The 35th Midwest Relativity Meeting & Eric Fest

31 Oct 2025, 07:55 → 2 Nov 2025, 17:00 Canada/Eastern

113 (MacKinnon Building)

Adam Pound (University of Southampton), Berend Schneider (University of Guelph), Béatrice Bonga (Radboud University), Eric Poisson, Luis Lehner, Philippe Landry (Canadian Institute for Theoretical Astrophysics), Roland Haas (University of Illinois), Tristan Pitre (University of Guelph)

The 35th Midwest Relativity Meeting will be held at the University of Guelph on October 31 to November 2 (Friday to Sunday), 2025. The last day will be a scientific celebration of Eric Poisson's 60th birthday with invited speakers.

The Midwest Relativity Meetings bring together researchers from across the Midwest and beyond to discuss a broad range of topics in gravitational physics, including cosmology, gravitational waves, numerical relativity, and quantum gravity. The primary focus of this meeting is on graduate students and postdocs, all of whom are encouraged to give talks. We encourage participation from under-represented groups, and we expect to accept all abstract submissions as long as time permits. The Blue Apple Award, sponsored by APS DGRAV, will be given for the best graduate student talk. 

Invited speakers at Eric Fest

Leor Barack
Patrick Brady
Katerina Chatziioannou
Eanna Flanagan
Scott Hughes
Sharon Morsink
Frans Pretorius
Jocelyn Read
Clifford Will

Funding sources

The 35th MWRM and EricFest are generously funded by the Canadian Institute for Theoretical Astrophysics, Perimeter Institute, the UKRI/ERC grant GWModels, and the University of Guelph through (1) the Department of Physics, (2) the College of Computational, Mathematical, and Physical Sciences, (3) the Vice-President (Research & Innovation), (4) the Provost and Vice-President (Academic), and (5) the Office of Graduate and Postdoctoral Studies. 

               

Friday 31 October

08:30 Registration

Friday Morning Session 1

1 Opening Remarks

2 Quasi-local conservation laws as a framework for boundary conditions in numerical relativity

In a series of previous works, several initial boundary value problems in numerical relativity were demonstrated as numerically stable even with boundaries placed in the strongly gravitating region of a dynamical spacetime. Now, care is being taken to define boundary conditions that can be placed on the radiating degrees of freedom based on quasi-local conservation laws. Incoming gravitational waves can be set by controlling the flux of quasi-local quantities, leading to a boundary condition framework first order in derivatives of the metric and with a physical interpretation valid in any location in a dynamical spacetime.

Speaker: Conner Dailey (Friedrich Schiller University)

3 Advances in Numerical Relativity for Modeling Compact Binaries and Gravitational Wave Emission

Numerical relativity (NR) has revolutionized our understanding of strong-field gravity, enabling high-fidelity simulations of compact binary systems. This paper reviews recent breakthroughs in NR methodologies, with emphasis on binary neutron star (BNS) and black hole–neutron star (BHNS) mergers. We discuss advancements in adaptive mesh refinement, constraint-damping formulations, and microphysical treatments of neutron star matter. These developments have refined predictions of gravitational waveforms, kilonova signatures, and remnant behavior, directly impacting multimessenger astronomy. We also present new results from simulations of high-spin BHNS systems and high-mass-ratio BNS mergers, highlighting implications for upcoming gravitational wave detectors (e.g., Einstein Telescope, Cosmic Explorer) and nuclear astrophysics.

Speaker: vahid taherkhani (phd student in astronomy &astrophysics in zanjan university)

4 High-Order Time Derivatives of Kerr Orbital Functionals

Functions of bound Kerr geodesic motion play a central role in many calculations in relativistic astrophysics, ranging from gravitational-wave generation to self-force and radiation-reaction modeling. Although these functions can be expressed as a Fourier series using the geodesic fundamental frequencies, reconstructing them in coordinate time is challenging due to the coupling of the radial and polar motions. In this talk, I will discuss two strategies for performing such reconstructions and their ability to estimate high-order coordinate-time derivatives of the orbital functional. The first method maps Fourier coefficients from Mino to coordinate time; the second method fits a sampled time series of the function to a truncated coordinate‑time Fourier series. While the latter method is prone to overfitting, it yields more accurate reconstructions and derivatives than the mapping, but completely misrepresents the harmonic content of the orbital functional. For the purpose of accurate coordinate-time derivative estimation, I will show how a hybrid method is more suitable to compute high-order derivatives. These results offer a general framework for accurately evaluating higher-order time derivatives along Kerr geodesic worldlines, with direct relevance to applications such as extreme-mass-ratio inspiral kludge waveform modeling, where such derivatives are key ingredients for precise gravitational-wave predictions.

Speaker: Alejandro Cardenas-Avendano (Wake Forest University)

5 Identifying spacetimes using neural networks

In the realm of general relativity, given two metric solutions in different gauges, determining whether they describe the same physical scenario poses significant challenges. This study proposes a novel application of machine learning techniques to address this issue within the context of numerical relativity. We introduce the first implementation of neural networks to learn the mapping between two metric solutions with identical manifold structure. We will also discuss potential application of how this approach can be used to compare various numerical relativity codes.

Speaker: Estuti Shukla (Penn State University)

6 3D Simulations of the Ekpyrotic Cosmological Scenario

In this work, we perform the first fully three-dimensional simulations of the ekpyrotic cosmological scenario. Our numerical relativity code is based on previous work that assumed some symmetry. In particular, we use a tetrad formulation of the Einstein-scalar field equations, but in this case using the parallel infrastructure PAMR. Our goal is to test the robustness of the ekpyrotic (slowly contracting) phase in smoothing and flattening the universe from highly inhomogeneous and anisotropic conditions. Our preliminary results suggest that in the fully generic case, the negative exponential potential of the ekpyrotic scenario is an efficient mechanism for homogenization and isotropization.

Speaker: Zachary Tyler (Oakland University)

7 Comparison of binary compact object merger populations from population-synthesis codes

Many binary population-synthesis (BPS) codes have been developed over the last few decades to evolve binary stars throughout their entire evolutionary history. BPS codes are extremely useful for investigating astrophysical processes we observe in the universe such as supernovae, x-ray binaries, generation of r-process elements, and gravitational-wave populations. However, many current BPS codes, such as COSMIC, rely on antiquated single-star evolutionary tracks and coarse prescriptions for binary physics. Newer BPS codes, such as POSYDON, instead use MESA binary sequences that self-consistently evolve each star's internal structure along with the binary's orbit. However, these BPS codes are computationally expensive to create and therefore are limited in their ability to explore uncertainties in the physics of binary evolution. In this work, we generate populations evolved from the same initial binary star population using three BPS codes used in the community – COSMIC with standard single-star evolutionary tracks, COSMIC updated with MESA single-star tracks, and POSYDON, which uses full MESA binary sequences – to determine the robustness of population predictions across different simulation methods. Using compact binary mergers as a test case, we find stark differences in the rates, properties, formation pathways, and progenitors.

Speaker: Alexandra Guerrero (University of Chicago)

10:30 Coffee Break

Friday Morning Session 2

8 Extracting Astrophysical Clues from Eccentric, Precessing Binary Black Hole Dynamics

The most general bound binary black hole (BBH) system has an eccentric orbit and precessing spins. Detecting a BBH with significant eccentricity near merger would be a clear signature of dynamical formation, while the spin–orbit misalignment angles (spin tilts) provide a powerful probe of different astrophysical formation channels. We have developed publicly available codes that evolve eccentric, precessing BBHs using orbit-averaged and precession-averaged post-Newtonian equations. These codes can evolve systems both backwards and forwards in time, enabling constraints on formation scenarios such as hierarchical mergers. Using them, we study the coupled evolution of orbital and spin-precession parameters, focusing on how eccentricity influences the separation at which spin morphology transitions occur. This framework can also inform the development of waveform models for eccentric, precessing BBHs, thereby enhancing our understanding of these systems through gravitational-wave observations.

Speaker: Anuradha Gupta (The University of Mississippi)

9 Geodesics and stability of a black hole surrounded by an anisotropic fluid

The Kiselev metric has gained increasing relevance in recent years due to its ability to model a variety of physical systems. It describes a black hole influenced by the presence of an anisotropic fluid—composed of the superposition of a perfect fluid, an electromagnetic field, and a scalar field. Depending on the equation of state parameter $\omega$, this metric can model dispersed gases in nebulae, accretion disks, and even dark matter. The metric becomes even more compelling for describing the environment around compact objects when a rotation parameter $a$ is introduced. In this work we analyze photon geodesics in de Sitter-type spacetimes described by the rotating Kiselev metric. Furthermore, we investigate the dynamical stability of this solution without rotation through the calculation of quasinormal modes and their corresponding quasinormal frequencies.

Speaker: Diogo Gripa Rodrigues (NASA Goddard Space Flight Center)

10 Dynamical quasinormal mode excitation

Current waveform models of binary black hole mergers incorporate a large amount of analytical information during the inspiral phase. On the other hand, post-merger templates use a phenomenological ansatz informed by numerical simulations, with the quasinormal frequencies as the only analytical input.
Working in the perturbative limit, we develop the first analytical model predicting the dynamical excitation of quasinormal modes during the plunge-merger-ringdown stages for generic orbits, including highly eccentric ones. We find that quasinormal mode amplitudes behave as activation functions near the waveform peak. Once the stationary ringdown regime is reached, a superposition of an infinite tower of non-oscillatory, exponentially damped terms appears. These terms are due to the source redshift at the horizon and can potentially swamp the overtone contributions. As a byproduct of our analysis, we explain from first principles the impact of the inspiral on the late-time, constant quasinormal mode amplitudes, which was previously observed a posteriori from numerical relativity fits.
Our model shows good agreement with a numerical solution of the binary merger. The proposed approach offers a highly promising way to extract physical quasinormal mode contributions, avoiding ambiguities and overfitting issues associated with phenomenological ansätze, and to construct analytically informed ringdown models.

Speaker: Marina De Amicis (Niels Bohr Institute)

11 Perturbed black holes and Integrability

Black Hole Perturbation Theory (BHPT) has been shown to be very successful in describing physical processes around BHs. In particular, the scattering of electromagnetic and gravitational waves (and other physical fields) and the so-called quasinormal mode (QNM) oscillations, the damped BH oscillations that dominate the ringdown phase of binary BH (BBH) coalescences.
A key feature of BHPT is that all the physically observable quantities can be obtained only in terms of (gauge-invariant) master functions that decouple the perturbative Einstein equations. In this talk, I will argue that, in the Schwarzschild case, this fact is connected to an underlying Darboux covariance structure of the space of master functions and equations.
Beyond this, one can deform the time-independent Schroedinger-type master equations along the flow of the completely integrable Korteweg–de Vries (KdV) hierarchy. This integrable structure generates an infinite tower of conserved quantities, the KdV integrals, which fully determine scattering amplitudes through a classical moment problem. In this way, integrability provides a new organizing principle for the perturbative dynamics of BHs.
Finally, I will discuss the additional integrability structures that appear and the important role that they can play in getting a deeper understanding of the general dynamics of BHs and what are the prospects of using these methods in the BBH problem.

Speaker: Carlos F. Sopuerta (Institute of Space Sciences of the Spanish National Research Council (ICE, CSIC))

12 Evolution of a black hole cluster in full general relativity

We evolve for the first time in full GR a small, collisional N-body black hole cluster of arbitrary total mass M. The bound cluster is initially compact (radius R/M≈10), stable, and consists of 25 equal-mass, nonspinning black holes. The dynamical interactions of compact objects in N-body clusters is of great interest for the formation of black holes in the upper mass gap as well as intermediate and supermassive black holes. These are potential sources of gravitational waves that may be detected by both current and future observatories. Unlike previous N-body Newtonian simulations, no “subgrid physics” is required. We can therefore confirm in full GR several key predictions: runaway growth of a large black hole via repeated mergers; spindown of the central black hole with increasing captures; the ejection of a black hole with a large asymptotic velocity due to a several-body interaction; and a regime where mergers occur primarily via direct collisions on highly eccentric orbits instead of quasicircular inspirals. We extract the GW signal and find it has several distinct features associated with the compact cluster regime. The signal is sufficiently loud that next generation observatories would likely be able to detect similar events across most of the observable universe. This work is a proof-of-principle study that we hope will open up a new arena for numerical relativity.

Speaker: Jamie Bamber (University of Illinois Urbana-Champaign)

13 Influence of observer’s inclination and spacetime structure on photon ring observables

Recent millimeter-wavelength observations of the near-horizon regions of
M87 and Sgr A by the Event Horizon Telescope have significantly advanced our understanding of gravity and astrophysics under extreme conditions. A key goal for future observations, with improved angular resolution and sensitivity, is the detection of the photon ring in these black holes. For Kerr black holes, photon rings exhibit universal scaling relations in terms of purely geometric observables: the demagnification exponent, time delay, and rotation parameter. Our study employs a nonperturbative, parametrized spinning spacetime to investigate how these observables vary with deviations from the no-hair theorem and the observer’s inclination. Particular emphasis is placed on polar observers, which is highly relevant for M87*. Our findings reveal that these observables encode vital information about the strong-field spacetime structure, including the morphology of the ergosphere. By analyzing specific non-Kerr spacetimes, we demonstrate that potential measurements of these observables, including the shadow size, can directly constrain the spin and Kerr-deviation functions. For polar observers, time delay measurements offer an independent estimate of the shadow size, providing a complementary probe of the black hole’s geometry. These results highlight the potential of using photon rings for precision tests of general relativity and for constraining strong-field deviations from Kerr spacetime.

Speaker: kiana salehi (perimeter Institute)

12:30 Lunch

Friday Afternoon Session 1

14 Testing Redshift Distributions in Gravitational-Wave Sources: A New Approach with V/V_max

The application of $V/V_{max}$ statistics to gravitational-wave cosmology aids in our understanding of compact object parameter evolution, particularly the redshift and mass distributions of binary systems. Transforming $V/V_{max}$ from a distance-dependent to a population-dependent diagnostic provides a direct pipeline for testing various gravitational-wave source distributions. Through this framework, we find support for a binary black hole source distribution favoring lower redshift evolution, modeled as $\rho(z) = (1+z)^k (dV_c/dz)$, particularly for lower values of $k$ (e.g., $k=1$ or $k=2$). Furthermore, we find that the $V/V_{max}$ methodology remains robust under minimal assumptions about the mass distribution, with negligible effect on $V/V_{max}$ values for individual events. Finally, we compare this methodology to Hierarchical Bayesian Inference, and why $V/V_{max}$ can act as a powerful tool for population statistics with O4 data.

Speaker: Chaitanya Khamar (University of Toronto-CITA)

15 Lensing-Induced Biases in Gravitational-Wave Inference and Tests of General Relativity

As gravitational-wave detectors reveal a growing population of black hole mergers, a hidden source of bias may already be shaping how we interpret them. Gravitational-wave lensing—an inevitable consequence of massive structures in the Universe—can distort signals in ways that mimic precession or apparent deviations from General Relativity. I will present a systematic study of lensing-induced biases in gravitational-wave parameter estimation and tests of General Relativity, showing how gravitational lensing can produce misidentified physical effects even when not confidently detected. Understanding and disentangling these effects will be critical to uncovering the true physics encoded in gravitational-wave signals.

Speaker: Ania Liu

16 Modeling Multimessenger White Dwarf Binaries Within The Milky Way

Multi-messenger astronomy is a new and promising area in astrophysics where the same phenomenon is seen using multiple methods. In this project we examine binary systems of dense stellar core remnants known as white dwarfs (WDs) which may be seen both in the electromagnetic spectrum as light and in gravitation as gravitational waves (GWs). Gravitational waves were first predicted by Albert Einstein in 1916; however, they weren’t directly detected up until very recently in 2015 by LIGO (Laser Interferometer Gravitational-wave Observatory). LISA (Laser Interferometer Space Antenna) is a future gravitational wave detector set to launch in 2034; it will allow the detection of GWs produced by objects such as white dwarfs whose frequency lay much lower than the LIGO band. In preparation for LISA, one must simulate these binary systems numerically to accurately estimate observational expectations. This project used a code known as COSMIC (Compact Object Synthesis and Monte Carlo Investigation Code) to simulate white dwarf populations. These populations were then mapped to a 3D distribution of HI and H2 gas within the Milky Way so that dust extinction approximations could allow for a more accurate calculation of apparent magnitude. Magnitudes were then compared to telescopes like the Vera Rubin Observatory to determine what of the population would be optically visible and qualify as multi-messenger sources.

Speaker: Ryan Beam (Grand Valley State University)

17 Using the LISA spacecraft to detect NEO

The Laser Interferometer Space Antenna (LISA) is scheduled to be launched in 2035 with the primary purpose of detecting gravity waves. This research analyzes LISA potential to detect Near Earth Objects (NEO) as a secondary purpose. This potential was assessed by examining the orbital paths of currently known asteroids to determine the minimal distance between LISA’s path and the NEOs. This distance was used to calculate gravitational accelerations on LISA and compared to its instrumental precision. Then, Monte-Carlo simulations were run in order to determine the much larger scope of yet undetected NEO’s. Through these methods, we found LISA’s ability to detect NEO’s to be limited; only Asteroids with a large mass and in close proximity are detectable.

Speaker: Brett Bolen

18 Neural Post-Einsteinian Test of General Relativity with the Third Gravitational-Wave Transient Catalog

Gravitational waves from compact binaries provide excellent opportunities for testing general relativity (GR) in the strong- and dynamical-field regime. So far, tests using inspiral signals have employed certain variants of the parametrized post-Einsteinian (ppE) framework to model deviations from GR. However, this approach has several limitations: the parameters introduced do not fully encompass all potential deviations from GR, and the estimation of these parameters is computationally expensive. To address the above issues, a neural post-Einsteinian (npE) framework has been developed by extending the ppE parametrization with a generative neural network, and the performance of the npE framework has been demonstrated using synthetic signals. In this talk, I will present recent results from applying the npE framework to the real data in the third Gravitational-Wave Transient Catalog. With a broader range of non-GR deviations incorporated in the npE parameter space, the test-of-GR conclusions can be drawn with greater robustness.

Speaker: Yiqi Xie (Canadian Institute for Theoretical Astrophysics)

19 Testing the Inspiral-Merger-Ringdown Test with Boson Stars

As detections of gravitational waves are increasing, they offer a testing ground for General Relativity. One test for deviations from General Relativity and the Standard Model is the inspiral-merger-ringdown test. In this project, we investigate the applicability and reliability of this test based on simulated observations of injected boson star waveforms. We find that the test in its most common form is is unreliable in this context, as it classifies injected binary boson stars as binary black holes. While adaptions of the test can increase the reliability to some extent, it also depends on the form of the injected waveform used for the simulated observation.

Speaker: Antonia Seifert (Perimeter Institute & University of Waterloo)

20 Numerical relativity in biscalar-tensor gravity

The search for modifications to Einstein’s theory of general relativity has become increasingly intriguing as recent and upcoming gravitational wave experiments open a new window to probe gravity with compact objects such as black holes. One natural extension to general relativity arises from the four-dimensional string-inspired effective field theory of gravity which introduces two scalar fields, (non)minimally coupled to curvature as well as coupled to each other. To consider the effect of these interactions in strong gravity environments, I will present numerical relativity simulations of two scalar fields evolving around single and binary black holes to consider the dynamical formation of the black-hole hairs using my open-source parameterized numerical relativity code, Canuda-AxiDil. We find an enhanced deviation from general relativity due to the coupling between the fields and demonstrate the presence of hair growth and radiative modes during the binary black hole inspiral, potentially leaving an imprint of the resulting gravitational wave.

Speaker: Chloe Richards

15:45 Coffee Break

Friday Afternoon Session 2

21 Towards Quantization of Horava Gravity

Horava Gravity is a field-theoretic model that shares many properties of GR. Namely it has metric as a basic degree of freedom and possesses spatial diffeomorphisms, as well as global time reparameterizations, as its symmetries. The feature of the model is the potentially much improved high energy behavior at the expense of forbidding the symmetry of local time reparameterizations and consequently Lorentz boosts. Heavily constrained as a phenomenological model, -- due to the constraints on Lorentz Violation, -- as a theoretical model it can provide a framework for asking questions in gravity-like theories that require access to the high energy regime. In the talk, I will report on the progress made on the understanding of its high energy properties, using QFT methods and tools.

Speaker: Jury Radkovski (Perimeter Institute, McMaster University)

22 Quantum nature of spacetime curvature in the deep quantum regime of a black hole

We investigate the quantum behavior of the Kretschmann curvature invariant $\mathcal{K}$ in the deep quantum regime near black hole singularities, where the classical expression diverges as $r^{-6}$ as $r \to 0$. Our goal is to understand whether this singular behavior persists under a quantum mechanical treatment. Due to the involvement of both first- and second-order time derivatives of metric variables in $\mathcal{K}$, formulating a corresponding quantum operator $\hat{\mathcal{K}}$ poses significant challenges. To address this, we consider the Wheeler-DeWitt equation within a Kantowski–Sachs minisuperspace framework in the presence of quantum vacuum fluctuations modeled by a Klein–Gordon field. The separable structure of the equation allows for a Bohmian quantum mechanical interpretation, enabling the derivation of quantum potentials and a procedure to represent second-order time derivatives as quantum operators. With the resulting Kretschmann operator $\hat{\mathcal{K}}$, we compute its expectation value $\langle \hat{\mathcal{K}} \rangle$ using regular wave function solutions (Class I and II) of the Wheeler–DeWitt equation. Our analysis reveals that $\langle \hat{\mathcal{K}} \rangle$ remains finite in certain quantum subregions, provided the wave function eigenvalues are subject to stricter constraints than those originally defining the solution classes. These findings suggest a potential resolution of curvature singularities within a quantum gravitational framework.

Speaker: HARPREET SINGH

23 Physical Foundations of Schrödinger and Dirac Wave Equations and their Implications to Quantum Cosmology and Quantum Gravity

Abstract
Introduction of stochastic definitions of universal Planck and Boltzmann constants (h, k) reveals hydrodynamic origins of both quantum mechanics and quantum field theory. Also, new perspectives regarding the quantum nature of internal atomic (spacetime, mass) versus external (space, time, mass), black hole, white hole, and Dirac anti-matter are discussed. The temperature limits (T = zero, T = infinity) of Huygens compressible ether are shown to correspond to formation of (black hole, white hole) at (circumference, center) of Poincaré disk. Also, the interesting problem of black-hole to white-hole transition of Rovelli due to Einstein time-reversible field equations is examined. Physical foundation of quantum mechanics, Schrödinger equation, and quantum mechanics wave function are examined. Also, physical foundation of quantum field theory and derivation of invariant Dirac relativistic wave equation from invariant modified equation of motion is presented. Identification of physical space as Huygens compressible ether, Casimir vacuum, leads to Poincaré-Lorentz dynamic as opposed to Einstein kinematic theory of relativity. The introduction of absolute frame suggests new perspectives regarding Einstein’s GTR and the principle of general covariance. Classical Newton law of gravitation is related to the pressure of Casimir vacuum. Thus, modified form of equation of motion is applied to derive both Friedmann-Lemaître equations that result in de Sitter universe even in the presence of matter and radiation. The hydrodynamics of universe is shown to follow either Schrödinger (non-dissipative, non-relativistic) or Dirac (dissipative and relativistic) wave equations with the latter in accordance with dissipative deterministic theory of quantum gravity introduced by ’t Hooft.

Keywords: Quantum mechanics . Quantum field theory. Quantum Cosmology . Quantum gravity . Black hole . White hole . T.O.E

Speaker: Siavash Sohrab (Northwestern University)

24 Quantum Gravity Predictions in Lorentz-Violating Gravity from Moving Black Holes

Speaker: Andrew Kovachik (kovachia@mcmaster.ca)

25 Wormholes in $f(R,T_{ab}T^{ab})$ modified-gravity theory

In this presentation, we investigate traversable wormhole spacetimes within the context of $f(R,T_{ab}T^{ab})$ modified-gravity theory. By considering a linear form of the function, we demonstrate the existence of numerous wormhole solutions where the matter fields satisfy all energy conditions. However, due to the lack of natural localization of these solutions, it is necessary to match them with an external vacuum spacetime. To address this, we derive the junction conditions for the theory and perform a matching of the interior wormhole spacetime with an exterior vacuum described by the Schwarzschild solution. Additionally, we show that this approach can be generalized to more complex dependencies of the function, provided there are no crossed terms between $R$ and $T_{ab}T^{ab}$, and that it is linear in $R$.

Speaker: Vladislav Ganiyev (Brock University)

26 Conformal Cores of Quantum Black Holes in Quadratic Gravity

We explore the possibility that quadratic gravity, as a renormalizable theory, describes the interior of quantum black holes. We find new exact power-law solutions to pure quadratic gravity under spherical symmetry, which are complex valued. The resulting solutions, dubbed powerballs, are horizonless compact objects that become Schwarzschild-like a small distance (of the order of the Planck length) outside the would-be Schwarzschild horizon. We present a description of the global eternal geometry, whose right and left exteriors are Lorentzian and Euclidean Schwarzschild-like regions, respectively, while the complex interior is a form of spiraling spacetime. We compute the total on-shell action integral as a saddle point to a gravitational path integral and discuss the Lorentzian and Euclidean interpretations thereof.

Speaker: Ruolin Liu (University of Waterloo, Perimeter Institute)

Saturday 1 November

Saturday Morning Session 1

27 The Entropy of Dynamical Black Holes and Apparent Horizons

We previously proposed a new formula for the entropy of a dynamical black hole (valid to leading order for perturbations off of a stationary black hole background) in an arbitrary classical diffeomorphism covariant Lagrangian theory of gravity in 𝑛 dimensions. Our formula in general relativity gives the entropy of a dynamical black hole as its area minus an integral involving the expansion of the null generators of the horizon. In stationary eras, this formula agrees with the usual area formula, but in nonstationary eras, we obtain a nontrivial correction term. We previously showed that this new formula in general relativity agrees with the apparent horizon area to leading order. In this talk, I will discuss the generalization of this result to more general theories of gravity, with the goal of finding an analog of the apparent horizon in more general theories.

Speaker: Victor Zhang

28 Causality and instability of the relativistic hydrodynamics from holography

We use holography as a tool to explore all-order in velocity gradients relativistic hydrodynamics. We demonstrate the link between causality and instability of the theory.

Speaker: Alex Buchel (UWO)

29 Sourced Carrollian Fluids Dual to Black Hole Horizons

The (degenerate) geometry of event horizons is linked to Carrollian fluids. We investigate the behaviour of event horizons via a perturbative coupling to a massless scalar field, making connections to Carrollian hydrodynamics with a driving source, and discuss the fluid equilibration in tandem with the horizon’s relaxation to equilibrium. We observe that after the perturbation dies off, the Carrollian fluid energy and momentum densities approach equilibrium as the horizon asymptotically becomes non-expanding. We connect the equilibration of the Carrollian fluid dual to the black hole horizon through the expansion of its background geometry.

Speaker: Sercan Hüsnügil (Perimeter Institute)

30 Guesswork in the gap: the impact of uncertainty in the compact binary population on source classification

The nature of the compact objects within the supposed 3-5M⊙ “lower mass gap” remains uncertain. Observations of GW190814 and GW230529 highlight the challenges gravitational waves face in distinguishing neutron stars from black holes. Interpreting these systems is especially difficult because classifications depend simultaneously on measurement noise, compact binary population models, and equation of state (EOS) constraints on the maximum neutron star mass. We analyze 66 confident events from GWTC-3 to quantify how the probability of a component being a neutron star, P(NS), varies across the population. The effects are substantial: the data reveals that P(NS) varies between 1% – 62% for GW230529’s primary and P(NS) ranges 39% – 99% for GW190425’s primary. By contrast, the asymmetric GW190814’s secondary varies P(NS) by ≤ 10%, demonstrating robustness at high SNR. Across our analysis, the dominant drivers of classification are the pairing preferences of neutron stars with other compact objects, and the neutron star spin distributions. Analysis using EOS information tends to affect P(NS) through the inferred maximum neutron star mass rather than the maximum spin. Without a systematic treatment of the numerous parameters influencing P(NS), its proposed role as a robust astrophysical observable is unlikely to withstand scrutiny, which may lead to ambiguous classifications of future GW events.

Speaker: Mr Utkarsh Mali (CITA (Canadian Institute for Theoretical Astrophysics))

31 Impact of nonlinearities on relativistic dynamical tides in compact binary inspirals

Speaker: Tristan Pitre (University of Guelph)

32 Scalar field perturbation of a rotating wormhole to obtain quasi-normal modes.

In this era of gravitational waves from blackhole and neutron star mergers, GW data is being collected at an amazing pace. There is a possibility of GWs of BH mergers scattering from wormholes and/or other exotic compact objects (ECOs), if they exist. Specialized groups are searching for wormhole signatures. We share the results of our study of scalar field perturbations and quasi-normal modes of the Teo rotating wormhole background via first and third order WKB approximations. Our goal is to move toward a high confidence level in the signatures to expect during the search and analysis of GW data.

Speaker: Ramesh Radhakrishnan (Baylor University)

10:30 Coffee Break

Saturday Morning Session 2

33 Reaching diffraction-limited resolution with coherent pulsar timing arrays

In Jow et al. 2025, we calculate the angular resolution of an incoherent PTA using polarized map-making techniques to reconstruct the GW sky. We extend this technique to a coherent analysis by exploring the effects of well constrained pulsar distance measurements on gravitational wave localization. We place estimates on the number of well constrained pulsar distances necessary to reach diffraction-limited localization of single, polarized GWs and find it to be a modest number. This work highlights a fundamental shift in PTA analysis as projected observational advancements move us beyond stochastic background detection and into the regime of PTAs as point source telescopes.

Speaker: Anna Tsai (CITA)

34 Conserved quasi-local multipole moments in full non-linear General Relativity

Formulating precise definitions of conserved charges like energy-momentum and angular momentum in General Relativity is extremely challenging. Instead of entering directly into the stress-energy tensor, gravitational energy manifests as an obstruction to integrating the local stress-energy conservation law to yield globally conserved charges. A very nice quasi-local definition for mass was found by Penrose, which is applicable in a wide variety of space-times, including static space-times. In this talk I define conserved quasi-local multipole moments in static space-times, inspired by Penrose's quasi-local mass. These moments can be computed in full non-linear General Relativity on any two-surface S, where they define the multipole moments of sources enclosed by S. The definition reduces to the ordinary Newtonian definition in the weak field limit, and it is suspected that the moments reduce to the Geroch-Hansen multipole moments when computed at infinity.

Speaker: Berend Schneider (University of Guelph)

35 Can Dirac Fields Source Traversable Wormholes?

Traversable wormholes have captivated the interest of the public and of theoretical physicists since their inception, with a recent resurgence of interest in the context of holography. While wormhole geometries are permissible within the framework of general relativity, they require matter with "negative energy" to sustain them; specifically, the matter must violate the null energy condition on a sufficiently large scale. Typical classical matter, however, does not possess the necessary properties to support such geometries.

Nonetheless, the question remains open as to whether a Dirac field can generate sufficient violations of the null energy condition to source a traversable wormhole. In this talk, I will present recent progress in exploring this possibility for positive-frequency Dirac fields within static, spherically symmetric wormhole spacetimes. I will examine whether solutions to the Dirac equation can provide the "negative energy" required to make static, spherically symmetric wormholes viable without the need to invoke nonphysical, exotic matter.

Speaker: Robert Weinbaum (University of Chicago)

36 Topology Change and Wormhole Nucleation via Topological Surgery in Lorentzian Geometry

Classical singularity theorems and topological‑censorship arguments seem to forbid the formation of a wormhole in 4‑dimensional general relativity, even at the kinematical level, through the emergence of naked singularities. I will present a concrete example, developed in [1], in which the would‑be singular set is excised and replaced by a controlled 0‑surgery on a compact spacetime neighborhood. Taking the connected sum with the complex projective plane yields a smooth Lorentzian cobordism that interpolates between two spacelike hypersurfaces of different topology. The singularity is traded for a finite “chronology‑violating bubble” containing closed timelike curves. The spacetime violates the standard energy conditions yet is entirely regular. After a concise review of Morse theory and the topological‑surgery toolkit, the talk will present the explicit metric construction and analyze its causal structure, energy‑condition violations, and consistency with topological obstructions. I will close with open questions and future directions, including possible extensions to more general Dehn surgeries.

[1] A. Pisana, B. Shoshany, S. Antoniou, L. H. Kauffman, S. Lambropoulou, “Wormhole Nucleation via Topological Surgery in Lorentzian Geometry,” arXiv:2505.02210."

Speaker: Alessandro Pisana (Brock University)

37 An Observer For Asymptotically Flat Spacetimes

Observers have always played a central role in quantum mechanics, yet their consistent incorporation into quantum gravity remains poorly understood. In asymptotically flat spacetimes, this issue becomes especially acute due to the infrared structure of gravity and the infinite-dimensional BMS symmetry group, which render the definition of observables highly nontrivial. In standard quantum field theory, local observables are required to commute with asymptotic symmetry charges. However, in quantum gravity, any attempt to construct such large-gauge–invariant observables encounters a fundamental obstruction: the observable algebra admits only a single pure state, namely the vacuum, making the formulation of nontrivial physical states seemingly impossible.

In this work, we propose a resolution by introducing an external observer degree of freedom into the boundary algebra of asymptotically flat quantum gravity. Remarkably, dressing the gravitational Fock space with the observer Hilbert space generates an entire sector of gauge-invariant quantum gravity states. A central feature of this construction is that the observer regulates ultraviolet divergences in frequency along $\mathscr{I}$, providing the first systematic avenue for gauging the full BMS supertranslation group. The resulting dressed Hilbert space naturally takes the form of a Faddeev–Kulish–like framework and gives rise to a type II von Neumann algebra of observables, offering a mathematically controlled and UV-finite formulation of the theory.

This approach establishes a precise setting for defining UV-finite gravitational observables and opens a path toward computing nonlinear local charges and entropies at null infinity.

Speaker: Murtaza Jafry (University of Chicago)

38 Spinoptics in the Kerr Spacetime: Polarized Wave Scattering

We study propagation of high-frequency electromagnetic and gravitational waves in the gravitational field of a rotating black hole. Due to the interaction of the spin of the field with the spacetime curvature, the standard geometric optics approximation that is used for obtaining the approximate high-frequency solutions of the wave equation should be modified. The corresponding modified spinoptics equations show that the worldline of the spinning massless particle is still null, but no longer a geodesic. We demonstrate that using the hidden symmetries of the Kerr metric one can obtain the corresponding spinoptics equations in the leading order of a 1/ω expansion in an explicit form. We focus on the case of the spinning massless fields scattering in the region near
the equatorial plane. We demonstrate that the asymptotic planes of the corresponding null ray’s motion are slightly tilted. We study this effect and its dependence on the spin of the black hole.

Speaker: Alex Koek (University of Alberta)

12:30 Lunch

Saturday Afternoon Session 1

39 Blue Apple Award

40 Initial data of slow fall off

The usual definition of asymptotic flatness at spatial infinity requires that the flat metric be approached at a particular rate. However, slower rates of fall off are compatible with well-defined evolution under the Einstein vacuum equations. Since initial data must satisfy the constraint equations, we want to know if we can specify the fall off rate that we want and still have compatibility with the constraint equations. I will show that we can, first in the context of Brill waves, and then more generally.

Speaker: David Garfinkle (Oakland University)

41 Asymptotically Euclidean Solutions of the Constraint Equations with Prescribed Asymptotics

I will discuss ongoing work on the construction of asymptotically flat vacuum initial data sets in General Relativity via the conformal method. My collaborators and I have demonstrated that certain asymptotic structures may be prescribed a priori through the method's seed data, including the ADM momentum components, the leading- and next-to-leading-order decay rates, and anisotropy in the metric's mass term, yielding a recipe to construct initial data sets with desired asymptotics. As an application, we discuss a simple numerical example, with stronger asymptotics than have been presented in previous work, of an initial data set whose evolution does not exhibit the conjectured antipodal symmetry between future and past null infinity.

Speaker: James Wheeler (University of Michigan)

42 A “dictionary” to test GR with GW: from observations to theory

GR, while one of the most successful and well-tested theories to date, is expected to receive corrections at high energies—through higher-curvature terms, additional degrees of freedom, or both. Given the vast landscape of possible extensions, how can we test them in a systematic way?

In this talk, I will present a general framework for interpreting deviations in gravitational wave data, focusing on the inspiral regime probed by LIGO.

The key idea is that genuine beyond-GR effects exhibit characteristic mass scalings, determined by the curvature order and field content of the underlying theory. Using techniques from Post-Newtonian Effective Field Theory (PN-EFT), we construct a dictionary between such corrections and their imprint on the waveform. This allows broad classes of higher-curvature modifications to be identified or constrained directly from data, without relying on specific models.

The talk will be based on arXiv:2507.17143.

Speaker: Suvendu Giri (Uppsala University, Sweden)

43 Measuring helicity of stochastic gravitational wave backgrounds

We review GW helicity probes from LVK through LISA, PTA to CMB, and distill the kinematic projection of the three vectors consisting of two polarizations and their spatial separation, identifying the requirement of non co-linearity. LISA and PTA in their current form are thus not helicity sensitive. We present extensions that enable a measurement.

Speaker: ueli pen (U)

44 A binary black hole metric approximation for all practical purposes (in magnetohydrodynamics)

Binary black holes can interact with surrounding matter, producing unique electromagnetic signatures and influencing their long-term evolution. Numerical simulations are crucial to understand the nonlinear behavior of gas and particles moving on this dynamical spacetime. We present a general binary black hole metric approximation valid at all binary separations for all practical purposes. We compare the metric against full numerical relativity in the context of General Relativistic Magnetohydrodynamical simulations. We show that the results are equivalent in many different scenarios, including merger and accretion from a circumbinary disk

Speaker: Luciano Combi Combi (Perimeter Institute)

45 A Yang-Mills-like formulation of General Relativity

I present a new formulation of General Relativity. The action is quadratic in the curvature and the equations of motion involve the divergence of the Riemann tensor. I show that this formulation is well posed and is equivalent to the Einstein equations. Overall, this formulation provides a surprising and peculiar new point of view on the Einstein equations.

Speaker: Erik Schnetter (Perimeter Institute for Theoretical Physics)

46 OGRePy: Object-Oriented General Relativity in Python

I will present a detailed introduction to my Python package, OGRePy: An Object-Oriented General Relativity Package for Python, which will be of great interest to anyone doing research in general relativity. I will demonstrate the package's usage and features, including its ability to calculate arbitrary tensor formulas involving any combination of addition, multiplication, trace, contraction, and partial and covariant derivatives, while automatically figuring out the proper index configuration and coordinate system to use for each tensor. I will discuss how this package has been used in research so far, as well as future plans.

Speaker: Prof. Barak Shoshany (Brock University)

47 GW250114 reveals black hole horizon signatures

The horizon of a black hole, the "surface of no return," is characterized by its rotation frequency $\Omega_H$ and surface gravity $\kappa$. A striking signature is that any infalling object appears to orbit at $\Omega_H$ due to frame dragging, while its emitted signals decay exponentially at a rate set by $\kappa$ as a consequence of gravitational redshift. Recent theoretical work predicts that the merger phase of gravitational waves from binary black hole coalescences carries direct imprints of the remnant horizon's properties, via a "direct wave" component that (i) oscillates near $2\Omega_H$, reflecting the horizon's frame dragging and the quadrupole nature of the gravitational radiation, and (ii) decays at an increasing rate characterized by $\kappa$, with additional screening from the black hole's potential barrier. In this paper, we report observational evidence for the direct wave in GW250114 with a matched-filter signal-to-noise ratio of $14.0^{+0.2}_{-0.1}$ ($13.5^{+0.1}_{-0.2}$) in the LIGO Hanford (Livingston) detector. The measured properties are in full agreement with theoretical predictions. These findings establish a new observational channel to directly measure frame-dragging effects in black hole ergospheres and explore (near-)horizon physics in dynamical, strong-gravity regimes.

Speaker: Sizheng Ma (Perimeter Institute)

48 Rotating black holes beyond Einstein

I will present a new general method to find rotating black hole solutions (analytically and in closed form) for a large class of beyond Einstein theories. These solutions are valid for any black hole spin (including near-extremal black holes) and converge exponentially fast with the number of basis functions kept in the solution. These new solutions can now be used to study extreme mass-ratio inspirals or quasinormal ringdown using analogous methods to those developed in general relativity for perturbations of Kerr black holes.

Speaker: Prof. Nicolas Yunes

16:20 Coffee Break

Saturday Afternoon Session 2

49 Relativistic Mechanisms for Entropy Reversal

This paper argues for a Relativistic light speed limit to escape velocity and a process that will transform higher elements into Hydrogen-1. It proposes a Relativistic process that will free energy to decay all aspects of the Periodic Table into a single Proton and Electron. They could then attract and form into Hydrogen-1. The relativistic slowdown will mean that Structural energies that maintain the stability of Atoms and Molecules will slow and lose mass. The matter particles would absorb that energy. So, both the mass of matter particles and their Kinetic Energy would increase. This process demonstrates how Matter and energy can reunite and reverse an aspect of entropy. Elements below Iron would absorb free energy to decay. Elements above Iron would release energy in fission.

Speaker: David Taylor (Relativistic Perspective)

50 Speed of light (electromagnetic wave) (c) is not constant in space and crisis on the theory of relativity.

The real working process of universal phenomenon is described in this paper and unraveling the misconceptions of the theory of relativity.Some experimental evidence and universal phenomenon indicates that the speed of light (c) is not constant in space.Speed of light varies one to another place in space and show relative speed in space.As which this effect predicts to existence of light wave (electromagnetic wave) propagating medium . We have called it aether medium. So this paper stablished aether theory again and discard to the theory of relativity because time dilation equation have derived from aether theory in this paper.Gravitational lensing gravitational redshift and blue shift, dark energy and dark matter etc phenomenon can solve easily by this innovative aether theory. New explanation of of gravity is provided by this innovative aether theory. This aether theory is compatible with quantum mechanics and unsolved problems of physics can solve by this aether theory

Speaker: Pramod Kumar (Not affiliated with institute)

51 Einstein's Missing Energy

Einstein worked on General Relativity for a decade before releasing it in 1916. For several of those years he struggled to include gravity’s own energy into his equation. He couldn’t get it right, so he just dropped it. How do we consistently put gravitational energy back into the equations? In this essay, Einstein’s own solution to this problem - the energy pseudo tensor, along with variants are quickly reviewed and found wanting. Quasilocal energy is thus used, and we include this energy into Einstein equations. Perhaps unsurprisingly, general covariance is broken. As an example, a Schwarzschild like solution is developed, but unlike a black hole, this solution shows no horizons and no massive singularity. This also allows for new polarization modes, namely monopolar radiation.

Speaker: Thomas Andersen (nSCIr.ca)

Oct2025EinsteinEnergy.pdf

52 Observations of Overlooked Power Flows Lets Us Describe the Beginning of our Physical Universe

Overlooked – 1st of 6 power flows:

Just strike a match and the ancient atom electrons, pushed by a strong energy force, slips from its harness, like a stone from a sling, emerge as light/photons, out instantaneously at 186,282 mps, requiring continuous dark energy1 .

1 [This energy enabled the electron to resist the powerful attraction of the proton by orbiting in a spirographic pattern since created eons ago. Visual proof of this spirograph orbit provided by the 1984 scanning electron microscope ‘pictures’ of ball bearing shaped atoms.]

2nd, 3rd, & 4th : Evidence of this instantaneous active dark energy is found operating continuously in firework displays, dynamite explosions, and very powerfully in chain reactions, atomic bombs, and in the light photons coming from the fusion/furnace of galactic star atoms.

5th: This evidence was overlooked by all up to now, this Active Dark Energy fueling every atom’s electrons found in all the electrons in every atom in all the stars, in all the galaxies out to the edge of space since created billions of years ago indicates that that field of dark energy must be far greater than our visible universe, perhaps infinite in order to power all galaxy star atoms, filling the whole visible universe, and consequently must have existed before the beginning of our physical universe.

The Beginning Initial (big bang like) Event:

6th: That active Dark Energy, prior to that Initial Event, permeated our universe running every kind of pattern until a sufficient amount – formed laser like beams that converged onto a small enough area (like that 1997 Stanford Lab Experiment, creating some very tiny atomic matter from huge laser energy beams) and explosively combined virtual particles creating all the atoms filling our Universe – the beginning of our Physical Universe replacing big bang from nothing.

Speaker: Charles Sven

Sunday 2 November

53 Eric and other bodies in free fall: does their acceleration depend on their internal structure?

Speaker: Prof. Clifford Will

54 Chasing Eric’s elegance for 3 decades

I began graduate studies in Fall 1993, arriving at Caltech vaguely interested in work being done in Kip Thorne’s group, but without a clear sense of what I really wanted to do. Kip’s advice to me was “talk to Eric.” Eric not only got me started thinking about subjects that continue to influence the calculations my group and I do today, but provided the standard that I have attempted to meet in almost all of the work I have done ever since. In this brief talk, I’ll describe the role Eric played in setting the initial data for my scientific career, and how he has continued to be the standard that I aspire to reach in the work that I enjoy the most.

Speaker: Scott Hughes (Massachusetts Institute of Technology)

55 Self-force now and then

Speaker: Prof. Leor Barack

10:35 Coffee Break

56 Black holes: From the inside, out

Speaker: Patrick Brady

57 The masses and radii of the neutron stars observed by NICER

Neutron stars are the densest known gravitationally-stable objects in the Universe. Their strong gravitational fields, rapid rotation rates, and supra-nuclear central densities allow for a fascinating interplay between general relativistic effects and nuclear physics theory. Pulse-profile modeling is a technique that uses the gravitationally-lensed X-ray flux emitted from hot spots on the neutron star's surface to infer its mass and radius. General relativity is a crucial ingredient in this analysis. The Neutron Star Interior Composition ExploreR (NICER) is a NASA X-ray telescope mounted on the International Space Station. NICER is a timing instrument designed to make the measurements required to implement pulse-profile modelling. In this talk, I will give an overview of how NICER data is used to infer a neutron star's radius (and its mass), along with the latest results and future observations of other pulsars.

Speaker: Sharon Morsink (University of Alberta)

12:25 Lunch

58 Following Eric's footsteps down the rabbit hole of the two body problem

I highlight some of Eric's seminal contributions to our understanding
of the two body problem in general relativity, from the 1990s at
Caltech until today, and use them as a springboard
for a discussion of two recent research directions. The first is the development of general
methods to define and derive coarse-grained variables to describe the
motion of macroscopic bodies, valid in contexts with nonlinear self-interactions. These
methods are complementary to the methods of effective field theory,
and build off earlier formalisms in the linear context by Detweiler and Whiting
and by Harte. The second is an effort to understand in what regimes
the dynamics of a two body system can be uniquely and/or usefully split
into a conservative, Hamiltonian sector and a dissipative sector.

Speaker: Prof. Eanna Flanagan

59 Neutron stars: Promises and challenges from gravitational-wave astronomy

Ground-based gravitational-wave observatories have detected more than three hundred gravitational-wave signals over the past decade. Most have been from binary black hole mergers, but the catalog of events also includes a handful of black-hole/neutron-star binaries and two or three double neutron star mergers. Observing these mergers is testing our understanding of gravity, revealing dense matter dynamics inside neutron-star cores, and tracing the endpoints of stellar evolution across cosmic time. They also tell us about the engines of transient astronomy and the sites where heavy elements are formed in our universe.

As observatory upgrades improve our sensitivity, and as new facilities come online, we expect both increased detection rates and higher-fidelity measurements of the loudest signals. The challenge of gravitational-wave astronomy is moving from detection to interpretation: understanding future observations will place stringent requirements on our modelling of the source systems and how well we understand the physics that describes them. In this talk, I’ll outline gravitational-wave measurements of neutron star properties, in particular the nuclear equation of state, and their implications. I'll show how simplifying assumptions used to analyze the first signals will break down with future observations, and how bringing in information from nuclear science can improve our interpretations. Finally, I’ll discuss how we can quantify and account for uncertainty in signal models in our analyses, reducing systematic biases and allowing unmodeled physics to be inferred from observational data.

Speaker: Prof. Jocelyn Read

60 Testing the nature of Kerr black holes and Hawking’s area theorem with gravitational waves

Speaker: Katerina Chatziioannou

15:45 Coffee Break + Group Photo

61 Insights into the rotating black hole interior from numerical solutions

Speaker: Prof. Frans Pretorius

62 Closing Remarks