### Scientific Program

The meeting takes place on Friday and Saturday, March 26 and 27, 2021. Registered participants have been sent the zoom meeting information by email.

The scientific program is displayed below in an interactive format. Alternatively, here is a hard copy. Talks are 12 minutes total, including questions. All times are given in the ** pacific time zone**.

Chair: Sam Gralla

speaker | title (click for abstract) | time | |
---|---|---|---|

1 | James Allen Isenberg |
History of PCGM
<Some remarks about the history of the pacific coast gravity meeting> |
9:00 |

Chair: Katerina Chatziioannou

speaker | title (click for abstract) | time | |
---|---|---|---|

1 | Lance Blagg |
The Extended Uncertainty Principle Kerr Black Hole
This project will apply the Extended Uncertainty Principle (EUP) to the Kerr black hole. The Kerr metric is a solution to Einstein’s equations of General Relativity, which describes an electrically neutral black hole rotating around an axis of symmetry. Since all black holes in nature are expected to be rotating, the Kerr solution offers a truly unique link between realistic astrophysics and fundamental gravitational physics. In fact, all current observations of black hole parameters, such as gravitational waves from LIGO and the black hole shadow from the Event Horizon Telescope, match those derived from the Kerr metric. The EUP introduces a quantum gravitational correction to the Heisenberg Uncertainty Principle. By adding a position-uncertainty correction term, the EUP allows for observation of quantum effects over macroscopic length scales. This thesis will explore differences in Kerr black hole horizons and the ergosphere, versus the same regions for the EUP enhanced Kerr black hole. EUP adjusted Schwarzschild metrics have shown differences in photon and particle orbits, and it is expected that differences in parameters will be found with the EUP enhanced Kerr metric as well. |
9:15 |

2 | Rhondale Tso* |
Testing Relativity with Gravitational Waves: The Next Generation Detectors
Ground-based interferometers have served as the most promising observatories for gravitational wave detections. As detectors advance there is an interest to probe frequency ranges outside conventional ground-based detector networks. This talk will overview tests of relativity in the deci-hertz range, looking at the proposed TianGO detector and other proposed detectors (e.g., GLOC). These would probe constraints on the weak-field (Brans-Dicke and EDGB), massive graviton dispersion effects, and gravitational wave memory. Models on massive graviton dispersion in the presence of screening mechanisms will also be discussed and its consequences for third-generation (Cosmic Explorer) detectors which will observe sources at high redshift. |
9:27 |

3 | Sam Gralla |
Photon Ring Tests of Gravity
General relativity predicts that emission from near black holes will be lensed into a series of narrow "photon rings" converging to a critical curve on the image plane. I will describe the basics of this phenomenon and review two potential photon ring tests of gravity, one performed by EHT using their current data, and one that we have proposed for a future space mission. I will argue that the EHT test is based on unjustified (almost certainly false) assumptions, while the space mission could test the Kerr hypothesis with sub-sub-percent precision. |
9:39 |

4 | Jose M. Ezquiaga |
Gravitational wave lensing meets tests of general relativity
Gravitational wave (GW) data is rapidly growing, allowing to explore new aspects of astrophysics, cosmology and fundamental physics. As GW detectors improve their sensitivity, they will be able to detect even more sources and at higher redshift sources. This will enhance significantly the chances of observing strongly lensed GWs. In this talk I will describe how, within general relativity (GR), certain strongly lensed signals could be distorted with respect to the unlensed predictions of GR. Then, I will discuss the new phenomenology when one considers GW lensing beyond GR, which could lead to birefringence and echoes. |
9:51 |

5 | Nima Laal* |
Pulsar Timing and Alternative Theories of Gravity
The unprecedented sensitivity to low-frequency gravitational waves (GWs) achieved by the most recent data set of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) attests to the value of pulsar timing in robust and sensitive tests of gravity, including tests for exotic polarizations of GWs. By providing a different line of sight to each pulsar along with an enhanced response to scalar-longitudinal modes, pulsar timing proves to be a fertile ground for studying alternative polarizations of gravity. In this regard, we have developed a statistical and theoretical framework to search for all alternative polarizations of gravity stemming from various metric theories of spacetime using NANOGrav’s 12.5 year data set. Using this advanced framework, we search for evidence of GW polarizations not present in Einstein’s general relativity and place constraints on the amplitudes of these GW modes |
10:03 |

6 | Zonghao Li* |
Diffeomorphism violation and local Lorentz violation in gravitational effective field theories
Deviations from General Relativity including diffeomorphism violation and local Lorentz violation are currently active topics in the search for new physics. We present a comprehensive method to systematically construct diffeomorphism-violating and local-Lorentz-violating terms in gravitational effective field theory. The implications of the possible presences of background fields in gravity are discussed, and the relation between different sets of theories is also studied. |
10:15 |

7 | Alyssa Johnson |
Progress on Novel Tests of Gravity at the Submillimeter Scale
The unification of the Standard Model and General Relativity may result in the failure of the Weak Equivalence Principle (WEP) and/or the gravitational Inverse-Square Law (ISL). Our fundamental understanding of gravity is questioned by these incongruities. However gravity is not well tested below the millimeter scale. Undergraduate researchers and faculty at Humboldt State University are using an experiment to measure gravitational interactions below 50 microns. The experiment uses a torsion pendulum with equal masses of two different materials arranged as a composition dipole. The twist of the torsion pendulum is measured as an attractor mass in a parallel-plate configuration is oscillated nearby. This oscillation creates a time dependent torque on the pendulum. The magnitude and size of this torque may be studied lead to test for deviations in the WEP or ISL at this untested scale. This talk will focus on the mitigation of the variations in the apparatus' inclination by use of an active leveling scheme that utilizes data from a tilt sensor that is fed to a PID loop. |
10:27 |

Chair: Jocelyn Read

speaker | title (click for abstract) | time | |
---|---|---|---|

1 | Jane Bright* |
Minidisk Dynamics in Accreting, Spinning Black Hole Binaries: Simulations in Full General Relativity
We perform magnetohydrodynamic simulations of accreting, equal-mass binary black holes in full general relativity focusing on the dynamical formation and evolution of minidisks. We find that during the late inspiral the sizes of minidisks are determined by the interplay between the tidal field and the effective innermost stable orbit around each black hole. In particular, we show evidence that minidisks form when the Hill sphere around each black hole is significantly larger than the black hole's effective innermost stable orbit. As the binary inspirals, the Hill sphere radius decreases, and hence minidisks and their associated electromagnetic signatures will disappear prior to merger when there are no more stable orbits within the Hill sphere. The disappearance of a hard electromagnetic component in the spectrum of such systems may provide a smoking gun signature of merging black hole binaries. |
11:15 |

2 | Pedro Espino* |
The Fate of Twin Stars on the Unstable Branch: Implications for the Formation of Twin Stars
We consider the dynamics of compact stars with hybrid hadron-quark equations of state. When the surface tension between the hadronic and quark phases is sufficiently strong, a first-order phase transition can be sustained over a large range of energy densities, leading to the emergence of a third family of stable compact stars (hybrid hadron-quark stars). The branch of stable hybrid stars is separated from the stable neutron star branch by a branch of unstable hybrid stars. Of particular interest are hybrid stars with the same masses as neutron stars but different radii (twin stars). We study the dynamics of unstable hybrid stars all the way down to the minimum mass twin star through 3-dimensional general relativistic hydrodynamic simulations of non-rotating and rotating unstable-branch twin stars. We find that unstable hybrid stars naturally migrate toward the hadronic regime and undergo strong (quasi)radial oscillations in the process. Our study suggests that it may be difficult to form stable twin stars, and hence it may be more likely that astrophysical hybrid stars have masses above the twin star regime. The oscillations between the two phases could provide a unique gravitational wave signature for a Quantum Chromodynamics deconfinement in hybrid star progenitors. |
11:27 |

3 | Nils Deppe |
Robust adaptive-order methods for relativistic magnetohydrodynamics
High-order numerical methods allow improved accuracy at reduced computational
cost when the solution is reasonably smooth. Except at the stellar surfaces,
this is the case during the inspiral of a neutron star binary. We describe a new
positivity-preserving adaptive-order method that combines discontinuous Galerkin
and finite-difference schemes. The method adjusts the order locally in space and
time to use the most efficient and highest-order method possible. At
discontinuities, it falls back to robust low-order methods that preserve the
positivity of quantities like density and pressure. We will show encouraging
preliminary results of our new method, which will soon be applied to
relativistic magnetohydrodynamics simulations of binary neutron stars mergers
and accretion disks. |
11:39 |

4 | Erik Wessel* |
NR Simulations of PPI-Unstable BH-Disk Systems: BH Spin and Gravitational Wave Detectability
Accretion disks around BHs are an under-studied potential GW source. The hydrodynamic Papaloizou-Pringle Instability (PPI) can cause persistent orbiting matter clumps to grow and produce copious GWs. Via full numerical relativity simulations of self-gravitating disks, we have extended the understanding of these BH-disk systems in two new ways. First, we conducted the first-ever study of the PPI around spinning BHs a/M = 0.7. We found that, in addition to slightly shifting orbital frequencies, prograde spin can reduce the accretion rate and extend GW signal lifetimes. Systems of 10 solar masses — relevant for BHNS mergers — could be detectable by Cosmic Explorer out to ~300 Mpc, while DECIGO (LISA) could detect systems of 1000 solar masses (10^5 solar masses) — relevant for disks forming in collapsing supermassive stars — out to cosmological redshift of z~5 (z~1). |
11:51 |

5 | Franklin Felber |
Dipole gravitational waves from compact binaries
During the sudden final inspirals of black-hole and neutron-star binaries, acceleration of the center of mass and other nonlinear relativistic effects produce powerful bursts of dipole gravitational radiation. The first observed inspiral event, GW150914, is estimated to have produced dipole gravitational radiation up to the order of 1 percent of the total radiated power and 10 percent of the strain signal. These estimates are not inconsistent with the observed unfiltered strain signals. |
12:03 |

Chair: Charles Torre

speaker | title (click for abstract) | time | |
---|---|---|---|

1 | Alexander Y. Yosifov* |
A New Look at the $C^{0}$-formulation of the Strong Cosmic Censorship Conjecture
We examine the $C^{0}$-formulation of the strong cosmic censorship conjecture (SCC) from a quantum complexity-theoretic perspective and argue that for suitable initial data, defined on a spacelike codimension-one submanifold within the black hole interior, the metric is $C^{0}$-extendable to a larger Lorentzian manifold across the Cauchy horizon. Two counterarguments are provided to demonstrate the pathologies associated with a hypothetical validity of the $C^{0}$ SCC. First, we examine the decay properties of solutions $\psi$ to the massive Klein-Gordon wave equation $\Box_{g}\psi+\frac{\alpha}{l^{2}}\psi=0$ in the interior of a Kerr-AdS black hole, and argue $\psi$ is asymptotically \textit{incomplete}. Second, we show a violation of the "complexity=volume" conjecture for a low-temperature hyperbolic AdS$_{d+1}$ black hole dual to a CFT living on a ($d-1$)-dimensional hyperboloid. |
1:00 |

2 | Nader Inan |
Maxwellian Mirages in GR
The linearized Einstein field equations of General Relativity exhibit a well-known Maxwell-like form similar to the 3-vector field equations of electromagnetism. Similarly, the linearized geodesic equation of motion leads to a Lorentz-like force equation. Typically the harmonic coordinate condition (similar to the Lorenz gauge in electromagnetism) is utilized to arrive at these results. However, the associated “gravito-electric” and “gravito-magnetic” fields are shown to be coordinate-dependent quantities which can be made to vanish with an appropriate coordinate transformation. Furthermore, the field equations falsely predict the existence of 3-vector gravitational waves. These “mirages” are mathematical artifacts that contradict the true physical nature of gravitational fields. In fact, they have been the cause for many erroneous results in the literature, particularly in gravitational wave research. An alternative approach is presented that also leads to Maxwell-like field equations, and a Lorentz-like force equation. However there are newly defined coordinate-invariant “gravito-electric” and “gravito-magnetic” 3-vector fields that cannot vanish by a coordinate transformation. There are also electric-like and magnetic-like tensor fields that are shown to correctly describe the tensor nature of gravitational waves. Lastly, transverse coordinates are introduced which map these coordinate-invariant degrees of freedom to particular components of the metric perturbation. As a result, it is argued that transverse coordinates expose the mirages found in harmonic coordinates, and better represent the true nature of gravity in linearized General Relativity. |
1:12 |

3 | Arun Ravishankar* |
Horizon Instability of the Extremal BTZ Black Hole
The general picture of the Aretakis instability (weak derivative instabilities) of extremal black hole horizons has emerged from a variety of techniques, whose domains of validity are largely non-overlapping. We study the a system where we have full analytical control - the BTZ black hole. We solve the null geodesic equation in full generality and show that the instability is associated with a class of null geodesics that orbit near the event horizon arbitrarily many times before falling in. |
1:24 |

4 | Kartik Prabhu |
Asymptotic quantum fields at spatial infinity
I will consider the asymptotic behaviour of massless scalar quantum fields at spatial infinity in Minkowski spacetime. The bulk scalar fields can be written in terms of massive scalar de Sitter fields on the hyperboloid of directions at spatial infinity. This gives a formulation of the QFT purely in terms of fields defined at spatial infinity which can be generalised to any asymptotically-flat spacetime. I will argue that quantum states with different values of the asymptotic "scalar charge" live in different unitarily-inequivalent Hilbert spaces. This gives a precise formulation of charge superselection. I will comment on generalization to asymptotic quantization in the gravitational case and the relation to null infinity. |
1:36 |

5 | James Allen Isenberg |
Stability of AVTD Behavior about Kasner Space Times in Polarized T^2-symmetric Vacuum Solutions
In collaboration with Ellery Ames, Florian Beyer, and Todd Oliynyk, we prove that within the class of Polarized T^2-symmetric vacuum solutions of the Einstein equations, for small perturbations of Kasner initial data, the space-time development in the contracting direction exhibits "asymptotically velocity term dominated" (AVTD) behavior in a neighborhood of the singularity. Such behavior is characterized by a congruence of time like observers approaching the singularity with each of them seeing an independent Kasner space-time. |
1:48 |

Chair: Kartik Prabhu

speaker | title (click for abstract) | time | |
---|---|---|---|

1 | Lintao Tan* |
Quantum Gravitational Corrections to Gravity during Inflation
Primordial inflation produces a vast ensemble of cosmological scale gravitons which can affect both the force of gravity and the propagation of gravitational radiation. I show how these effects can be studied by using the graviton self-energy to quantum correct the linearized Einstein equations. I demonstrate that the graviton self-energy has nine structure functions in a general cosmological background and I show how these structure functions change the linearized Einstein equations. I also present explicit one loop results for the nine structure functions on de Sitter background. |
2:30 |

2 | John Botke |
Accelerating scaling and the origin of all cosmic structures, both large and small
In a recent paper, we present a new cosmology based on the idea of a universe dominated by vacuum energy with time-varying curvature. In this model, the universe began with an exponential Plank era inflation before transitioning to a spacetime described by Einstein’s equations. While no explicit model of the Plank era is yet known, we do establish a number of properties that the vacuum of that time must have exhibited. In particular, we show that structures came into existence during that inflation that were later responsible for all cosmic structures. A new solution of Einstein's equations incorporating time-varying curvature is presented which predicts that the scaling was initially power law with a parameter of before transitioning to an exponential acceleration of the present-day scaling. A non-conventional model of nucleosynthesis provides a solution for the matter/antimatter asymmetry problem and a non-standard origin of the CMB. The CMB power spectrum is shown to be a consequence of uncertainties embedded during the initial inflation and the existence of superclusters. Using Einstein’s equations, we show that so-called dark matter is, in fact, vacuum energy. |
2:42 |

3 | Philip Tee |
Dynamics in Emergent Spacetime Geometries
Recent advances in network science have permitted the formulation of models of emergent geometry based upon an Ising model. The ground state properties of these models exhibit many interesting characteristics, including low dimensionality, area entropy laws, and global Ricci flatness. In this talk I will describe recent advances that incorporate matter and dynamics into these models as topologically stable excitations of the ground state. We are able to demonstrate that in the continuum limit a minimal dynamical Hamiltonian prescription recovers non relativistic quantum mechanics. |
2:54 |

4 | Abhijit Chakraborty* |
Conformal aspects of near-horizon acceleration radiation of an atom freely falling into a black hole
A cloud of two-level atoms falling freely into a Schwarzschild black hole was recently shown to detect radiation in the Boulware vacuum using a quantum optics approach. In this model, the atoms interact with a mode of the scalar field via a dipole interaction. The relative acceleration between the field and the atom causes it to detect the radiation. The detected radiation has a thermal spectrum with a Planck factor which depends upon the frequency of the field mode. In this talk, we show that the probability of detecting radiation is dominated by the conformal aspects of near-horizon physics. The results indicate the relation of the near-horizon conformal quantum mechanics (CQM) with the spectrum of the detected thermal radiation with Hawking temperature and reinforce its relevance to all other thermodynamic properties of the black hole. Additionally, this insight about the effect of the near-horizon CQM enables us to tackle the same problem for a variety of spacetime backgrounds and general initial conditions. |
3:06 |

5 | Sanjib Katuwal* |
Inflaton Effective potential from Photons for General ϵ
We accurately approximate the contribution that photons make to the effective potential of a charged inflaton for inflationary geometries with an arbitrary first slow roll parameter ϵ. We find a small, nonlocal contribution and a numerically larger, local part. The local part involves first and second derivatives of ϵ, coming exclusively from the constrained part of the electromagnetic field which carries the long range interaction. This causes the effective potential induced by electromagnetism to respond more strongly to geometrical evolution than for either scalars, which have no derivatives, or spin one half particles, which have only one derivative. For ϵ=0 our final result agrees with that of Allen on de Sitter background, while the flat space limit agrees with the classic result of Coleman and Weinberg. |
3:18 |

6 | Michael Bishop |
Modified commutators are not sufficient to determine a quantum gravity minimal length scale
In quantum gravity, it is expected that their will be some minimum length scale on possible states of the system. This is derived from modifying the standard quantum position and momentum operators to get a strictly positive lower bound on the uncertainty of position. It is generally thought that a modified commutator of the form $[{\hat x}, {\hat p}] = i \hbar (1 + \beta p^2)$ is sufficient to give rise to a minimum length scale. We test this assumption by presenting several different families of modified operators which all lead to the same modified commutator and demonstrating that each family has a different minimum length scale and even no minimal length. We do this by checking the uncertainty in position directly. This is due to the modified operators a subtly different uncertainty principle. The conclusion is that the modification of the operators is the main factor in determining whether there is a minimal length. This fact - that it is the specific form of the modified operators which determine the existence or not of a minimal length scale - can be used to keep or reject specific modifications of the position and momentum operators in theory of quantum gravity. This is joint work with Joey Contreras, Jaeyeong Lee, and Douglas Singleton from the Fresno State physics department. |
3:30 |

7 | Diandian Wang* |
4D physics from 2D: Descendants in celestial CFT and emergent multi-collinear factorization
Recent years has seen a fast development of flat space holography, in the sense that scattering amplitudes on 4D Minkowski background when written in a certain basis behaves like a 2D CFT named the celestial CFT. An important issue is to establish some dictionary between the two descriptions. I will talk about how to obtain multicollinear limits of the 4D scattering amplitudes by bootstrapping the celestial CFT. It turns out that asymptotic symmetries alone can predict the correct poles and residues, which boosts our confidence in the possibility of obtaining the full 4D physics from 2D. |
3:42 |

Chair: Gary Horowitz

speaker | title (click for abstract) | time | |
---|---|---|---|

1 | Douglas Singleton |
A Non-Abelian Firewall Solution
A simple, closed-form solution to the Yang–Mills field equations is presented which has a non-Abelian firewall — a spherical “horizon” where the energy density diverges. By the gravity/gauge duality, this non-Abelian firewall implies the existence of a gravitational firewall. Gravitational firewalls have been proposed as a way of resolving the information loss paradox, but at the cost of violating the equivalence principle. |
11:00 |

2 | Aasmund Folkestad* |
Holography Abhors Visible Trapped Surfaces
In this talk, I will explain how the consistency of the holographic dictionary implies a hallmark prediction of the weak cosmic censorship conjecture: that in classical gravity, trapped surfaces lie behind event horizons. We will see that causal wedge inclusion requires the formation of event horizons outside of strong gravity regions. Few assumptions are made beyond the absence of evaporating singularities in strictly classical gravity obeying the null curvature condition. Finally, I comment on the implication that spacetimes with naked trapped surfaces do not admit a holographic dual and speculate on the dual CFT interpretation of a trapped surface. |
11:12 |

3 | Jie-qiang Wu |
Peierls bracket and gravitational dressing in Jackiw-Teitelboim gravity
How to deal with diffeomorphism symmetries is one of the difficult problem in general relativity. Because of the diffeomorphism symmetries, we need to consider diffeomorphism invariant operators and gravitational dressing. In this work, we consider a special gravitational dressing which is to locate the operator by shooting geodesic from the spatial boundary. We try to use Peierls bracket to study the commutator between this gravitational dressing operator and the ADM energy operator. We found that the ADM energy increase/decrease when the extra created out-going particle is in front of/behind the event horizon. Our result strengthens the Marolf-Polchinski firewall argument in some sense. |
11:24 |

4 | Henry Maxfield |
Gravitating spinning strings in AdS3
Single trace operators of large-N gauge theories at large spin (which control deep inelastic scattering, for example) can be described through the AdS/CFT correspondence by classical spinning strings. But in three dimensional AdS, the spectrum of these spinning strings is modified by gravity, because the gravitational force does not decay at long distances. In fact, at sufficiently large spin the strings gravitate so strongly that they become black holes. |
11:36 |

5 | David Grabovsky* |
The Tortoise and the Hare: A Causality Puzzle in AdS/CFT
We pose and resolve a holographic puzzle regarding an apparent violation of causality in AdS/CFT. If a point in the bulk of AdS moves at the speed of light, the boundary subregion that encodes it may need to move superluminally to keep up. With AdS3 as our main example, we prove that the finite extent of the encoding regions prevents a paradox. We show that the length of the minimal-size encoding interval gives rise to a tortoise coordinate on AdS that measures the nonlocality of the encoding. We use this coordinate to explore circular and radial motion in the bulk before passing to the analysis of bulk null geodesics. For these null geodesics, there is always a critical encoding where the possible violation of causality is barely avoided. We show that in any other encoding, the possible violation is subcritical. |
11:48 |

6 | Sergio Hernandez-Cuenca* |
Boundary Causality Violating Metrics in Holography
The full bulk path integral in a Lorentzian formulation of holography includes metrics that violate boundary causality. This leads to the following puzzle: The commutator of two field theory operators at spacelike-separated points on the boundary must vanish. However, if these points are causally related in a bulk metric, then the bulk calculation of the commutator will be nonzero. It would appear that the integral over all metrics of this commutator must vanish exactly for holography to hold. This is puzzling since it must also be true if the commutator is multiplied by arbitrarily many other operators. We give a prescription for the bulk path integral in which this puzzle is resolved. In this approach, the bulk dual of the boundary commutator is not the limit of the naive bulk commutator. |
12:00 |

Chair: Vasileios Paschalidis

speaker | title (click for abstract) | time | |
---|---|---|---|

1 | Zoheyr Doctor |
The Population of Remnant Black Holes From Stellar Mass Black Hole Mergers
The mass, spin, and kick velocity of a black hole formed from the collision of two smaller black holes can be calculated with general relativity. For stellar mass remnants, these remnants will persist for much longer than the age of the universe, assuming no interactions with other astrophysical bodies. In this work, we use the inferred population of in-spiraling black hole masses and spins to infer the population properties of the remnant black holes and their current day number density. We find that a Milky-Way-equivalent galaxy at redshift zero should contain O(5000) remnant black holes. |
1:00 |

2 | Yangyang Cai* |
Consistent Blandford-Znajek expansion
The Blandford-Znajek mechanism is the continuous extraction of energy from a rotating black hole via plasma currents flowing on magnetic field lines threading the horizon. In the discovery paper, Blandford and Znajek demonstrated the mechanism by solving the equations of force-free electrodynamics in a perturbative expansion valid at small black hole spin. Attempts to extend this perturbation analysis to higher order have encountered inconsistencies. We overcome this problem using the method of matched asymptotic expansions, taking care to resolve all of the singular surfaces (light surfaces) in the problem. Working with the monopole field configuration, we show explicitly how the inconsistencies are resolved in this framework and calculate the field configuration to one order higher than previously known. However, there is no correction to the energy extraction rate at this order. These results confirm the basic consistency of the split monopole at small spin and lay a foundation for further perturbative studies of the Blandford-Znajek mechanism. |
1:12 |

3 | Will Lockhart* |
Re-analyzing Data from the Event Horizon Telescope: An Alternative Liklihood Function for Geometric Model Fitting
In 2017 the Event Horizon Telescope (EHT) recorded the first interferometric measurements of an astrophysical black hole candidate, M87*. We have re-analyzed this data set, fitting a parameterized geometric model of the source to the data and reproducing the results of the EHT collaboration. I will compare these results with an alternative analysis using a modified likelihood function in the fitting algorithm. This comparison helps to quantify potential biases in image reconstruction. Finally I will discuss what this data set can tell us about the nature of M87*, and what it cannot. |
1:24 |

4 | Daniel Palumbo* |
Discriminating Accretion States via Rotational Symmetry in Simulated Polarimetric Images of M87
Polarized images of black holes carry information about magnetic field morphology on event horizon scales. We describe a modal decomposition of linear polarized images into basis functions with varying polarization over azimuthal angle. We apply this decomposition to analyze ray traced images of general relativistic magnetohydrodynamics simulations of the Messier 87* (M87*) accretion flow. We show that the dimensionless Fourier coefficient associated with rotational symmetry, $\beta_2$, is a strong discriminator of accretion magnetization states for models of M87* that are consistent with the total intensity images produced by the Event Horizon Telescope (EHT). For simulated images viewed at the resolution of the EHT, we find that $|\beta_2|$ is greater than 0.2 only for models with dynamically important magnetic fields in the accretion flow. We also find that higher black hole spins produce increasingly radial polarization patterns. |
1:36 |

5 | Nathan June |
Roadblocks to Detecting Continuous Gravitational Waves
The search for continuous gravitational waves (GW) is an exciting and fruitful area of research. As we continue to push the boundaries of its capabilities, the Laser Interferometer Gravitational Observatory (LIGO) picks up GW signals fainter and further away than ever. Nevertheless, that sensitivity level makes it incredibly susceptible to interference from transient and persistent noise sources ranging from environmental sources to power source coupling from electronic devices. These noises show up in the primary channels of the detector as tall, narrow peaks called 'lines,' which can occur individually or as multiples in evenly spaced sets representing the harmonics of the source, aptly referred to as 'combs.' Combs are particularly detrimental to GW searches because they have the potential to pollute wide frequency bands, each line mimicking non-terrestrial sources. However, LIGO also has dozens of additional auxiliary channels through which it collects data. Often, the noise sources recorded on these separate channels are independent of the primary channel's noise. However, occasionally, it is possible to detect coherence among the sources. This correlation is significant because it provides investigative efforts a solid starting point from which to determine the source of the noise, narrowing the field of their search and potentially saving hours of time and wide frequency ranges in which we detect legitimate wave signatures. Our work developed software tools to, among other things, cross-examine existing coherence dictionaries with various portions of LIGO's O3 observing run. These examinations returned data on physical locations on-site, at which we could find sources of noise artifacts. |
1:48 |

Chair: Sam Gralla

speaker | title (click for abstract) | time | |
---|---|---|---|

1 | Gabriele Bozzola* |
Advancements in general-relativistic simulations of charged black holes
The past years have seen a growth in interest in modified theories of gravity,
which, due to the numerous pathologies, are challenging to study with numerical
methods. On the other hand, Einstein-Maxwell's theory is well-posed, shares many
non-trivial properties with other modified theories of gravity (i.e., emission
of dipole radiation), and in certain regimes even model modified gravity.
Despite that, the non-linear dynamics of charged black holes is uncharted
territory. In this talk, we will discuss our recent advancements in performing
numerical-relativity simulations of charged black holes. We will also present
some first applications, including a constraint of black-hole charge as inferred
from gravitational waves. |
2:30 |

2 | Xiang Li* |
Angular emission patterns of remnant black-holes
The final stage of a binary black-hole merger, the ringdown, is described by the Teukolsky Equation, which predicts both the temporal and angular dependencies. Many studies have focused on black-hole spectroscopy, while the angular distribution has not been extensively investigated. In this work, by introducing a novel global fitting procedure over both time and angular dependencies, we further study the spatial distribution of ringdown waveforms. We show that spin-weighted spheroidal harmonics are a better representation of angular emission pattern when compared to spin-weighted spherical ones, and that their differences are distinguishable in numerical relativity waveforms. In the presentation, we will qualitatively draw the relation between the progenitor binary properties and the excitation of quasinormal modes, including higher-order angular modes and overtones. Specifically, we show that the retrograde modes will be excited when the primary black hole's spin in a large mass ratio binary is not aligned with the orbital angular momentum. Our work seeks to inspire a strategy of testing the ringdown angular emission pattern by stacking multiple gravitational-wave events, as a single event cannot be observed from multiple directions to allow for a global fitting. |
2:42 |

3 | Keefe Mitman* |
Gravitational Memory in Numerical Relativity: Computing Memory Effects and Correcting Existing Waveforms
Gravitational memory is a phenomenon induced by the passage of gravitational waves that corresponds to persistent, physical changes to spacetime. We present advances in resolving the two primary gravitational memory effects—displacement and spin memory—in numerical simulations of binary black hole mergers produced by SXS's Spectral Einstein Code. We show that the waveforms extracted using Cauchy-characteristic extraction (CCE) obey the Bondi-Metzner-Sachs (BMS) balance laws to a high degree of accuracy, unlike previous waveforms produced by numerical relativity simulations. We also show that the waveforms in all publicly available waveform catalogs, which do not exhibit memory effects, can be corrected to include such features. |
2:54 |

4 | Alireza Rashti* |
Elliptica: a new code for construction of initial data
We have developed a new infrastructure, named Elliptica, to make
initial data of various numerical relativity systems such as
binary neutron stars and black hole neutron star binaries.
Using multi-domain spectral method with Schur complement domain
decomposition, Elliptica iteratively solves elliptic equations and
adjusts various parameters to construct the initial data which
represent the physical system of interest.
Here, we present the backbone of Elliptica and, as an example, test
initial data made for a black hole neutron star binary system. |
3:06 |

5 | Vasileios Paschalidis |
Black hole-neutron star coalescence: Effects of the neutron star spin on jet launching and dynamical ejecta mass
Black hole-neutron star (BHNS) mergers are thought to be sources of gravitational waves (GWs) with coincident electromagnetic (EM) counterparts. To further probe whether these systems are viable progenitors of short gamma-ray bursts (SGRBs) and kilonovae, and how one may use (the lack of) EM counterparts associated with LIGO/Virgo candidate BHNS GW events to sharpen parameter estimation, we study the impact of neutron star spin in BHNS mergers. Using dynamical spacetime magnetohydrodynamic simulations of BHNSs initially on a quasicircular orbit, we survey configurations that differ in the BH spin (aBH/MBH=0 and 0.75), the NS spin (aNS/MNS=-0.17 , 0, 0.23, and 0.33), and the binary mass ratio (q ≡MBH:MNS=3 ∶1 and 5 ∶1 ). The general trend we find is that increasing the NS prograde spin increases both the rest mass of the accretion disk onto the remnant black hole, and the rest mass of dynamically ejected matter. Magnetically driven jets are launched only for q =3 ∶1 regardless of the initial NS spin. The lifetime of the jets and their outgoing Poynting luminosity are consistent with typical SGRB luminosities and expectations from the Blandford-Znajek mechanism. By the time we terminate our simulations, we do not observe either an outflow or a large-scale magnetic-field collimation for the other systems we consider. The mass of dynamically ejected matter is such that kilonovae can be powered with peak bolometric luminosities potentially detectable by the LSST. |
3:18 |

Chair: Sam Gralla

speaker | title (click for abstract) | time | |
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1 | Sam Gralla |
Presentation of the DGRAV award for the best student talk
Our secret panel of judges will reveal themselves and announce the winner. |
3:30 |