Diego Blas, Institut de Física d'Altes Energies (IFAE)

Title: Closing gaps in the GW spectrum: Ideas to detect microHz and high frequency GWs

Abstract: In this talk I will describe some recent ideas to find GWs in two bands which are not covered by traditional methods, and that have a rich potential for astrophysics and fundamental physics.

Slides can be found here

Stephen Green, University of Nottingham 

Title: Simulation-Based Inference for Gravitational Waves

Abstract: During the first half of the fourth LIGO-Virgo-KAGRA observing run, we observed gravitational waves from merging binaries roughly every three days. While this routine detection promises exciting results, it is becoming a significant challenge to analyze all events using our most sophisticated theoretical models. In this talk, I will describe how to overcome these challenges using deep learning techniques for rapid, amortized Bayesian inference. This approach uses simulated data to train neural networks (such as normalizing flows) to represent the Bayesian posterior. Once trained, sampling becomes extremely fast. I will also describe how to establish full confidence in results using importance sampling, as well as initial results on population inference and future prospects to treat realistic noise.

Slides can be found here

Syksy Räsänen, University of Helsinki

Title: Walking on a bed of nails - effect of dark matter discreteness on light propagation

Abstract: In general relativity, light travels on null geodesics when the curvature of the wavefront and the spacetime are much smaller than photon energy. Localised dark matter particles can source large spacetime curvature, pushing light off null geodesics. I discuss how to take this effect into account, and how it may affect cosmological redshift and distance. 

Slides can be found here

Fabian Bautista, Institut de Physique Théorique (IPhT), Saclay 

Title: From two-dimensional CFTs to Kerr Binary Systems

Abstract: In this talk we will discuss a new window into the solution of Heun differential equations arising in black hole perturbation theory using the tools of two-dimensional CFT. Kerr Compton amplitudes for massless perturbation of generic spin-weight s, are written in compact form in terms of the so-called Nekrasov-Shatashvili functions; their symmetry properties are also discussed. These are then used as building blocks to study the scattering of two Kerr black holes with generic spin orientation. Comparison to conservative observables for bounded systems computed via first-order gravitational self-force methods are shown.

Title: Probing the host environments of compact binaries

Abstract: When, where, and how compact binaries form is still a mystery, and observations from the LIGO-Virgo-KAGRA detectors are starting to shed light on this. In this talk, I will discuss ways of probing host environments of these binaries. First, I will show how the redshift evolution of the binary black hole merger rate can be used to constrain the parameter space of the host galaxies of such mergers. Then, I will discuss how a nonzero centre of mass acceleration affects the observed signal in our detectors, and how one can differentiate between candidate host environments by measurements of acceleration. I will end with some prospects along these lines for the future.

Slides can be found here

Sofie Koksbang, Southern University of Denmark (SDU)

Title: Observational effects of cosmic backreaction with concrete abstract cosmology and machine learning

Abstract: Cosmic backreaction is a concept from inhomogeneous cosmology which originally gained attention because the phenomenon can lead to apparent accelerated expansion of the universe without dark energy. However, it is at this point unclear to what extent cosmic backreaction is relevant for our universe. One of the methods for revealing its importance (or lack thereof) is using observational tests that can either reveal a deviation from FLRW behavior or directly reveal effects of cosmic backreaction.

 In this talk, I will give a brief review of cosmic backreaction. I will then introduce "concrete abstract cosmology" and discuss the observational signatures of backreaction that have been uncovered with this approach and what possibilities the approach gives when combined with machine learning.

Ira Rothstein, Carnegie Mellon University

Title: Environmental Effects on Binary Inspirals

Abstract: The problem of predicting the gravitational wave signal from a binary inspirals involves solving a complicated set of non-linear differential equations. There has been great progress made in making such predictions analytically in Post-Newtonian limit (small relative velocities) using techniques originally developed for quantum field theory. In this talk I will discuss how one can extend these ideas to include the effects of an inviscid fluid environment by utilizing field theoretic techniques developed to study non-equilibtrium systems. I will begin by building a point particle action for a compact object moving in an perfect fluid. By doing so we effectively trade a set of boundary conditions for a coefficient in the action, which greatly simplifies the problem. I will then demonstrate how this approximation can reproduce the full answer at distances large compared to the radius of the object.

This will lead naturally to a discussion of "Dalamberts’ paradox'' which states that an object moving at constant velocity through fluid with no viscosity feels no drag force. I will then show how to introduce viscosity using the closed-time path integral formulation of field theory. By covariantizing this theory I will show how one can systematically include Post-Newtonian corrections to the action for the purpose of making precision gravitational wave predictions which are sensitive to environmental conditions.

Slides can be found here

Amanda Farah, University of Chicago

Title: What the population of gravitational wave sources can do for you

Abstract: The LIGO-Virgo-KAGRA collaboration’s growing catalog of gravitational wave signals is gradually illuminating the astrophysical population of binary neutron stars and black holes. Features in this population contain information about a wide range of physical phenomena, including the processes that lead to the formation of merging compact objects and the conditions of the universe through which the gravitational waves propagated before reaching earth. It is therefore a powerful tool for understanding the astrophysics of compact object formation, measuring cosmological parameters, and probing the matter distribution between gravitational-wave sources and earth. I will present recent insights into the population of compact object masses inferred from gravitational-wave data and discuss their impacts on each of these topics.

Mikołaj Korzyński, Polish Academy of Sciences

Title: Optical Drift Effects In General Relativity

Abstract: I will discuss the redshift and the position drifts in general relativity, i.e. the temporal variations of the redshift and the position on the sky of a light source, as registered by an arbitrary observer. I will present the derivation of exact relativistic formulas for the drifts, in which they are expressed in terms of the kinematical variables characterising the motions of the source and the observer, i.e. their momentary 4-velocities and 4-accelerations, as well as the spacetime curvature along the line of sight. The formulas we derive are completely general and involve automatically all possible GR effects. They may be regarded as the counterpart of the Sachs optical equations for temporal variations of the standard observables. I will discuss their physical consequences, especially interesting in spacetimes with caustics, and their possible applications to the gravitational lensing theory, astrometry and cosmology. The talk should be accessible to anyone with a background in general relativity.

Nicola Gaspari, Radboud University

Title: Kicks and mergers of binary neutron stars

Abstract: Neutron stars, black holes, and the compact binaries they dwell in, can experience an impulsive change of velocity (a velocity kick) at some point in their evolution. The compact remnants born in supernovae for instance are kicked by asymmetric explosions, while the remnants of binary black hole mergers can recoil due to the anisotropic emission of gravitational waves. As a result of the kick, the galactic trajectories of these objects change and their kinematics results distinct from that of the progenitors, to the point that some might even unbound from the host galaxy. In this talk I will illustrate the impact of kicks on kinematics bringing binary neutron stars (BNSs) as a case study. I will discuss some recent results on the observed systemic velocities and merger locations of BNSs, both Galactic and in extragalactic transients, and show the implications for observation and interpretation of compact object mergers.

Saptaswa Gosh, Indian Institutes of Technology

Title: Classical black hole scattering from Worldline QFT in modified theories of gravity

Abstract: In this talk, we will briefly describe a technique useful for the computation of gravitational observables, impulse and waveform in a black hole scattering event, namely "Worldline Quantum Field theory." We then show how to apply this formalism for a theory beyond GR  involving additional scalar degrees of freedom, namely the Scalar-Tensor theory and the higher curvature terms (e.g. dynamical Chern-Simons theory). We will show how the impulse and waveform get corrected due to the extra scalar degree of freedom up to 2PM in the Scalar-Tensor theory.  Furthermore, we will point out several intricacies when the mass of the scalar field is non-vanishing. We also show how the eikonal phase gets corrected up to 3PM for spinning two body scattering events in dCS gravity.

Riccardo Gonzo, University of Edinburgh

Title: The on-shell space of scattering and bound classical observables

Abstract: We will show how scattering orbits can inform bound-orbit models, allowing to harness the power of the S-matrix to construct gravitational waveforms relevant for the dynamics of compact binary systems. First, I will derive the so-called radial action from the worldline formalism, focusing for simplicity on the probe limit in a Kerr background. Then, I will show that such radial action (and the S-matrix) is a natural generating functional of classical observables, which provide a direct analytic continuation between a novel on-shell basis of scattering (coordinate time delay, elapsed proper time and deflection angle) and bound (radial frequency, averaged redshift and periastron advance) observables. Including radiation, we will then derive a new surprising map between scattering and bound waveforms, which is inspired and confirmed by Post-Newtonian calculations with the standard time-domain multipoles. Finally, we will emphasize the need to resum the perturbative series and to study non-local-in-time effects to make contact with phenomenological waveforms.