**Olakanmi Akinto**, Comsats Institute of Information Technology

*Strong Gravity Approach to QCD and General Relativity*

A systematic study of a Weyl type of action, which is scale free and quadratic in the curvature, is undertaken. The dynamical breaking of this scale invariance induces general relativity (GR) as an effective long distance limit of the theory. We prove that the corresponding field equations of the theory possess an effective pure Yang - Mills potential, which describes the asymptotic freedom and color confinement properties of QCD. This inevitably leads to the solutions of quantum Yang - Mills existence on R4 (with its characteristic mass gap), and dark matter problems. The inherent Bern - Carrasco - Johansson (BCJ) double - copy and gauge - gravity duality properties of this formulation lead to the solutions of neutrino mass and dark energy problems. This approach provides a strong gravity basis for the unification of quantum Yang - Mills theory (QYMT) with GR.

**Natacha Altamirano**, Perimeter Institute

*Emergent dark fluid from decoherence in quantum interactions*

Much effort has been devoted into understanding the quantum mechanical properties of gravitational interactions. Here we explore the recent suggestion that gravitational interactions are a fundamental classical channel that is described by continuous quantum measurements and feedforward (CQMF). Specifically, we investigate the possibility that some properties of our universe, modeled using a Friedman-Robertson-Walker metric, can emerge from CQMF by introducing an underlying quantum system for the dynamical variables, avoiding well known difficulties in trying to quantize the spacetime itself. We show that the quantum decoherence necessary in such a measurement model manifests itself as a dark energy fluid that fills the spacetime and whose equation of state asymptotically oscillates around the value w = −1/3, regardless of the spatial curvature.

**Dimitry Ayzenberg**, Montana State University

*Testing General Relativity with Black Hole Continuum Spectrum Observations:*

*Einstein-dilaton-Gauss-Bonnet Gravity and Chern-Simons Gravity*

Observations of continuum spectra of black holes allow us to study the physics and properties of accretion disks and black holes. These observations have been used to determine the masses and spin angular momenta of black holes, as well as the temperature, evolution, and magnetic field structure of accretion disks. Continuum spectrum observations can also be used, at least in principle, to test General Relativity in the strong-field regime. In this talk I will present our method and results of using the continuum spectrum to test General Relativity and place constraints on modified gravity theories, specifically Einstein-dilaton-Gauss-Bonnet gravity and Chern-Simons gravity.

**Swetha Bhagwat**, Syracuse University

*Spectroscopic analysis of stellar mass black-hole mergers in our local universe with ground-based gravitational wave detectors*

**Pablo Bosch**, Perimeter Institute

*Nonlinear evolution and final fate of charged AdS black hole superradiant instability*

We describe the full nonlinear development of the superradiant instability for a charged massless scalar field, coupled to general relativity and electromagnetism, in the vicinity of a Reissner-Nordstrom-AdS black hole. The presence of the negative cosmological constant provides a natural context for considering perfectly reflecting boundary conditions and studying the dynamics as the scalar field interacts repeatedly with the black hole.

**Alejandro Cardenas-Avendano**, Montana State University

*A study for testing the Kerr metric with AGN iron line eclipses:*

Recently, two of us have studied iron line reverberation mapping to test black hole candidates, showing that the time information in reverberation mapping can better constrain the Kerr metric than the time-integrated approach. Motivated by this finding, here we explore the constraining power of another time-dependent measurement: an AGN iron line eclipse. An obscuring cloud passes between the AGN and the distant observer, covering different parts of the accretion disk at different times. Similar to the reverberation measurement, an eclipse might help to better identify the relativistic effects affecting the X-ray photons. However, this is not what we find. In our study, we employ the Johannsen-Psaltis parametrisation, but we argue that our conclusions hold in a large class of non-Kerr metrics. We explain our results pointing out an important difference between reverberation and eclipse measurements. (JCAP 1604:054, 2016)

**Shira Chapman**, Perimeter Institute

*Complexity of Formation in Holography - part I*

It has recently been conjectured that computational quantum complexity can be computed for a holographic state by evaluating the gravitational action on a region in the bulk of the dual gravitational theory bounded by null surfaces which goes under the name of the Wheeler-DeWitt patch. We use a set of rules for evaluating the contributions of the null surfaces and joints to compute the action on such a region anchored at the boundary at t = 0 for certain black hole spacetimes. We compare this result to that of two copies of empty AdS. We find that for boundary dimension d > 2 in the limit of large horizon radius the action difference grows linearly as the horizon entropy with proportionality coefficient (d − 2)/d · cot(π/d). For BTZ black holes the action difference is independent of the black hole mass.

**Katerina Chatziioannou**, CITA

*Probing the Internal Composition of Neutron Stars with Gravitational Waves*

Gravitational waves from neutron star binaries carry information about the equation of state of supranuclear matter through a parameter called tidal deformability. Its measurability has been assessed in a number of studies, concluding it could provide important information about the equation of state of neutron star matter. In this talk, I will describe a complimentary approach to the problem of equation of state determination, one which focuses on how information from gravitational waves can be translated in ways that could be of direct benefit to nuclear physicists. Specifically, I will talk about what gravitational waves can tell us about the internal composition of neutron stars, information that is directly applicable to equation of state modeling.

**Hsin-Yu Chen**, University of Chicago

*Observational Selection Effects with Ground-Based Gravitational Wave Detectors*

Ground-based interferometers are not perfectly all-sky instruments, and it is important to account for their behavior when considering the distribution of detected events. In particular, the LIGO detectors are most sensitive to sources above North America and the Indian Ocean and, as the Earth rotates, the sensitive regions are swept across the sky. However, because the detectors do not acquire data uniformly over time, there is a net bias on detectable sources’ right ascensions. Both LIGO detectors preferentially collect data during their local night; it is more than twice as likely to be local midnight than noon when both detectors are operating. We discuss these selection effects and how they impact LIGO’s observations and electromagnetic follow-up. Beyond galactic foregrounds associated with seasonal variations, we find that equatorial observatories can access over 80% of the localization probability, while mid-latitudes will access closer to 70%. Facilities located near the two LIGO sites can observe sources closer to their zenith than their analogs in the South, but the average observation will still be no closer than 44◦ from zenith. We also find that observatories in Africa or the South Atlantic will wait systematically longer before they can begin observing compared to the rest of the world, although there is a preference for longitudes near the LIGOs. These effects, along with knowledge of the LIGO antenna pattern, can inform electromagnetic follow-up activities and optimization, including the possibility of directing observations even before gravitational-wave events occur.

**Anne-Sylvie Deutsch**, Pennsylvania State University

*Inflation, cosmic variance, and the bispectrum in the squeezed limit*

**Zoheyr Doctor**, University of Chicago

*Gravitational Wave Emulation Using Gaussian Process Regression*

Parameter estimation (PE) for gravitational wave signals from compact binary coalescences (CBCs) requires reliable template waveforms which span the parameter space. Waveforms from numerical relativity are accurate but computationally expensive, so approximate templates are typically used for PE. These 'approximants', while quick to compute, can introduce systematic errors and bias PE results. We describe a machine learning method for generating CBC waveforms and uncertainties using existing accurate waveforms as a training set. Coefficients of a reduced order waveform model are computed and each treated as arising from a Gaussian process. These coefficients and their uncertainties are then interpolated using Gaussian process regression (GPR). As a proof of concept, we construct a training set of approximant waveforms (rather than NR waveforms) in the two-dimensional space of chirp mass and mass ratio and interpolate new waveforms with GPR. We demonstrate that the mismatch between interpolated waveforms and approximants is below the 1% level for an appropriate choice of training set and GPR kernel hyperparameters.

**Ariel Edery**, Bishop's University

*Generating Einstein gravity, cosmological constant and Higgs mass from Restricted Weyl Invariance*

Recently, it has been pointed out that dimensionless actions in four dimensional curved spacetime possess a symmetry which goes beyond scale invariance but is smaller than full Weyl invariance. This symmetry was dubbed {\it restricted Weyl invariance}. We show that starting with a restricted Weyl invariant action that includes pure $R^2$ gravity and a Higgs sector with no explicit mass, one can generate the Einstein-Hilbert action with cosmological constant and a Higgs mass. The model also contains an extra massless scalar field which couples to the Higgs field (and gravity). If the coupling of this extra scalar field to the Higgs field is negligibly small, this fixes the coefficient of the nonminimal coupling $R\Phi^2$ between the Higgs field and gravity.

**Joshua Faber**, Rochester Institute of Technology

*Multidomain spectral methods for initial data and data compression*

Spectral methods solvers are widely used throughout numerical relativity to generate initial data. Here, we compare the performance of multidomain spectral techniques with single-domain versions when applied to the binary black hole problem, for a variety of configurations including differing masses and arbitrary spins. We then discuss the prospect for using multidomain spectral methods as a data compression technique for post-processing applications, and discuss the potential accuracy that can be achieved for binary black hole simulations.

**Maya Fishbach**, University of Chicago

*Final black hole spins from hierarchical mergers*

I will discuss the hierarchical merger model for the formation of stellar mass black holes (such as the binary black holes observable by LIGO). In the hierarchical merger model, each black hole in a black hole binary is the result of a merger of two lesser black holes from a previous generation, and the previous generation's black holes may themselves be merger products of an even earlier generation. I will use the formulas of Hoffman, Barausse & Rezzolla (2016) to show that if black holes form in this hierarchical merger scenario, their spin magnitudes follow a certain probability distribution. This spin distribution can be compared to LIGO spin measurements to constrain the hierarchical merger scenario.

**Heather Fong**, CITA

*Error analysis of numerical gravitational waveforms from coalescing binary black holes*

The Advanced Laser Interferometer Gravitational-wave Observatory (Advanced LIGO) has finished a successful first observation run and will commence its second run this summer. Detection of compact object binaries utilizes matched-filtering, which requires a vast collection of highly accurate gravitational waveforms. This talk will present a set of about 100 new aligned-spin binary black hole simulations. I will discuss their properties, including a detailed error analysis, which demonstrates that the numerical waveforms are sufficiently accurate for gravitational wave detection purposes, as well as for parameter estimation purposes.

**John Friedman**, University of Wisconsin-Milwaukee

*Can magnetic-field windup kill the r-mode instability of neutron stars?*

At second order in perturbation theory, the unstable r-mode of a rotating star includes growing differential rotation whose form and growth rate are determined by gravitational radiation reaction. With no magnetic field, the angular velocity of a fluid element grows exponentially until the mode reaches its nonlinear saturation amplitude and remains nonzero after saturation. With a background magnetic field, the differential rotation winds up and amplifies the field, and previous work suggests that the amplification may damp out the instability. A background magnetic field, however, turns the time-independent perturbations corresponding to adding differential rotation into perturbations whose characteristic frequencies are of order the Alfven frequency. We argue that magnetic field growth stops soon after the mode reaches its saturation amplitude. We show that this is the case for a toy model, where magnetic amplification for small saturation amplitude is too small to damp the r-mode. For a more realistic model of a cold, rotating neutron star, an analogous upper limit depends on the assumption that there are no marginally unstable perturbations.

**David Garfinkle**, Oakland University

*A simple estimate of gravitational wave memory in binary black hole systems.*

**Daniel George**, University of Illinois

*An inspiral-merger-ringdown waveform model for compact binaries on eccentric orbits*

The detection of compact binaries with significant eccentricity in the sensitivity band of gravitational wave detectors will provide critical insights on the dynamics and formation channels of these events. In order to search for these systems and place constraints on their rates, we present a time domain, inspiral-merger-ringdown waveform model that describes the gravitational wave emission from compact binaries on orbits with low to moderate values of eccentricity. We use this model to explore the detectability of these events in the context of advanced LIGO.

**Roman Gold**, Perimeter Institute

*Black hole accretion flows as seen by the Event Horizon Telescope*

Accreting black holes (BHs) are at the core of relativistic astrophysics as messengers of the strong-field regime of General Relativity that carry complementary information to Gravitational Waves. In particular, the black holes in M87 and Sgr A* constitute prime targets for the Event Horizon Telescope, which among other things aims at imaging the imprint of the black hole (its "shadow") on the surrounding relativistic gas.

I will present results from general-relativistic, polarized radiative transfer models for the inner accretion flow in Sgr A*. The models use time-dependent, global GRMHD simulations of hot accretion flows including standard-and-normal-evolution (SANE) and magnetically arrested disks (MAD). I present comparisons of these synthetic data sets to the most recent observations with the Event Horizon Telescope and show how the data distinguishes the models and probes the magnetic field structure.

**Elizabeth Gould**, Perimeter Institute

*Observational Constraints of Holographic Cosmology from Planck Data*

The holographic cosmology framework expresses inflationary predictions in terms of the observables of a 3D quantum field theory (QFT). It predicts two possible regimes for power spectra of primordial curvature perturbations. The first is well approximated by the standard power law expansion (for strongly coupled QFT), while the second is a new holographic expansion, dual to a weakly coupled QFT in the UV, but strong coupling in the IR. We compare the two regimes against the current cosmological observations and show that they do equally well at l>30, where a holographic perturbative expansion can be trusted. However, the (naive) holographic expansion is disfavored by data at low l's at ~2 sigma level.

**Daniel Guariento**, Perimeter Institute

*Self-gravitating fluid solutions of Shape Dynamics*

Shape Dynamics is a 3D conformally invariant theory of gravity which possesses an increasingly large set of solutions in common with General Relativity. Upon close inspection, these solutions behave in surprising ways, so in order to probe the fitness of Shape Dynamics as a viable alternative to General Relativity one must understand increasingly complex solutions, on which to base perturbative studies and numerical analyses. We show that a class of time-dependent exact solutions of Shape Dynamics exists from first principles, representing a central inhomogeneity in an evolving cosmological environment. By assuming only a perfect fluid source in a spherically symmetric geometry we show that this fully dynamic non-vacuum solution satisfies in all generality the Hamiltonian structure of Shape Dynamics. The solutions are characterized by shear-free flow of the fluid and admit an interpretation as cosmological black holes.

**Lucas Hackl**, Pennsylvania State University

*Entanglement production in cosmology*

Entanglement entropy provides a measure to quantify correlations in a quantum state, for instance of a scalar field representing matter on a cosmological background geometry. In this talk, I will discuss the time evolution of the entanglement entropy during inflation and reheating. I show that instabilities and parametric resonance can lead to a linear growth of the entanglement entropy in generic subsystems.

**Theodore Halnon**, Pennsylvania State University

*Covariance and Quantum Cosmology*

In relativity, time is relative between reference frames so we can exclude coordinate independence. However when general relativity is combined with quantum mechanics, the problem is that quantum mechanics requires a time coordinate in order to write an evolution equation for wave functions. One method to study relativity is to interpret the dynamics of a matter field as a clock. We look at an isotropic cosmological model with two matter ingredients. One is given by a scalar field and one by vacuum energy or a cosmological constant. In our systems, there are two matter fields and thus two clock rates. This paper considers two Hamiltonians derived from their respective clock rates. We find semi-classical solutions for these equations and compare the physical predictions that they imply.

**Stephen Harnish**, Bluffton University

*Analog gravity: computational analysis for acoustic metrics*

Mathematical analysis of sonic wave simulations from NCSA Blue Waters determines acoustic metrics controlled by pressure and temperature within an LJ lattice. These techniques offer reverse engineering of acoustic metrics for models of analog gravity.

**Carl-Johan Haster**, CITA

*Fast and Accurate Inference on Gravitational Waves from Precessing Compact Binaries*

After a detection candidate has been observed with gravitational waves, the next task becomes to identify the inference of possible combinations of source parameters. These parameter estimation studies of compact binary gravitational wave events are heavily limited in their flexibility and usefulness by computational cost. Using ROQ (Reduced Order Quadrature) methods it is however possible to bring this cost down by several orders of magnitude, while not sacrificing any accuracy, therefore enabling parameter estimation studies at a completely new level. In this talk, I will showcase the capabilities of the underlying ROQ formalism and give highlights of the parameter estimation studies now within reach.

**Robie Hennigar**, University of Waterloo

*Thermodynamics of hairy black holes in Lovelock gravity*

I will discuss the thermodynamics of a class of hairy black holes in Lovelock gravity from the perspective of black hole chemistry. These analytic black hole solutions arise from the conformal coupling of a real scalar field to the dimensionally extended Euler densities and provide an ideal testbed for examining the effects of scalar hair on black hole thermodynamics. Studying the linearized field equations about a maximally symmetric background reveals that the theory is free from ghost and tachyon instabilities provided constraints are enforced on the coupling constants. The black hole solutions display a variety of interesting thermodynamic behaviour including (virtual) triple points, reentrant phase transitions, isolated critical points, and include the newly discovered "superfluid black holes".

**Matthew Hogan**, Wells College

*Diffeomorphism invariant cosmological symmetry in full quantum gravity*

This talk summarizes a new proposal to define rigorously a sector of loop quantum gravity at the diffeomorphism invariant level corresponding to homogeneous and isotropic cosmologies, thereby enabling a detailed comparison of results in loop quantum gravity and loop quantum cosmology. The key technical steps we have completed are (a) to formulate conditions for homogeneity and isotropy in a diffeomorphism covariant way on the classical phase-space of general relativity, and (b) to translate these conditions consistently using well-understood techniques to loop quantum gravity. We also describe, as a proof of concept, a complete analysis of an analogous embedding of homogeneous and isotropic loop quantum cosmology into the quantum Bianchi I model of Ashtekar and Wilson-Ewing.

**Florian Hopfmueller**, Perimeter Institute

*The Null Canonical Pairs of Gravity*

The symplectic potential of Einstein gravity is integrated on a null hypersurface to obtain the canonical pairs of configuration and momentum variables, without introducing any gauge fixing. Corner degrees of freedom and the connection with boundary terms in the Lagrangian are discussed. Some of the remaining degrees of freedom are pushed to the corner, using diffeomorphisms.

**Uzair Hussain,** Memorial University

*Extreme mass ratio merger and horizon deformation*

We study the dynamical spacetime that arises when a small object radially plunges into a large Schwarzschild black hole. We assume that the mass of the small object, \mu, is much smaller than the black hole mass M. Under this assumption we can employ the Zerilli formalism modified to include a source term which arises from the energy-momentum tensor of the small object. We solve the Zerilli equation by numerically evolving initial data for various low-\ell modes of the spherical harmonics. Then, we ray trace null geodesics of the event horizon after the merger backward in time to extract the geometry of the perturbed event horizon. Further, we take advantage of the axisymmetry of the setup to locate the apparent horizon and its geometry.

**Matthew Johnson,** Perimeter Institute & York University

Constraining cosmological ultra-large scale structure using numerical relativity

Cosmic inflation, a period of accelerated expansion in the early universe, can give rise to large amplitude ultra-large scale inhomogeneities on distance scales comparable to or larger than the observable universe. The cosmic microwave background (CMB) anisotropy on the largest angular scales is sensitive to such inhomogeneities and can be used to constrain the presence of ultra-large scale structure. I will present the results of simulations in a variety of scenarios that illustrate how numerical relativity can be useful for observational cosmology in determining the history of the very early Universe.

**Darsh Kodwani**, CITA

*Longitudinal gravitational memory*

It is well known that gravitational waves leave a permanent displacement between two free-falling masses - a memory effect. We present a similar effect caused by neutrino shells that leads a change in relative velocity between two freely falling masses and the potential detection of this effect using pulsar timing and interferometers (pulsar scintillation in particular).

**Ikjyot Singh Kohli,** York University

*Stochastic Eternal Inflation in Heisenberg Universes*

I will discuss the stochastic dynamics and the implications for a minimally coupled scalar field in a Bianchi Type II / Heisenberg universe.

**David Kubiznak,** Perimeter Institute

*On Thermodynamics of Accelerating Black Holes*

I will discuss how to formulate consistent thermodynamics for accelerating black holes described by the C-metric.

**Prayush Kumar**, CITA

*Measuring neutron star tidal deformability with Advanced LIGO with neutron star - black hole binaries*

The pioneering discovery of gravitational waves (GW) by Advanced LIGO has ushered us into an era of observational GW astrophysics. Compact binaries remain the primary target sources for GW observation, of which neutron star - black hole (NSBH) binaries form an important subset. GWs from NSBH sources carry signatures of (a) the neutron star's tidal distortion by the companion black hole during inspiral, and (b) its potential tidal disruption near merger. In this talk, I will discuss how well we can measure the leading order tidal effects from individual, as well as populations of, LIGO observations of disruptive NSBH mergers. I will also discuss how our measurements of non-tidal parameters can get affected by ignoring tidal effects in LIGO's parameter estimation analyses.

**Phil Landry,** University of Guelph

*Dynamical Tidal Response of a Rotating Neutron Star*

A neutron star (NS) subject to a gravitomagnetic tidal field (associated with mass currents) develops internal fluid motions through gravitomagnetic induction; the fluid motions are irrotational, provided the star is non-rotating.

When the NS is allowed to rotate, the tidal field couples to the star's spin; the coupling is tractable in the slow-rotation limit. In this case, the fluid motions induced by an external gravitomagnetic field are fully dynamical, even if the tidal field is stationary: interior metric and fluid variables are time-dependent, and vary on the timescale of the rotation period. Remarkably, the exterior geometry of the NS remains time-independent.

**Jacob Lange,** Rochester Institute of Technology

*Comparing Synthetic Gravitational Wave data from Binary Black Hole Coalescence Directly to Numerical Solutions of Einstein’s Equation*

We compare synthetic data directly to complete numerical relativity simulations of binary black holes. In doing so, we circumvent ad-hoc approximations introduced in semi-analytical models previously used in gravitational wave parameter estimation and compare the data against the most accurate waveforms including higher modes. In this talk, we focus on the synthetic studies that test potential sources of systematic errors. We also run "end-to-end" studies of intrinsically different synthetic sources to show we can recover parameters for different systems.

**Adam Lewis,** CITA

*Fundamental Frequency Extraction from Precessing Eccentric BBH Simulations*

Despite considerable interest, the study of eccentric binary black hole inspirals remains highly underdeveloped on both numerical and analytic fronts. We therefore report here on a series of very long (12000M, where M is the total mass of the binary) “generic” fully nonlinear BBH simulations, with initial eccentricities of about 0.1 and 0.2, mass ratios of 5 and 7, primary spin magnitudes of 0.6 and 0.8, and spin-separation inclination angles ranging between 0 and 80 degrees. We use these runs to showcase techniques for extracting the fundamental coordinate frequencies of the motion. These yield new gauge-insensitive methods for analytic-numeric comparisons. Our techniques also allow for the detection of orbital resonances, which we detect but measure no effect from.

**Steve Liebling,** Long Island University

*Pushing GR's Limits With LIGO's Detections*

**Hyun Lim,** Brigham Young University

*A wavelet approach for binary compact object merger*

Highly accurate simulations of binary black holes and neutron stars are needed to address a variety of interesting problems in relativistic astrophysics. We present a new method for solving the Einstein equations in the BSSN and CCZ4 formulations using iterated interpolating wavelets. Wavelet coefficients provide a direct measure of the local approximation error for a solution and place collocation points that naturally adapt to features of the solution. Further, they exhibit exponential convergence on unevenly spaced collection points. The parallel implementation of the wavelet simulation framework presented here deviates from conventional practice in combining multi-threading with a form of message-driven computation sometimes referred to as asynchronous multitasking.

**Carlos Lousto,** Rochester Institute of Technology

*Modeling the source of GW150914 and GW151226 with targeted numerical-relativity simulations*

The theoretical gravitational-wave signal for merging black holes, as predicted by general relativity, can be computed only by full numerical relativity, because analytic approximations fail near the time of merger. In this talk, we report the modeling of GW150914 and GW151226 with numerical-relativity simulations, using black-hole masses and spins consistent with those inferred from LIGO's measurement. In particular, we employ two independent numerical-relativity codes that use completely different analytical and numerical methods to model the same merging black holes and to compute the emitted gravitational waveform; we find excellent agreement between the waveforms produced by the two independent codes.

**Wayne Lundberg **

*A Comprehensive Theory Scorecard*

A broad perspective on theoretical physics is achieved by simply scoring success at addressing various problems. Such problems are generally distinct areas in the ontology of physics research…but each claim to be seeking insight into a “common mathematical foundation.” A concerted search is also motivated by the discovery of Higgs Boson, together with new empirical constraints. The ‘scorecard’ approach inter-relates the collected body of results, particularly efforts to explain gravitational anomalies. It is important to diagnose inconsistency of theoretical formulae with those that govern other space-time scales. Inconsistent formulation frustrates existing theories’ success at explaining anomalous particle, gravitational and cosmological observations. The key is to determine which theory must be reformulated- and how! A well-founded theory applicable across all physical scales results which yields qualitative agreement with all observations.

**Morgan Lynch,** University of Wisconsin-Milwaukee

*Temperatures of Renormalizable Quantum Field Theories in Curved Spacetime*

We compute the temperature registered by an Unruh-DeWitt detector coupled to a Hadamard renormalizable quantum field and moving along an arbitrary accelerated trajectory in curved spacetime.

**Robert Mann,** University of Waterloo

*Noisy Quantum Cosmology*

**Hugo Marrochio**, Perimeter Institute

*Complexity of Formation in Holography - part II*

It has recently been conjectured that computational quantum complexity can be computed for a holographic state by evaluating the gravitational action on a region in the bulk of the dual gravitational theory bounded by null surfaces which goes under the name of the Wheeler-DeWitt patch. We use a set of rules for evaluating the contributions of the null surfaces and joints to compute the action on such a region anchored at the boundary at t = 0 for certain black hole spacetimes. We compare this result to that of two copies of empty AdS. We find that for boundary dimension d > 2 in the limit of large horizon radius the action difference grows linearly as the horizon entropy with proportionality coefficient (d − 2)/d · cot(π/d). For BTZ black holes the action difference is independent of the black hole mass.

**Raissa Mendes,** University of Guelph

*Testing scalar-tensor theories with highly compact neutron stars*

Scalar-tensor theories of gravity are extensions of General Relativity including an extra, non-minimally coupled scalar degree of freedom. A wide class of these theories, albeit indistinguishable from GR in the weak field regime, can predict a radically different phenomenology for neutron stars, due to the existence of a non-perturbative, strong-field effect referred to as spontaneous scalarization. This effect is known to occur for theories where the linear effective coupling $\beta$ between the scalar and matter fields is sufficiently negative, and has been strongly constrained by pulsar timing observations. In this talk, I discuss the possibility of testing scalar-tensor theories in the highly unconstrained region of coupling functions with $\beta>0$, based on the fact that sufficiently compact neutron stars in these theories would be subject to a tachyonic-like instability. I will discuss the results of numerical simulations determining the various end-states of this instability and their observational signatures.

**James Mertens,** Case Western Reserve University

*Deviations from Homogeneity in an Inhomogeneous Universe*

It has long been wondered to what extent the observable properties of an inhomogeneous universe will be measurably different from a corresponding FLRW model. Here, we use tools from numerical relativity to study the properties of photons traversing an inhomogeneous universe. We evolve the full, unconstrained Einstein field equations for a spacetime containing dust and vacuum energy in proportions similar to our Universe, with a spectrum of long-wavelength density perturbations similar to the observed one. We then integrate the optical scalar equations along paths through this numerical spacetime, with all paths terminating at an observer situated similarly to ourselves, and construct the resulting Hubble diagrams.

**Vassilios Mewes,** Rochester Institute of Technology

*Numerical relativity simulations of tilted BH-torus systems*

We present results from three-dimensional, numerical relativity simulations of a {\it tilted} black hole-thick accretion disc system. The simulations are analysed using tracer particles in the disc which are advected with the flow. Such tracers, which we employ in these new simulations for the first time, provide a powerful means to analyse in detail the complex dynamics of tilted black hole-torus systems. We show how its use helps to gain insight in the overall dynamics of the system, discussing the origin of the observed black hole precession and the development of a global non-axisymmetric m=1 mode in the disc. Our three-dimensional simulations show the presence of quasi-periodic oscillations (QPOs) in the instantaneous accretion rate, with frequencies in a range compatible with those observed in low mass X-ray binaries with either a black hole or a neutron star component. The frequency ratio of the dominant low frequency peak and the first overtone is o1/f∼1.9, a frequency ratio not attainable when modelling the QPOs as p-mode oscillations in axisymmetric tori.

**Jonah Miller,** Perimeter Institute

*Simulating the Ejecta of Binary Neutron Star Coalescence at High Resolution*

Observational signatures of binary neutron star mergers include gravitational waves and faint supernova-like transients powered by radioactive decay of freshly synthesized heavy elements. We use smoothed particle hydrodynamics, which is well suited for such problems, and adapt the highly scalable 2HOT code to simulate these mergers. Retaining performance while adding new physics provides a unique opportunity to exercise the principles of co-design and for a collaboration between domain and computer scientists.

**Elliot Nelson,** Perimeter Institute

*Quantum Information of the Inflationary Wavefunction*

During inflation, the degrees of freedom of metric fluctuations in different regions of space become entangled, and share information that leads to decoherence of quantum fluctuations into classical field perturbations. We apply quantum information tools to study this process and relate the spread of redundant information to the dynamics of the inflationary wave functional, commenting on the role of gravity and on branching of the wave function.

**Keith Ng,** University of Waterloo

*The equivalence principle and QFT: Can a particle detector tell if we live inside a hollow shell?*

We show that a particle detector can distinguish the interior of a hollow shell from flat space for switching times much shorter than the light-crossing time of the shell, even though the local metrics are indistinguishable. This shows that a particle detector can read out information about the non-local structure of spacetime even when switched on for scales much shorter than the characteristic scale of the non-locality.

**Alejandro Satz,** Pennsylvania State University

*Entropy of observable subalgebras and quantum field entanglement*

A new perspective on the entanglement entropy of quantum fields restricted to a spatial region can be attained by seeing it as a limiting case of a well-defined and more general quantity, the entropy of a subalgebra of smeared field observables. We introduce this notion and discuss various examples, including the recovery from it of the entanglement entropy of a sphere in Minkowski space.

**Zachary Silberman,** Rochester Institute of Technology

*Generation of Vector Potentials for Numerical Relativity Initial Data*

In studies of highly relativistic magnetized accretion flows around black holes, many different numerical codes are employed; while some codes evolve the magnetic field vector B, others evolve the magnetic vector potential A, the two being related by the curl: B=curl(A). Here, we discuss how to generate vector potentials corresponding to specified magnetic fields on staggered grids, a surprisingly difficult task on finite cubic domains. The code we have developed solves this problem in two ways: a direct linear algebra approach and a brute-force method whose scaling is nearly linear in the number of grid cells. We discuss the success both algorithms have in generating smooth vector potential configurations, how they scale for various problem sizes, and how both may be extended to more complicated cases involving multiple mesh-refinement levels.

**Alexander Smith,** University of Waterloo

*Spacetime Topology and Vacuum Entanglement*

Questions about the topological structure of our Universe are unanswered by the theory of relativity and are expected to be explained by a full theory of quantum gravity. In this talk, we will analyze how the topology of our Universe influences both vacuum fluctuations and vacuum entanglement. We will see in what ways and to what extent particle detectors are sensitive to the underlying topology of the Universe, and we will discuss how to use them to distinguish universes with identical local geometry but differing global topology. Further, we will see that if the spacetime topology induces a preferred direction, the vacuum entanglement harvesting protocol becomes sensitive to it.

**Sotaro Sugishita,** Osaka University

*Entanglement entropy for free scalar fields in AdS:*

We compute entanglement entropy for free massive scalar fields in AdS. The entangling surface is a minimal surface whose boundary is a sphere at the boundary of AdS. We also evaluate 1-loop quantum corrections coming from the scalar fields to holographic entanglement entropy. Applying the results, we compute the leading difference of entanglement entropy between two holographic CFTs related by a renormalization group flow triggered by a double trace deformation. This talk is based on arXiv:1608.00305.

**Alexandra Terrana,** Perimeter Institute & York University

*Mapping the Universe with the large-scale kinetic Sunyaev-Zel'dovich effect*

In this talk, I describe how the kinetic Sunyaev-Zel'dovich (kSZ) effect, cosmic microwave background (CMB) anisotropies induced by the Compton scattering of CMB photons by free electrons undergoing bulk motion, can be a tool for studying the observable Universe on the largest scales. In this regime, the kSZ effect is a census of the CMB dipole observed by free electrons on our past light cone in the post-reionzation Universe. Long wavelength modes of the gravitational potential induce a power asymmetry in the cross correlation of the kSZ CMB anisotropies and the electron density field as a function of redshift. We forecast the ability of future experiments to detect this signal, and the possibility of using this signature to reconstruct the 3D gravitational potential on large scales.

**David Wenjie Tian,** ICN-UNAM

*The very early Universe in modified gravity*

In modified gravities and with respective to the minimal standard model, one can study the gravitational baryogenesis and leptogenesis induced by nonstandard cosmic expansion; investigate hot, warm and cold dark matter as thermal relics of the very early Universe; calculate the primordial abundances of D, T, He-3, He-4, Li-6, Li-7, Be-7 from the semi-analytical approach; and look into hydrogen recombination and the cosmic microwave background. We will take power-law f(R) and nonminimally coupled f(R,Tm) gravities as examples to illustrate that, nonstandard behaviors of the very early Universe in different eras exert joint constraints on the viability of modified gravity theories.

**Erickson Tjoa,** University of Waterloo

*Superfluid Black Holes*

In this talk I will discuss the recently discovered "superfluid black holes". These are the first example of black holes which exhibit a ``$\lambda$-line" phase transition, i.e. a line of second order (continuous) phase transitions. The transition closely resembles those found in condensed matter systems which, in the case of liquid $^4$He marks the normal fluid/superfluid transition. This lambda transition occurs within the context of black hole chemistry for a class of asymptotically anti-de Sitter hairy black holes in cubic (and higher) order Lovelock gravity where a real scalar field is conformally coupled to gravity. During my talk I will discuss the model, the phase transition, and the necessary conditions a black hole equation of state must satisfy to admit $\lambda$-lines.

**Alexander Tolish,** University of Chicago

*The Cosmological Memory Effect:*

The "memory effect" is the permanent change in the relative separation of test particles resulting from the passage of gravitational radiation. We investigate the memory effect for a general, spatially flat FLRW cosmology by considering the radiation associated with emission events involving particle-like sources. We find that if the resulting perturbation is decomposed into scalar, vector, and tensor parts, only the tensor part contributes to memory. Furthermore, the tensor contribution to memory depends only on the cosmological scale factor at the source and observation events, not on the detailed expansion history of the universe. In particular, for sources at the same luminosity distance, the memory effect in a spatially flat FLRW spacetime is enhanced over the Minkowski case by a factor of (1+z).

**Eric Van Oeveren,** University of Wisconsin-Milwaukee

*A Constraint on the Tidal Deformability of Neutron Stars*

LIGO will soon detect gravitational waves sourced by the inspiral and merger of binary systems that include neutron stars. These signals will include information about the neutron star equation of state; in particular, the phase evolution will give a constraint on the tidal deformability of neutron stars. We place a theoretical constraint, based on causality, on the tidal deformability and estimate the resulting effect on the gravitational waves from a black hole-neutron star binary.

**Trevor Vincent,** CITA

*Computing Initial Data with Discontinuous Galerkin Methods*

Discontinuous Galerkin (DG) finite element methods have been used to solve hyperbolic PDEs in relativistic simulations and offer advantages over traditional discretization methods. Comparatively little attention has been given towards using the DG method to solve the elliptic PDEs arising from the Einstein initial data equations. We describe how the DG method can be used to create a parallel, adaptive solver for initial data. We discuss the current state of the dG code we are developing.

**Shouhong Wang,** Indiana University

*Law of gravity, dark matter and dark energy*

In this talk, we demonstrate that the presence of dark matter and dark energy requires that the variation of the Einstein-Hilbert action be taken under energy-momentum conservation constraint. This gives rise to a new set of field equations, altering the Einstein equations with a new term analytically derived from the constraints. Then we show that with the gravitational field equations, gravity behaves like the Einstein gravity in the solar system, and it has more attraction in the galactic scale (dark matter), and it becomes repulsive over very large scale (dark energy).

**I-Sheng Yang,** Perimeter Institute & CITA

*Gravitational rotation of polarization from pulsar binaries.*

**Aaron Zimmerman,** CITA

*Extracting the redshift factor in binary black hole simulations*

The redshift factor of a black hole is an invariant quantity of fundamental interest in PN and self-force descriptions of circular binaries. We have implemented a novel method for extracting the redshift factor in simulations, confirming a conjectured relationship between it and the surface gravity of a black hole. This redshift factor allows for an array of new comparisons between analytical approximations and numerical simulations, and it also allows us to test a generalization of the first law of black hole mechanics to binaries. I will present our method and initial comparisons between analytics and numerics.

**Peter Zimmerman,** University of Arizona

*Horizon Instability of Extremal Black Holes*

Aretakis's discovery of a horizon instability of extremal black holes came as something of a surprise given earlier proofs that individual frequency modes are bounded. Is this kind of instability invisible to frequency-domain analysis? The answer is no: We show that the horizon instability of the Kerr black hole can be recovered in a mode analysis as a branch point at the horizon frequency. We use the mode approach to generalize to nonaxisymmetric perturbations, finding an enhanced growth rate. In the electromagnetic and gravitational cases, the field strength and curvature exhibit unbounded growth in time. Finally, by studying charged scalar perturbations of the extremal Reissner–Nordströ m solution, we connect the enhanced growth to the existence of a superradiant bound.

**Yosef Zlochower,** Rochester Institute of Technology

*Puncture-Based Evolutions of Highly Spinning Black-Hole Binaries*

We recently developed a code for solving the 3+1 system of constraints for highly-spinning black-hole binary initial data in the puncture formalism. Here we explore how different choices of gauge can be used to efficiently evolve binaries with near maximal spins.

**Nosiphiwo Zwane,** Perimeter Institute

*Cosmological test of Everpresent Lambda*

Everpresent Lambda is a cosmological scenario inspired by the causal set approach to quantum gravity, which predicts that the observed "cosmological constant" fluctuates between positive and negative values with a vanishing mean. This implies that the "cosmological constant" is a stochastic function of cosmic time, with a standard deviation comparable to the critical density of the universe at any epoch. By exploring the space of cosmological parameters and stochastic realizations of dark energy via Monte Carlo Markov chains, we show that Everpresent Lambda can fit cosmological observations as well as the standard LCDM model. Furthermore, it can potentially ease some high redshift tensions with CDM model, such as the Baryonic Acoustic Oscillations (BAO) in Lyman- forest at z 2~3, the ultra-massive black holes at z ~ 7, and the primordial Lithium abundance.