection/Series:

## The thick sandwich problem in (2+1)-dimensional causal dynamical triangulations

Thursday Jun 01, 2017

Causal dynamical triangulations (CDT) is a sum-over-histories approach to quantum gravity which leverages the techniques developed in lattice quantum field theory. In this talk, I discuss the thick sandwich problem in CDT: Given initial and final spacelike hypersurfaces, each with a fixed geometry, what is the transition amplitude for one transitioning into the other? And what geometries dominate the associated path integral? I discuss preliminary studies performed in this direction. I also highlight open problems and interesting directions for future research.

Collection/Series:

## Rethinking time at the Big Bang

Thursday Jun 01, 2017
Speaker(s):

I will discuss two ways in which revising the notion of time at the Big Bang will lead to testable predictions. I will then contrast these predictions against standard ΛCDM scenario, and cosmological observations. The first model, Holographic Cosmology, is based on a 3d quantum field theory without time, suggesting the possibility of nonperturbative effects on large angles (l

Collection/Series:

## Dualities for chiral algebras from S-duality

Thursday Jun 01, 2017
Speaker(s):

During past few decades, string theory has been used as a source of conjectural dualities in various areas of physics and mathematics. We have extended these applications of string dualities to the study of chiral algebras in 2d CFT. In this talk, I will sketch how to use S-duality of D3-D5-NS5 systems to shed some light on already known dual constructions of chiral algebras and generate huge amount of new dualities.

Collection/Series:

## Entanglement entropy of scalar fields in causal set theory

Thursday Jun 01, 2017

Entanglement entropy is now widely accepted as having deep connections with quantum gravity. It is therefore desirable to understand it in the context of causal sets, especially since they provide in a natural and covariant manner the UV cutoff needed to render entanglement entropy finite. Defining entropy in a causal set is not straightforward because the usual hypersurface data on which definitions of entanglement typically rely is not available.

Collection/Series:

## The holographic dual to general covariance

Thursday Jun 01, 2017
Speaker(s):

One of the defining features of holography is the geometerization of the renormalization group scale. This means that when a quantum field theory is holographically dual to a bulk gravity theory, then the direction normal to the boundary in the bulk (the radial' direction) is to be interpreted as the energy scale of the dual quantum field theory. So this direction can be seen to be `emergent', and the evolution of bulk fields along this direction is equated with the renormalization group flow of sources or couplings of boundary operators.

Collection/Series:

## Lorentzian quantum cosmology

Thursday Jun 01, 2017
Speaker(s):

Using Picard-Lefschetz theory we show that the Lorentzian path integral forms a good starting point for quantum cosmology which avoids the conformal factor problem present in Euclidean gravity. We study the Lorentzian path integral for a homogeneous and isotropic model with a positive cosmological constant. Applied to the “no-boundary” proposal, we show that this leads to the inverse of the result obtained by Hartle and Hawking. Including a inflation field, the Lorentzian path integral prefers to start at the 'top of the hill' leading to good initial conditions for slow roll inflation.

Collection/Series:

## Entanglement structure and UV regularization in cMERA

Thursday Jun 01, 2017
Speaker(s):

We give an introduction to cMERA, a continuous tensor networks ansatz for ground states of QFTs. We also explore a particular feature of it: an intrinsic length scale that acts as an ultraviolet cutoff. We provide evidence for the existence of this cutoff based on the entanglement structure of a particular family of cMERA states, namely Gaussian states optimized for free bosonic and fermionic CFTs. Our findings reflect that short distance entanglement is not fully present in the ansatz states, thus hinting at ultraviolet regularization.

Collection/Series:

## Gravity degrees of freedom on a null surface

Thursday Jun 01, 2017
Speaker(s):

A canonical analysis for general relativity is performed on a null surface without fixing the diffeomorphism gauge, and the canonical pairs of configuration and momentum variables are derived. Next to the well-known spin-2 pair, also spin-1 and spin-0 pairs are identified. The boundary action for a null boundary segment of spacetime is obtained, including terms on codimension two corners.
FH, Laurent Freidel arXiv:1611.03096, Phys. Rev. D 95, 104006 (2017)

Collection/Series:

## Topological field theories and gapped phases of matter

Thursday Jun 01, 2017
Speaker(s):

I will introduce the idea that topological field theories describe the low-energy properties of gapped local quantum systems. This idea has proved fruitful in recent studies of gapped phases of matter.

Collection/Series:

## Pages

Scientific Organizers:

• Lauren Hayward Sierens, Perimeter Institute
• Lee Smolin, Perimeter Institute
• Miroslav Rapcak, Perimeter Institute
• Nick Van den Broeck, Perimeter Institute
• Yigit Yargic, Perimeter Institute