1. | Naresh Adhikari, University of Wisconsin-Milwaukee Enabling Multi-Messenger Astronomy through Low-Latency GW-GRB Coincidence Ranking and Inference. November 17, 2023 (1:54 PM - 2:06 PM) The joint detection of the gravitational wave event GW170817, its associated gamma-ray burst GRB170817A, and subsequent observations of the kilonova AT2017gfo ushered in a new era of multi-messenger astronomy. This single event refined constraints on gravitational wave speed, the Hubble constant, and rapid neutron-capture nucleosynthesis. My presentation will detail results from comprehensive testing of a low-latency pipeline for identifying GW-GRB triggers in preparation of O4. Using LVK-endorsed O3 replay mock data, we evaluated search latency metrics and GCN alert issuance times. Our end-to-end testing demonstrates the feasibility of real-time multi-messenger searches to uncover novel astrophysics from concurrent GW-GRB events. I will also present rapid Bayesian inference of GW-GRB afterglow parameters by jointly fitting simulated signals. |

2. | Mohamed Fawzy Abbas Aly, SUNY at Buffalo On the Generalization of the Kruskal-Szekeres Coordinates: A Global Conformal Charting of the Reissner–Nordström Spacetime November 18, 2023 (11:36 AM - 11:48 AM) The Kruskal-Szekeres coordinates construction for the Schwarzschild spacetime could be viewed geometrically as a squeezing of the $t$-line associated with the asymptotic observer into a single point, at the event horizon $r=2M$. Starting from this point, we extend the Kruskal charting to spacetimes with two horizons, in particular the Reissner-Nordström manifold, $mathcal{M}_{RN}$. We develop a new method for constructing Kruskal-like coordinates and find two algebraically distinct classes charting $mathcal{M}_{RN}$. We pedagogically illustrate our method by constructing two compact, conformal, and global coordinate systems labeled $mathcal{GK_{I}}$ and $mathcal{GK_{II}}$ for each class respectively. In both coordinates, the metric differentiability can be promoted to $C^infty$. The conformal metric factor can be explicitly written in terms of the original $t$ and $r$ coordinates for both charts. |

3. | Jayana Antunes Saes de Lima, University of Illinois at Urbana-Champaign Why is the average stiffness inside neutron stars approximately universal? November 18, 2023 (9:36 AM - 9:48 AM) The accurate observations of neutron stars have deepened our knowledge of both general relativity and the properties of nuclear physics at large densities. Relating observations to the microphysics that govern these stars can sometimes be aided by approximate universal relations. One such relation connects the central pressure to the central energy density and the compactness of the star, and it has been found to be insensitive to the equation of state to a 10% level. In this talk, I will show that this relation can be interpreted as the average of the speed of sound squared in the interior of a star of a given compactness. Furthermore, I will show that the universality is rooted in the behavior of the equation of state at energy densities below nuclear saturation through a geometric argument. This implies the approximate universality will deteriorate as the compactness increases, and the interior of the star becomes more sensitive to energy densities above nuclear saturation. |

4. | Nijaid Arredondo, University of Illinois at Urbana-Champaign Highly eccentric amplitudes in the efficient fully-precessing eccentric waveform model November 17, 2023 (1:30 PM - 1:42 PM) Future gravitational wave detectors, especially the Laser Interferometer Space Antenna (LISA), will be sensitive to black hole binaries formed in astrophysical environments that promote large eccentricities and spin precession. Gravitational wave models that include both effects have only recently been developed, in particular the Efficient Fully Precessing Eccentric (EFPE) model, which covers the inspiral stage with small-eccentricity-expanded gravitational wave amplitudes accurate for eccentricities e < 0.3. New approaches are needed to cover the full range of eccentricity. In this work, we develop such a method, pushing the leading post-Newtonian (PN) order gravitational wave amplitudes of the EFPE model to high eccentricities. The new model is able to accurately represent the gravitational wave amplitudes to e ≤ 0.8. Comparing the small-eccentricity amplitudes to our fully-eccentric amplitudes in the LISA band, however, reveals that radiation reaction circularizes binaries too quickly for there to be a significant difference between the amplitudes. This suggests that the EFPE model may have a larger regime of validity in eccentricity space than previously thought, making it suitable for inspiral parameter estimation with LISA data. |

5. | Vishal Baibhav, Northwestern University Which came first? Black-hole Spin or Supernova Kick November 18, 2023 (2:54 PM - 3:06 PM) While the origins of black hole spins remain a mystery, it's commonly assumed that if black holes come from isolated massive star binaries, their spins should align with orbital angular momentum. However, this notion is often in conflict with observations. We will question this long-held viewpoint and explore various mechanisms that can spin up BHs before or during supernovae. In addition to natal spins, we will discuss methods that can spin BHs isotropically, parallel to supernova kicks, and perpendicular to supernova kicks. These different mechanisms leave behind distinct imprints in the observable distributions of spin magnitudes, spin-orbit misalignments and the effective inspiral spin of merging binaries. In particular, these mechanisms allow even the binaries originating in the field to exhibit precession and retrograde spin. This broadens the parameter space allowed for isolated binary evolution, which was previously thought to be exclusive to dynamically assembled binaries. |

6. | Quentin Bailey, Embry-Riddle Aeronautical University Towards a multipole expansion for testing spacetime symmetry November 17, 2023 (10:24 AM - 10:36 AM) In the last 3 decades, theorists and experimentalists have been increasingly interested in precision tests of core principles of General Relativity, most notably local Lorentz symmetry. I briefly summarize the state-of-the-art results with the broad range of tests so far. Recent work with colleagues on a multipole expansion including the effects of hypothetical spacetime symmetry breaking will be discussed. |

7. | Jamie Bamber, University of Illinois Urbana-Champaign Fundamental physics with black holes and scalar fields November 18, 2023 (4:06 PM - 4:18 PM) Novel scalar fields are of significant interest to gravitational physics, both in the context of scalar field dark matter and scalar-tensor modified gravity. I will discuss my work exploring the interaction of black holes and black hole binaries with massive scalar field dark matter environments in both the inspiral, merger and ringdown regimes using both analytic methods and numerical relativity, the challenges of building realistic GW templates, and how we can extend this work to baryonic matter environments and non-minimal couplings. |

8. | Lydia Bieri, University of Michigan Asymptotics of Radiative Spacetimes November 18, 2023 (3:54 PM - 4:06 PM) The null asymptotic behavior of spacetimes is important to read off information from gravitational waves and to investigate scattering problems. We study radiative spacetime solutions of the Einstein equations describing various physical scenarios. Then we derive and discuss the null asymptotic structures of these solutions. Thereby we find interesting classifications based on the asymptotic behavior of these spacetimes. |

9. | Sylvia Biscoveanu, Northwestern University Probing Correlations in the Binary Black Hole Population with Flexible Models November 18, 2023 (2:06 PM - 2:18 PM) The astrophysical formation channels of binary black hole systems predict correlations between their mass, spin, and redshift distributions, which can be probed with gravitational-wave observations. Population-level analysis of the latest LIGO-Virgo-KAGRA catalog of binary black hole mergers has identified evidence for such correlations assuming linear evolution of the mean and width of the effective spin distribution as a function of the binary mass ratio and merger redshift. However, the complex astrophysical processes at play in compact binary formation do not necessarily predict linear relationships between the distributions of these parameters. In this talk, we relax the assumption of linearity and instead search for correlations using a more flexible cubic spline model. We report evidence for a nonlinear correlation between the width of the effective spin distribution and redshift, highlighting the valuable role of such flexible models in population analyses of compact-object binaries in the era of growing catalogs. |

10. | Alex Buchel, UWO/Perimeter Institute Gravitational reheating at strong coupling from holography November 18, 2023 (4:42 PM - 4:54 PM) We use gauge/gravity correspondence to study the gravitational reheating of strongly coupled gauge theories in the rapid exit from the long inflationary (de Sitter) phase. We estimate the maximal reheating temperature of holographic models. |

11. | Daniel Caballero, Univeristy of Illinois Urbana-Champaign Stability of Dark Matter Admixed Neutron Stars November 18, 2023 (9:48 AM - 10:00 AM) Dark matter is an essential ingredient in our understanding of the Universe. Although most studies have focused on investigating dark matter at large scales, recently there has been an exploration of the effect that dark matter can have on neutron stars, mostly referring to their instability. However, these studies have used a stability criterion that makes a number of, until now, unproven assumptions about the normal modes of the system. In this presentation, I will prove the first few of these assumptions (finding a canonical energy and lagrangian) and discuss the remaining necessary steps to fully prove the validity of the stability criterion. |

12. | Rohit Chandramouli, University of Illinois at Urbana-Champaign Probing internal dissipative processes of neutron stars with gravitational waves during the inspiral III: Implications from GW170817 November 18, 2023 (9:24 AM - 9:36 AM) In a neutron star binary, each neutron star gets tidally deformed due to the gravitational field of the companion. The tidal response of each star depends on the internal nuclear properties of the star. Since the tidal response will affect the inspiral of the binary, the internal properties of the neutron stars get encoded in the gravitational waveform. The tidal response consists of a conservative and dissipative contribution, both of which contribute to the gravitational wave phase. Each contribution is described by a respective tidal deformability. The emph{conservative tidal deformabilities} (that enters at 5PN in the phase) have already been constrained from gravitational wave observations. For the first time, using data from the GW170817 binary neutron star event, we place constraints on the emph{dissipative tidal deformability} $overline{Xi}$ (that enters at 4PN in the phase). Using the constraint on $overline{Xi}$, we place (conservative) constraints on the effective shear and bulk viscosity of the neutron stars. We discuss implications for the underlying equation of state of the neutron stars. In addition, we perform Bayesian parameter estimation on future GW170817-like events and demonstrate the potential measurability of $overline{Xi}$. |

13. | Bryce Cousins, University of Illinois Urbana-Champaign Cosmic anisotropy with networks of next-generation gravitational-wave detectors November 17, 2023 (9:48 AM - 10:00 AM) The standard cosmological model requires the assumption of isotropy and homogeneity, a principle that is generally well-motivated but is now in conflict with various anisotropies found using independent astrophysical probes. These anisotropies tend to be dipolar in nature, but their origins are not fully understood. For example, the cosmic microwave background possesses a kinematic dipole that is explained by Earth’s relative motion, but the analogous late-universe dipoles cannot be entirely accounted for by kinematics. One effect of a generic, non-kinematic anisotropy is a dipole in luminosity distance measurements. We demonstrate here how such a dipole could be measured using binary neutron star mergers from six networks of next-generation ground-based gravitational-wave detectors. For a dipole of amplitude g = 0.01 located at angular coordinates (81◦, 170◦), we find that the best-case network of three next-generation detectors could constrain the amplitude to 0.010 ± 0.001 and the location to (81 ± 7◦, 170 ± 8◦). We also assess the best-case network’s directional sensitivity across the sky by varying the dipole’s location on a grid; we find that the amplitude constraints vary only by ≲ 2.5%. Our findings indicate that next-generation bright standard siren binary neutron stars could result in either equivalent or improved constraints of a generic dipole when compared to existing methods. |

14. | Conner Dailey, Perimeter Institute Reflecting boundary conditions in numerical relativity as a model for black hole echoes November 18, 2023 (11:48 AM - 12:00 PM) Recently, there has been much interest in black hole echoes, based on the idea that there may be some mechanism (e.g., from quantum gravity) that waves/fields falling into a black hole could partially reflect off of an interface before reaching the horizon. There does not seem to be a good understanding of how to properly model a reflecting surface in numerical relativity, as the vast majority of the literature avoids the implementation of artificial boundaries, or applies transmitting boundary conditions. Here, we present a framework for reflecting a scalar field in a fully dynamical spherically symmetric spacetime, and implement it numerically. We study the evolution of a wave packet in this situation and its numerical convergence, including when the location of a reflecting boundary is very close to the horizon of a black hole. This opens the door to model exotic near-horizon physics within full numerical relativity. |

15. | Daine Danielson, The University of Chicago Asymptotic Charge Induced Decoherence in QED and Quantum Gravity November 17, 2023 (5:18 PM - 5:30 PM) In QED and (linearized) quantum gravity, we show that any localized charge will eventually decohere in the momentum basis in an asymptotically flat spacetime. This places an upper bound on the size of any coherent quantum superposition in space, and also generates an enhanced rate of wavepacket spreading. We estimate the size of these effects, which arise because any massive (or charged) particle necessarily radiates soft, entangling gravitons/photons to null infinity as it evolves. In the limit of infinite time—such as in QED scattering theory—this soft radiation gives rise to superselection in the electron momentum basis, with the result that almost all scattering states exhibit total delocalization of the charges. It is an experimental fact that this does not obstruct accurate predictions for collider experiments, where the central-momentum dependence of scattering cross sections can still be calculated. Nevertheless, in regimes where quantum coherence of charged particles becomes important, this total loss of coherence in traditional scattering theory is a fundamental obstacle to realistic predictions. In QED scattering, realistic physics only survives within a small class of carefully dressed states. In (nonlinear) quantum gravity, the conclusion is different, and suggests that valid physical states in quantum-gravitational scattering theory can only be described in terms of relational observables, e.g. by the introduction of extended objects. |

16. | Alex Deich, UIUC/ICASU Lyanpunov exponents to test general relativity November 17, 2023 (11:00 AM - 11:12 AM) Photon rings are key targets for near-future space-based very-long baseline interferometry missions. The ratio of flux measured between successive light-rings is characterized by the Lyapunov exponents of the corresponding nearly-bound null geodesics. Therefore, understanding Lyapunov exponents in this environment is of crucial importance to understanding black hole observations in general, and in particular, they may offer a route for constraining modified theories of gravity. While recent work has made significant progress in describing these geodesics for Kerr, a theoryagnostic description is complicated by the fact that Lyapunov exponents are time-parameterization dependent, which necessitates care when comparing these exponents in two different theories. In this work, we present a robust numerical framework for computing and comparing the Lyapunov exponents of null geodesics in Kerr with those in an arbitrary modified theory. We then present results obtained from calculating the Lyapunov exponents for null geodesics in two particular effective theories, scalar Gauss-Bonnet gravity and dynamical Chern-Simons gravity. Using this framework, we determine accuracy lower-bounds required before a very-long baseline interferometry observation can constrain these theories. |

17. | Guillaume Dideron, Perimeter Institute Looking for unmodelled physics in gravitational waves November 17, 2023 (11:12 AM - 11:24 AM) In anticipation of the increased precision and detection rate of the next generation of Gravitational Waves (GW) detectors, we want to refine our methods for identifying and interpreting unmodeled (or mismodeled) but physical signals in the data. I describe how to search for unmodeled signals in GW data by using the cross-correlated power of the residual strains in pairs of GW detectors. I forecast the precision with which certain models of deviations from General Relativity can be recovered with this technique from a population of events detected by 3rd-generation GW detectors. |

18. | Ariel Edery, Bishop's University Phase transition at a critical coupling $xi_c$ for a vortex nonminimally coupled to Einstein gravity in AdS$_3$ November 18, 2023 (4:54 PM - 5:06 PM) A Nielsen-Olesen vortex (Abelian-Higgs model) is nonminimally coupled to Einstein gravity with cosmological constant $Lambda$. The nonminimal coupling term $xi,R,|phi|^2$ (where $R$ is the Ricci scalar and $xi$ a dimensionless coupling constant) plays a dual role: it contributes to the potential of the scalar field $phi$ and to the Einstein-Hilbert term for gravity. This leads to a novel feature: there is a critical coupling $xi_c$ where the VEV is zero for $xige xi_c$ but becomes non-zero when $xi$ crosses below $xi_c$ and the gauge symmetry is spontaneously broken. Moreover, we show that the VEV near the critical coupling has a power law behaviour proportional to $|xi-xi_c|^{1/2}$. Therefore $xi_c$ can be viewed as the analog of the critical temperature $T_c$ in the Ginzburg-Landau mean-field theory of second-order phase transitions. The critical coupling exists only in an AdS$_3$ background. However, in an asymptotically flat spacetime (topologically a cone), the deficit angle depends on the coupling $xi$ and is no longer determined solely by the mass; remarkably, a higher mass does not necessarily yield a higher deficit angle. |

19. | Amanda Farah, University of Chicago Cosmology with the mass distribution of GW sources November 17, 2023 (10:12 AM - 10:24 AM) Gravitational waveforms contain information about both the luminosity distance and redshift to their sources, making them clean probes of the expansion history of our universe, H(z). However, redshift information is completely degenerate with the masses of the objects that created the waveform, making it impossible to know the redshift of a single source without knowledge of the mass that produced it. In this talk, I will show how simultaneously inferring the mass distribution of gravitational wave sources along with cosmological parameters breaks this degeneracy. By using a flexible, non-parametric model for the mass distribution, I will additionally demonstrate that the morphology of this mass distribution does not need to be known a priori in order to preform this measurement. Non-parametric cosmological probes such as these will allow future ground-based gravitational wave detectors to make cosmological measurements to higher redshifts than any other instruments, with less systematic uncertainty. |

20. | Tomás Ferreira Chase, Universidad de Buenos Aires Ultralight vector dark matter, anisotropies and cosmological adiabatic modes November 17, 2023 (9:24 AM - 9:36 AM) Although LCDM does a great work at fitting cosmological observables, it does not explain the nature of the dark sector. In this talk I will consider dark matter is described by an ultra-light vector field. Vector fields source anisotropies in the early universe characterized by a shear tensor which rapidly decays once the fields starts oscillating, making them viable dark matter candidates. I will present the set of equations needed to evolve scalar cosmological perturbations in the linear regime, both in Synchronous gauge and Newtonian gauge. Finally, I will show that the shear tensor has to be taken into account in the calculation of adiabatic initial conditions. |

21. | Maya Fishbach, Canadian Institute for Theoretical Astrophysics (CITA) LIGO-Virgo-KAGRA's Oldest Black Holes November 18, 2023 (2:18 PM - 2:30 PM) In their third observing run, the LIGO-Virgo-KAGRA gravitational-wave observatory was sensitive to binary black hole mergers out to redshifts z ~ 1. However, some of these binary black holes likely experienced long delay times between the formation of their progenitor stars at their gravitational-wave merger. We use delay time distributions predicted by binary population synthesis to infer the formation redshifts of the ~70 black hole events reported in the gravitational-wave catalog GWTC-3. We find that at least one of these black hole systems probably formed before redshift z > 4, and discuss implications for the star formation rate and the cosmic metallicity evolution. These results highlight the promise of current gravitational-wave observatories to probe high-redshift star formation. |

22. | Gabriel Freedman, University of Wisconsin-Milwaukee Cross Validating the Inter-pulsar Correlations in the NANOGrav 15 yr Dataset November 17, 2023 (2:54 PM - 3:06 PM) The pulsar timing array (PTA) community has found evidence for a correlated stochastic signal following the Hellings-Downs pattern indicative of an isotropic stochastic gravitational-wave background (GWB). In this talk, I will introduce a method that addresses the now important task of validating these results: a Bayesian leave-one-out analysis aimed at assessing the significance of the GWB in individual pulsars in the array. This technique specifically explores the inclusion of inter-pulsar correlations in PTA data. I will present the results of using this method to cross validate the NANOGrav 15 yr results, and end by discussing the extension of this technique to other correlation signatures. |

23. | David Garfinkle, Oakland University Gravitational wave memory and the wave equation November 18, 2023 (1:54 PM - 2:06 PM) Gravitational wave memory and its electromagnetic analog are shown to be straightforward consequences of the wave equation. From Maxwell's equations one can derive a wave equation for the electric field, while from the Bianchi identity one can derive a wave equation for the Riemann tensor in linearized gravity. Memory in both cases is derived from the structure of the source of those wave equations |

24. | Shohreh Gholizadeh Siahmazgi, Wake Forest University Infrared Effects and the Unruh State November 17, 2023 (4:42 PM - 4:54 PM) The late-time behavior of some of the modes of quantum fields in the Unruh state in two-dimensional black hole spacetimes are studied, along with their contribution to the symmetric two-point function. The modes that are positive frequency with respect to the Kruskal time coordinate on the past horizon are composed of packets of modes that constitute the Boulware state. It is found that the infrared behavior of these Boulware modes determines the late-time behaviors of the Kruskal modes and the symmetric two-point function. |

25. | Ifigeneia Giannakoudi, Perimeter Institute Ultralight Boson Probe-ability with LISA November 17, 2023 (9:36 AM - 9:48 AM) The emission of gravitational waves from black hole superradiance can serve as a probe of massive ultralight bosons. In this work, we examine the possible detection of vector ultralight bosons in follow-up searches on massive black hole binary mergers with LISA. To this end, we make use of the massive black hole binary merger catalogs regarding massive black hole formation for three different ``seed'' mechanisms. Assuming vector bosons of optimal mass, the mass of the boson that corresponds to the highest strain amplitude at the peak, we compute the signal-to-noise ratio for LISA. For a 4-year mission time, we find that all three models predict systems that can emit signals with signal-to-noise ratio greater than 10, which is our observational threshold. We use these signals to make a rough prediction for the range of vector boson masses that can be probed or constrained. |

26. | Andrew J. S. Hamilton, U. Colorado, Boulder Spin(11,1) String Theory November 18, 2023 (5:06 PM - 5:18 PM) Spin(10), the covering group of SO(10), is a well-known promising grand unified group. Remarkably, Spin(10) chirality coincides with Dirac chirality, pointing to a nontrivial unification of Spin(10) and spacetime groups in Spin(11,1) that does not violate the Coleman-Mandula no-go theorem. The 11+1 dimensions of Spin(11,1) do not separate into a direct product of internal and spacetime dimensions. Rather, the 12 dimensions separate into a fermionic 10 dimensional internal compact manifold embedded inside 3+1 large spacetime dimensions. The 10 compact dimensions separate into a 4 dimensional weak manifold and a 6 dimensional color manifold that transform differently under Lorentz transformations. After symmetry breaking to the standard model, the weak and color manifolds together form a 10 dimensional Calabi-Yau manifold. The proposed Spin(11,1) string theory is a 26 dimensional tachyonic, nonsupersymmetric, anomaly-free bosonic string theory compactified to 12 dimensions on the self-dual maximal torus of the group SU(8)xSU(8). Weak and color gauge fields are carried by open bosonic strings whose ends attach to the fermionic weak and color subbranes of the Calabi-Yau manifold. Being nonsupersymmetric, the theory does not predict unobserved super-partners. |

27. | Abhishek Hegade Kumbale Raveesha, UIUC Probing dissipative effects in neutron stars using gravitational waves-II November 18, 2023 (9:12 AM - 9:24 AM) Tidal interactions in binary neutron star systems allow us to extract information about the equation of state inside a neutron star from gravitational wave observations. In this talk, we discuss how one could potentially probe out-of-equilibrium effects inside a neutron star by modeling the effects of tidal dissipation during the inspiral of a binary neutron star system. We describe our ongoing work on computing dissipative tidal love numbers due to shear and bulk viscosity in full general relativity. |

28. | George Hrabovsky, Midwest Area Science an d Technology Constructing Classical Fields from a Lagrangian in Mathematica November 18, 2023 (4:18 PM - 4:30 PM) I will demonstrate how to construct a scalar field theory in Mathematica in specific coordinate bases. |

29. | Aurora Ireland, University of Chicago Supermassive Primordial Black Holes from Inflation November 17, 2023 (9:00 AM - 9:12 AM) Much remains to be understood about the origin and evolution of our universe's largest supermassive black holes (SMBHs). In this talk, I motivate the possibility that some fraction of these SMBHs may be primordial in origin, having formed from the direct collapse of density perturbations seeded by inflation. Such a scenario is naively in conflict with constraints from CMB spectral distortions but can be made viable for a distribution of curvature perturbations which is sufficiently non-Gaussian. I present a concrete model of multi-field inflation capable of yielding such dramatic non-Gaussianities and calculate the maximal abundance of SMBHs, finding it to be consistent with the population observed at high-redshift. This result has a number of interesting implications and is especially timely in light of recent evidence from the NANOGrav collaboration and other pulsar timing arrays for a stochastic gravitational wave background consistent with SMBH mergers. |

30. | Nima Laal, Vanderbilt The NANOGrav 15-year data set: Search for Transverse Polarization Modes in the Gravitational-Wave Background November 18, 2023 (3:30 PM - 3:42 PM) In their most recent data set, NANOGrav found compelling evidence for a gravitational wave background with Hellings and Downs(HD) correlations. These correlations describe gravitational waves as predicted by general relativity, which has two transverse polarization modes. However, more general metric theories of gravity can have additional polarization modes which produce different correlation patterns. In this talk, I will report on what the recent NANOGrav efforts can reveal about the possibility of the observed correlations to be described by a mix of quadrupolar Hellings and Downs (HD) and Scalar Transverse (ST) correlations. |

31. | Haoyang Liu, UCAS/UIUC Probing higher curvature effects through an effective-field-theory consistent gravitational wave model November 17, 2023 (11:48 AM - 12:00 PM) Effective Field Theory (EFT) allows us to modify general relativity while keeping Lorentz invariance and without introducing extra degrees of freedom. Previous work have considered EFT corrections to gravitational waves in the inspiral or in the ringdown stages separately. In our work, we construct a full inspiral-merger-ringdown waveform model within the EFT extension to general relativity, by combining the inspiral and ringdown corrections. Using this waveform, we first explore the region in the EFT parameter space that could lead to constraints when comparing the model to data. We then find a conservative constraint on the EFT model through Bayesian parameter estimation on advanced LIGO/Virgo events. We also investigate how the choice of different priors affects our conclusions. |

32. | Kris Mackewicz, University of Chicago Gravity of Gluonic Fluctuations and the Value of the Cosmological Constant November 17, 2023 (9:12 AM - 9:24 AM) We analyze the classical linear gravitational effect of an idealized pion-like dynamical system where uniform gluonic stress-energy fills a spherical volume bounded by a 2D surface comprising the quarks' rest mass. In one orbit of a system of total mass $M$, quarks of mass $m<<M$ expand apart initially with $v/csim 1$, slow due to the gluonic attraction, reach a maximum size $R_0 sim hbar/ Mc$, then recollapse. We solve the linearized Einstein equations and derive the effect on freely falling bodies whose wordlines pass through the causal diamond of the gluonic bubble. The bubble model is shown to produce a secular mean outward residual velocity of test particles that lie within its orbit. It is shown that the mean gravitational repulsion of bubble-like virtual-pion vacuum fluctuations agrees with the measured value of the cosmological constant, for a bubble with a radius equal to about twice the pion de Broglie length. These results support the view that the gravity of standard QCD vacuum fluctuations is the main source of cosmic acceleration. |

33. | Utkarsh Mali, Canadian Institute for Theoretical Astrophysics (CITA) Gravitational Wave Cosmology using Spectral Sirens November 17, 2023 (10:00 AM - 10:12 AM) Cosmological parameters, such as the Hubble constant, have been a focused area of study for the astrophysics community. We aim to estimate the expansion rate using the spectral sirens method. This involves using features in the full parametric mass distribution, such as gaps, dips and peaks. By studying these features evolution with redshift, we are able to extract cosmological parameters. Using hierarchical Bayesian inference we simultaneously fit for the optimal mass distribution and cosmology, finding agreement with previous results. We then study the effects of model parameters on the cosmological inference. Our study will provide insight into the impact of different astrophysical formation channels on the expansion rate. |

34. | John Joseph Marchetta, Baylor University Supertranslations at spatial infinity in terms of Ashtekar-Barbero variables. November 17, 2023 (5:06 PM - 5:18 PM) It was recently shown that strengthening the standard parity conditions at spatial infinity in the Hamiltonian framework by requiring the leading order terms in the constraints to vanish yields non-vanishing BMS charges, in contrast to the standard parity conditions in which only the charges associated to the Poincare subgroup are non-vanishing. It has been understood for some time that with the standard parity conditions, the asymptotic region expressed in ADM variables and Ashtekar-Barbero variables are equivalent. In this talk we expressed the new boundary conditions in terms of Ashtekar-Barbero variables. Although the super translation charges coincide between the two formulations, in the Ashtekar-Barbero variables there are irremovable divergences plaguing the rotation and boost charge, rendering this formalism distinct from ADM. We discuss our goal of finding boundary conditions in Ashtekar-Barbero variables in which the asymptotic structure and generators are well defined and the full BMS charges are non-vanishing with the incentive of quantizing these charges using methods of loop quantum gravity. |

35. | Taillte May, Perimeter Institute Non-linear contribution to black hole ringdown November 18, 2023 (11:24 AM - 11:36 AM) Linear perturbations on black holes have very distinctive behaviour. Detecting this distinctive behaviour would be a strong test of the nature of black holes. In order to make this detection it is important to understand exactly how and when a black hole binary merger exhibits linear ringdown behaviour. I will discuss our work on quantifying a non-linear contribution (Amplitude Induced Mode Excitation) to the ringdown gravitational wave signal. |

36. | Tyler McMaken, JILA, University of Colorado Boulder Pancakification in quantum Kerr black holes November 17, 2023 (4:30 PM - 4:42 PM) Kerr black holes possess two horizons, the inner of which has the oft-overlooked property that gravitational tidal forces initially spaghettifying a freely falling observer will eventually change signs and flatten the observer like a pancake. When the effects of a quantum field are included on the metric, these compressive tidal forces at face value coincide with the perception of negative-temperature Hawking radiation for an infalling observer close enough to the inner horizon. Here I will give a rigorous analysis of the perceived Hawking spectrum in the Kerr interior and discuss its implications for the viability of the Kerr model in light of the effective field theory approach to quantum gravity. |

37. | Simone Mezzasoma, University of Illinois Urbana-Champaign Mitigating Waveform Model Uncertainties in Binary Black Hole Parameter Estimation November 17, 2023 (2:30 PM - 2:42 PM) The analysis of gravitational waves (GW) from coalescing binaries has to grapple with the challenge of waveform model uncertainties, potentially introducing biases in parameter estimation (PE). This study focuses on mitigating these inaccuracies within the context of inspiral-merger-ringdown (IMR) phenomenological models by employing a more informed IMRPhenomD waveform (WF) model. This model accounts for and marginalizes over the IMR fitting coefficients that embody the uncertainties in creating such a WF approximant. Initially, we generate probability distributions for the WF model fitting coefficients by training the model against a set of highly accurate waveforms. Subsequently, we plan to integrate these distributions as priors to enable simultaneous sampling of astrophysical parameters and IMR fitting coefficients during the PE of known GW events. By embracing the inherent variability of fitting phenomenological coefficients, this approach promises more reliable and unbiased astrophysical parameter estimates. |

38. | Soumodeep Mitra, University of South Dakota Probing Quantum Nature of Black holes with Ultralight Dark Matter November 17, 2023 (4:18 PM - 4:30 PM) We have studied the motion of a compact object through a cloud of ultralight scalar field, mimicking the dark matter. Due to the scattering of the scalar dark matter from the compact object, there will be a net force acting on the compact object, as well as some of these dark matter particles, will be absorbed by the compact object. This will result in Dynamical Friction experienced by the compact object, leading to modifications to the trajectory of the compact object. As we demonstrate, the dynamical friction, as well as the energy absorbed by the compact object depends crucially on the reflectivity of the surface of the compact object and can be used to distinguish classical black holes from exotic compact objects, including quantum black holes. |

39. | Ian Newsome, Wake Forest University Quantum Effects in 3+1 Schwarzschild-de Sitter Spacetime: Properties of the Hadamard Two-Point Function November 17, 2023 (4:54 PM - 5:06 PM) Schwarzschild-de Sitter spacetime offers a classical background structure in which quantum fields and their correlations can be studied in a case where there is both a black hole and cosmological horizon. To investigate these correlations, the Hadamard two-point function is of particular interest. It has been previously found in two dimensions, for spatially separated points on a coincident time hypersurface, that the Hadamard function grows linearly with respect to time in the region between the two horizons, indicating an instability, with a rate of growth proportional to the sum of the black hole and cosmological horizon surface gravities. To determine whether this linear growth persists in four dimensions, where scattering effects associated with the quantum field occur due to the presence of an effective potential, the contribution of s-wave sector (l=0) modes to the Hadamard function has been computed for a massless minimally coupled scalar field in the Unruh state. Results for the Hadamard two-point function, as well as properties of the quantum field modes from which it is comprised, will be presented. |

40. | Kellie O'Neal-Ault, Embry-Riddle Aeronautical University Recent Spacetime symmetry tests in Gravity November 18, 2023 (10:24 AM - 10:36 AM) A brief overview into recent tests of gravity and its spacetime symmetries is given. We work with the Standard-Model Extension, an agnostic, effective field-theory framework that allows for analysis of foundational spacetime symmetries. There have been a wide range of experiments and theory developments that have helped contribute toward constraining terms within the framework, providing clues to a possible underlying, unified theory of physics. |

41. | Caroline Owen, University of Illinois at Urbana-Champaign Waveform accuracy and systematic uncertainties in current gravitational wave observations November 17, 2023 (2:18 PM - 2:30 PM) The post-Newtonian formalism plays an integral role in the models used to extract information from gravitational wave data, but models that incorporate this formalism are inherently approximations. Disagreement between an approximate model and nature will produce mismodeling biases in the parameters inferred from data, introducing systematic error. Through an injection and recovery campaign, we undertake a proof-of-principle study of such systematic error. In particular, we study how unknown, but calibrated, higher-order post-Newtonian corrections to the gravitational wave phase impact systematic error in recovered parameters. We consider injected data of non-spinning binaries as detected by a current, second-generation network of ground-based observatories and recover them with models of varying PN order in the phase. We will show that the truncation of higher order (>3.5) post-Newtonian corrections to the phase can produce significant systematic error even at signal-to-noise ratios of current detector networks. Additionally, we will present a method to mitigate systematic error by marginalizing over our ignorance in the waveform through the inclusion of higher-order post-Newtonian coefficients as new model parameters and show that this method can reduce systematic error greatly at the cost of increasing statistical error. |

42. | Frederick Pardoe, University of Illinois at Urbana Champaign Hyperbolic encounters of binary black holes in general relativity, and scalar Gauss-Bonnet fields November 17, 2023 (3:30 PM - 3:42 PM) I will present simulations of hyperbolic encounters between equal-mass black holes that source a scalar field quadratically coupled to the Gauss-Bonnet invariant. Depending on the impact parameter, the black holes scatter, merge, or exhibit zoom-whirl orbits. I will discuss the evolution of the scalar field and present the scalar energy flux. Working at the decoupling limit, the background spacetime represents the evolution of black hole binaries in General Relativity. I will also comment on changes to the black hole masses and a set of spin-spin simulations with results pertinent to classical General Relativity. |

43. | Nihan Pol, University of Wisconsin Milwaukee The NANOGrav 15 yr dataset: Evidence for a nanohertz gravitational wave background November 18, 2023 (3:42 PM - 3:54 PM) In this talk, I will give a brief overview of the NANOGrav 15 yr dataset, followed by the results from the search for a nanohertz gravitational wave background. I will compare the results from the NANOGrav 15 yr analysis with those from other pulsar timing arrays, and end by illustrating what is in store for the next few years of nanohertz gravitational wave science. |

44. | Ramesh Radhakrishnan, Baylor University Simplest set of modifications to Einstein's Gravity to construct stable traversable wormholes November 17, 2023 (3:42 PM - 3:54 PM) Many authorities on Gravity have proved that it is not possible to construct a stable traversable wormhole within the framework of Einstein Gravity. In particular, the weak energy condition must be satisfied as a minimum for a viable traversable wormhole. It is also well established that certain modified gravities allow the construction of a stable wormhole that may not require exotic matter at least near the vicinity of the wormhole throat. Most of the issues as it relates to stabilizing a wormhole in Einstein gravity can be resolved if we were to use such modified gravity frameworks. With this in mind, we have embarked on an effort to find the simplest set of modifications to Einstein Gravity that may be necessary to construct a stable traversable wormhole. We first explore wormholes in some established modified gravity theories such as the Lovelock Gravity, Einstein-Gauss-Bonnett Gravity, and a few wormhole geometries where the stress tensor is modified to avoid the need for negative energy material as much as possible. In each case we describe the modification required to Einstein Gravity and how these modifications help with stabilizing the wormhole. We also attempt to standardize stable wormhole validation procedures. |

45. | Justin Ripley, University of Illinois, Urbana-Champaign Probing internal dissipative processes of neutron stars with gravitational waves during the inspiral November 18, 2023 (9:00 AM - 9:12 AM) A long-standing goal in astrophysics and nuclear particle physics has been to determine the neutron star equation of state, i.e., the relation between the pressure and the energy density in the interior of a neutron star. We find that modeling internal dissipative effects in neutron stars binaries during the inspiral requires that one introduce a new tidal deformability parameter--the dissipative tidal deformability. We show that (as is the case for black holes) the dissipative tidal deformability corrects the gravitational-wave phase at 4 post-Newtonian order for quasi-circular binaries. This correction receives a large finite-size enhancement by the stellar compactness, analogous to the case of the tidal deformability, which makes the parameter potentially measurable with ground-based gravitational wave detectors. The correction is not degenerate with the time of coalescence, which also enters at 4PN order, because it contains a logarithmic frequency-dependent contribution that breaks the partially correlations. We briefly discuss our derivation of this result, our ongoing work on computing and measuring the dissipative tidal deformability of neutron stars. |

46. | Tamal RoyChowdhury, University of Wisconsin-Milwaukee High accuracy post-Newtonian and numerical relativity comparisons involving higher modes for eccentric binary black holes and a dominant mode eccentric inspiral-merger-ringdown model November 17, 2023 (2:42 PM - 2:54 PM) Spherical harmonic modes of gravitational waveforms for inspiraling compact binaries in eccentric orbits from post-Newtonian (PN) theory accurate to third post-Newtonian order and those extracted from numerical relativity (NR) simulations for binary black holes (BBHs) are compared. We combine results from the two approaches (PN and NR) to construct time-domain hybrid waveforms that describe the complete evolution of BBH mergers through inspiral-merger-ringdown (IMR) stages. These hybrids are then used in constructing a fully analytical dominant mode (ℓ=2, |m|=2) eccentric IMR model. A simple extension to a multimode model based on this dominant mode model is also presented. Overlaps with quasicircular IMR waveform models, including the effect of higher modes, maximized over a time and phase shift, hint at the importance (mismatches>1%) of including eccentricity in gravitational waveforms when analyzing BBHs lighter than ∼80 M⊙, irrespective of the binary’s eccentricity (as it enters the LIGO bands) or mass ratio. The combined impact of eccentricity and higher modes seems to become more apparent through smaller overlaps with increasing inclination angles and mass ratios. |

47. | Pranav Satheesh, University of Florida Studying gravitational wave and slingshot recoil of massive black holes in cosmological simulations November 18, 2023 (10:12 AM - 10:24 AM) A galaxy merger can result in the formation of a massive black hole (MBH) binary system. The merger of an MBH binary system is characterized by the emission of gravitational waves (GW) and can result in the recoiling of the merger product. The gravitational recoil could displace the black hole from the host galaxy and can be observed as an offset active galactic nuclei (AGN). Systems where the binary inspiral time is long enough for another galaxy merger could result in a triple massive black hole system. The triple interactions can cause the ejection of the lightest black hole through gravitational slingshot effect. We study such triple and binary massive black hole systems using the Illustris cosmological hydrodynamical simulation. We do a comparative analysis of GW-induced recoil kicks and slingshot effect kicks in our fiducial model. Since GW recoil kicks are spin-dependent, we consider random and aligned spin models for the black hole spins. We find that slingshot kicks are more prevalent than GW kicks if the binary black holes have spins aligned before the merger. Observations of offset AGNs will tell us more about the spin alignment of the progenitor black holes. |

48. | Aryanna Schiebelbein, Canadian Institute for Theoretical Astrophysics Populations of Binary Black Holes Over Cosmic Time November 17, 2023 (1:42 PM - 1:54 PM) There is no consensus on the exact mechanism by which binary black hole mergers form. As the number of binary black hole mergers observed increases, population studies can be used to learn about the origins of these systems. We use the binary black hole population to inform a model for the rate of mergers, which is dependent on formation environments and histories. By looking at the merger rate over cosmic time, we place constraints on any metallicity preferences or the time delay between star formation and black hole merger. We also use the rate to measure the black hole mass density as a function of cosmic time. |

49. | Kristen Schumacher, University of Illinois Urbana-Champaign Better early than never: Testing GR with GW Polarizations November 17, 2023 (11:24 AM - 11:36 AM) In addition to the two polarization modes present in general relativity, some modified theories of gravity may contain up to four additional polarizations, which are allowed to travel at different speeds. Detecting these additional modes would be clear evidence of new physics, while non-detections may constrain such modified theories. Here we present a model-independent method to compute the different polarizations directly from the metric perturbation in theories where additional modes may travel at speeds different from the speed of light. We further extend the ppE framework to apply to such theories. Finally, we discuss how faster propagation speeds, which result in early arrival times at the detector, would impact the constraints we would be able to place by searching for these polarizations. |

50. | Jacob Sprague, Northwestern University Gravitational wave signals from axion clouds in the LSD band November 17, 2023 (2:06 PM - 2:18 PM) A population of axion clouds in the Milky Way would produce a cacophony of GW signals. The Levitated Sensor Detector (LSD) is a high-frequency GW instrument being developed at Northwestern, and in this talk I will discuss our most current predictions for resolvable signals, as well as for the confusion foreground produced by the many unresolved sources. |

51. | Christopher Stith, University of Michigan Black hole formation with null dust November 17, 2023 (3:54 PM - 4:06 PM) We extend the 2009 result of Christodoulou on black hole formation in vacuum to a certain mildly symmetric class of null dust spacetimes. That is, we construct a set of characteristic initial data on a past null boundary, and we prove that the future development of this spacetime (which satisfies the Einstein-null dust equations) contains a closed trapped surface. The mechanism by which this occurs is the focusing of incoming energy from the null dust itself. If the null dust is set to zero, the vacuum result of Christodoulou is recovered. In future work, we hope to remove the mild symmetry assumption. |

52. | Charles Sven, Independent Researcher The Copernican Cosmological Principle Revisited. November 18, 2023 (5:18 PM - 5:30 PM) Modern telescopic studies including NASA’s COBE satellite measurements shed much light on this Copernican Cosmological Principle. |

53. | Darsan Swaroop Bellie, CIERA, Northwestern University The unresolved stochastic background from compact binary mergers detectable by next-generation ground-based gravitational-wave observatories November 18, 2023 (10:00 AM - 10:12 AM) The next generation of ground-based gravitational-wave detectors will look much deeper into the Universe and have unprecedented sensitivities and low-frequency capabilities. Especially alluring is the possibility of detecting an early-Universe cosmological stochastic background that could provide important insights into the beginnings of our Universe and fundamental physics at extremely high energies. However, even if next-generation detectors are sensitive to cosmological stochastic backgrounds, they will be masked by more dominant astrophysical backgrounds, namely the residual background from the imperfect subtraction of resolvable compact binary coalescences (CBCs) as well as the CBC background from individually unresolvable CBCs. Using our latest knowledge of masses, rates, and delay time distributions, we present a data-driven estimate of the unresolvable CBC background that will be seen by next-generation detectors. Accounting for statistical and systematic errors, this estimate quantifies an important piece in the CBC noise budget for next-generation detectors and can help inform detector design and subtraction algorithms. We compare our results with predictions for backgrounds from several cosmological sources in the literature, finding that the unresolvable background will likely be a significant impediment for many models. This motivates the need for simultaneous inference methods or other statistical techniques to detect early-Universe cosmological backgrounds. |

54. | Colm Talbot, University of Chicago Preparing for gravitational-wave population inference at scale November 18, 2023 (2:42 PM - 2:54 PM) Abstract to follow |

55. | Zachary Tyler, Grand Valley State Physics Department Numerical Solutions to McVittie Timelike Geodesics November 18, 2023 (11:00 AM - 11:12 AM) This project provides numerical solutions to timelike geodesics within the Schwarzschild and McVittie metrics. The Schwarzschild metric represents a static, non-spinning black hole. The McVittie metric appears to be Schwarzschild close to the origin, but an expanding FLRW space (cosmology) far away. The main goal of this research is to show the difference in the or- bital and gravitational wave patterns between static and expanding spacetimes. Both FLRW and Schwarzschild-DeSitter spacetimes are discussed within the numerical context of calculating geodesics. The numerical method used is a Hamiltonian approach and an 8th order Runge-Kutta, coded used python. |

56. | Gayathri Vivekananthaswamy, University of Wisconsin Milwaukee Gravitational wave source populations: Disentangling an AGN component November 18, 2023 (2:30 PM - 2:42 PM) The astrophysical origin of the over 90 compact binary mergers discovered by the LIGO and Virgo gravitational wave observatories is an open question. While the unusual mass and spin of some of the discovered objects constrain progenitor scenarios, the observed mergers are consistent with multiple interpretations. A promising approach to solve this question is to consider the observed distributions of binary properties and compare them to expectations from different origin scenarios. Here we describe a new hierarchical population analysis framework to assess the relative contribution of different formation channels simultaneously. For this study we considered binary formation in AGN disks along with phenomenological models, but the same framework can be extended to other models. We find that high-mass and high-mass-ratio binaries appear more likely to have an AGN origin compared to the same origin as lower-mass events. Future observations of high-mass black hole mergers could further disentangle the AGN component from other channels. |

57. | Robert Weinbaum, University of Chicago The Averaged Null Energy Condition for Classical Dirac Fields November 18, 2023 (11:12 AM - 11:24 AM) Assessing whether known matter fields satisfy or violate the averaged null energy condition, or ANEC, is of central importance — both for theoretical results in general relativity, and for restricting the class of physically allowed spacetimes, such as traversable wormholes. In this talk, I will discuss recent progress in determining the validity of ANEC for the case of a classical Dirac field in static, spherically symmetric spacetimes. |

58. | James Wheeler, University of Michigan, Ann Arbor Generic Naked Singularities in Vaidya Spacetimes November 18, 2023 (4:30 PM - 4:42 PM) The incoming Vaidya spacetimes are perhaps the simplest toy models for the dynamical formation of a Schwarzschild black hole from nonsingular initial data, arising from the collapse of a spherically symmetric cloud of null dust. While it has long been recognized that these can exhibit the dynamical formation of globally naked singularities given a self-similar mass profile, I will present recent work indicating that this formation is generic, in a natural sense, within the full class of Vaidya spacetimes with arbitrary mass profile. I will discuss this observation's significance in the context of weak cosmic censorship. |

59. | Christopher Winfield, Oakland University Dept. of Mathematics and Statistics Analysis of Large Harmonic-Degree Asymptotics for Non-Radial Stellar Pulsation Models: Explorations of Numerical and Symbolic Computation via Mathematica November 18, 2023 (1:30 PM - 1:42 PM) This work is a continuation of a study of the Cowling approximation, a common ansatz used in linearized perturbation models of non-radial stellar pulsation (classical gravity). We demonstrate how to apply the methods of the study to approximate general solutions of the model and we check the methods against specific cases, such as those of Lane-Emden background pressure-density and those of adiabatic equilibrium. In turn, this work involves refined estimates which we develop via symbolic computation, some of which are motivated by numerical instabilities which arise in numerical checks. |

60. | Alan Wiseman, University of Wisconsin -- Milwaukee The self-force on a static charge in Schwarzschild spacetime using the method of images November 18, 2023 (1:42 PM - 1:54 PM) One of the most basic self-force problems (some times called radiation reaction force) is that of an electric charge held fixed in Schwarzschild spacetime. There are many ways to solve the problem, but the result is still somehow "non intuitive". I will use some simple arguments to derive the force using the method of images -- and hopefully restore the intuitive nature of the problem. I will also discuss how to extend this method to moving charges. |

61. | Yiqi Xie, University of Illinois Urbana-Champaign Neural Post-Einsteinian Framework for Efficient Theory-Agnostic Tests of General Relativity with Gravitational Waves November 17, 2023 (11:36 AM - 11:48 AM) The parametrized post-Einsteinian (PPE) framework and its variants are widely used by the gravitational-wave community to probe gravity through tests that apply to a large class of theories beyond general relativity. However, PPE is not truly theory-agnostic as it only captures certain types of deviations from general relativity: those that admit a post-Newtonian series form. Moreover, each type of deviation in PPE has to be tested separately, making the whole process computationally inefficient and expensive, possibly obscuring the theoretical interpretation of potential deviations detected. In this talk, I will present the neural post-Einsteinian (NPE) framework, an extension to PPE that overcomes the above weaknesses using deep-learning neural networks. I will showcase the application of NPE to future tests of general relativity during the fifth observing run of the LIGO-Virgo-KAGRA collaboration. In particular, I will demonstrate the use of NPE to efficiently explore deviations from general relativity beyond what can be mapped to PPE, including those coming from the higher-order corrections of the Einstein-dilaton-Gauss-Bonnet gravity and from dark-photon interactions in the hidden sector of matter. |

62. | Victor Zhang, University of Chicago The Entropy of Dynamical Black Holes and the Generalized Second Law November 17, 2023 (4:06 PM - 4:18 PM) We propose 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. In stationary eras, this formula agrees with the usual Noether charge formula, but in nonstationary eras, we obtain a nontrivial correction term. In particular, in general relativity, our formula gives the entropy of a dynamical black hole as its area minus an integral involving the expansion of the null generators of the horizon. Our formula for entropy in a general theory of gravity is obtained from the requirement that a ``local physical process version'' of the first law of black hole thermodynamics hold for perturbations of a stationary black hole. It follows immediately that for first order perturbations sourced by external matter that satisfies the null energy condition, our entropy obeys the second law of black hole thermodynamics. For vacuum perturbations, the leading order change in entropy occurs at second order in perturbation theory, and the second law is obeyed at leading order if and only if the ``modified canonical energy flux'' is positive (as is the case in general relativity but presumably would not hold in more general theories of gravity). Our formula for the entropy of a dynamical black hole differs from the formula proposed independently by Dong and by Wall. We obtain the relationship between their formula and ours, thereby generalizing their results to our class of Lagrangians. We then consider the generalized second law in semiclassical gravity for first order perturbations of a stationary black hole. We show that the validity of the quantum null energy condition (QNEC) on a Killing horizon is equivalent to the generalized second law using our notion of black hole entropy but using a modified notion of von Neumann entropy for matter. On the other hand, the generalized second law for the Dong--Wall entropy is equivalent to an integrated version of QNEC, using the unmodified von Neumann entropy for the entropy of matter. |

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