Strong Gravity

The study of compact objects in the strong field regime needs a thorough understanding of the initial value problem in general relativity at the resence of hydrodynamical or magnetohydrodynamical sources. This is a twofold problem that includes general relativistic solutions that represent realistic astrophysical systems at a given moment in time as well as their subsequent evolutions. In this talk I will present the fundamental principles of this endeavor as well as efforts in understanding a great variety of astrophysical systems from binary neutron stars to ergostars and black hole-disks through numerical relativity.

The velocity of a gravitational wave (GW) source provides crucial information about its formation and evolution processes.

Previous studies considered the Doppler effect on the phase of GWs as a potential signature of a time-dependent velocity of the source. However, the Doppler shift only accounts for the time component of the wave vector, and in principle motion also affects the spatial components. In my talk I discuss the latter effect, known as “aberration” for light, for GWs and how it affects the waveform modeling of an accelerating source. I show that the additional aberrational phase shift could be detectable in two astrophysical scenarios, namely, a recoiling binary black hole (BBH) due to GW radiation and a BBH in a triple system.

Further, I discuss how adding the aberrational phase shift in the waveform templates could significantly enhance the detectability of moving sources.

Compact white dwarf (WD) binaries are important sources for space-based gravitational-wave (GW) observatories, and an increasing number of them are being identified by surveys like ELM and ZTF. We study the effects of nonlinear dynamical tides in such binaries. We focus on the global three-mode parametric instability and show that it has a much lower threshold energy than the local wave-breaking condition studied previously. By integrating networks of coupled modes, we calculate the tidal dissipation rate as a function of orbital period. We construct phenomenological models that match these numerical results and use them to evaluate the spin and luminosity evolution of a WD binary. While in linear theory the WD's spin frequency can lock to the orbital frequency, we find that such a lock cannot be maintained when nonlinear effects are taken into account. Instead, as the orbit decays, the spin and orbit go in and out of synchronization.  Each time they go out of synchronization, there is a brief but significant dip in the tidal heating rate. While most WDs in compact binaries should have luminosities that are similar to previous traveling-wave estimates, a few percent should be about ten times dimmer because they reside in heating rate dips. This offers a potential explanation for the low luminosity of the CO WD in J0651. Lastly, we consider the impact of tides on the GW signal and show that LISA and TianGO can constrain the WD's moment of inertia to better than 1% for centi-Hz systems.

To predict the gravitational waves emitted by a black hole binary, one needs to understand the dynamics of the binary in general relativity. No closed form solutions of this problem exist. Instead one must introduce some form of approximation. One such approximation, can be made if one of the components is much heavier than the other, suggesting a perturbative expansion in the mass-ratio. I will review this small mass-ratio (SMR) expansion of the dynamics, and the progress that has been made over the last two decades. In particular, I will discuss some recent results on the convergence of this series, suggesting that at relatively low orders this SMR expansion can be used to model even equal mass binaries.

We comment on the recently introduced Gauss-Bonnet gravity in four dimensions. We argue that it does not make sense to consider this theory to be defined by a set of D->4 solutions of the higher-dimensional Gauss-Bonnet gravity. We show that a well-defined D->4 limit of Gauss-Bonnet Gravity is obtained generalizing a method employed by Mann and Ross to obtain a limit of the Einstein gravity in D=2 dimensions. This is a scalar-tensor theory of the Horndeski type obtained by dimensional reduction methods. By considering simple spacetimes beyond spherical symmetry (Taub-NUT spaces) we show that the naive limit of the higher-dimensional theory to four dimensions is not well defined and contrast the resultant metrics with the actual solutions of the new theory. Theory and solutions in lower dimensions will also be briefly discussed.

We discuss several numerical and analytical studies of the modified gravity theory Einstein dilaton Gauss-Bonnet (EdGB) gravity. This class of modified gravity theories admit scalarized black hole solutions. The theory may then provide significantly different gravitational wave signatures during binary black hole merger as compared to general relativity, so that gravitational wave observations may provide new stringent constraints on EdGB gravity. The theory is also an important member of the Horndeski theories, which have been used to model theories of the early universe, including inflation and ekpyrosis. We describe our investigations of the self-consistency of EdGB gravity for spherically symmetric solutions that differ significantly from those in GR, including scalarized black hole solutions, and find that for solutions where the Gauss-Bonnet corrections to general relativity are not small, the theory no longer admits hyperbolic evolution. We discuss how an effective field theory interpretation of the equations of motion though should always guarantee (locally) hyperbolic evolution for the theory.

With the impressive number of binary black hole mergers observed by the LIGO-Virgo detector network in the recent years, it is now important to understand the formation channels of these systems. This talk focuses on the common envelope phase, crucial to the formation of compact object binaries. During this phase, the two companions evolve inside a shared envelope, with the secondary object orbiting towards the core of the primary star. Drag forces in the stellar envelope pull the two stellar cores into a tighter orbit. Additionally, the embedded object can be modified by accretion from the flow around it. I will present local simulations explaining the hydrodynamics of the common envelope inspiral phase, and highlight the effects of the full set of flow parameters on accretion and drag forces in these episodes. I will then discuss the transformation of binaries in common envelope phases and the effect of this phase on the properties of stellar-mass black hole populations

Observations have shown that nearly all galaxies harbor massive or supermassive black holes at their centers. Gravitational wave (GW) observations of these black holes will shed light on their growth and evolution, and the merger histories of galaxies. Massive and supermassive black holes are also ideal  laboratories for studying strong-field gravity. Pulsar timing arrays (PTAs) use observations of millisecond pulsars to detect low-frequency GWs with frequencies ~1-100 nHz, and can detect GWs emitted by supermassive black hole binaries, which form when two galaxies merge. I will discuss source modeling and detection techniques for PTAs, as well as present limits on nanohertz GWs from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration.

In this talk I will review the state of the art in PN gravity, and in particular its significant advancement via the EFT of spinning gravitating objects. First, I will introduce the concept of a tower of EFTs for the binary inspiral problem. I will then present the intricate formulation of the EFT of spinning objects. Finally, I will present some advanced results accomplished within this framework.

Recent observations of gravitational waves represent a remarkable success of our theoretical models of relativistic binaries. However, accurate models are largely restricted to binaries in which the two members have roughly equal masses; for binaries with more disparate masses, modelling is less mature. This is especially relevant for extreme-mass-ratio inspirals (EMRIs), in which a stellar-mass object orbits a supermassive black hole in a galactic core. EMRIs are uniquely precise probes of black hole spacetimes, and they will be key targets for the space-based detector LISA. They are best modelled by gravitational self-force theory, in which the smaller object generates a small gravitational perturbation that reacts back on it to exert a "self-force", accelerating the object away from geodesic motion. For LISA science, we must work to second order in this perturbative treatment. In this talk, I discuss the foundations of self-force theory, its application to EMRIs, and the current status of first- and second-order models.

Merging compact objects encode a vast deal of information about their progenitor stellar systems, such as the types of galactic environments they were born in, the intricacies of stellar evolution the persisted throughout their lives, and the physics of the supernovae that marked their deaths. In this talk, I will highlight multiple open questions that can be illuminated through a combination of compact objects observations (via gravitational waves and/or electromagnetic radiation) and computational modeling of environments that lead to the formation of black holes and neutron stars. I first focus on how the growing catalog of black hole mergers observed by the LIGO/Virgo network can place constraints on binary black hole formation scenarios, both by uncovering gravitational-wave sources that have features specific to a subset of formation scenarios (such as spins that are anti-aligned from the orbital angular momentum, high masses that occupy the pair instability mass gap, and distinguishable eccentricities at merger) and through pairing Bayesian hierarchical inference with state-of-the-art astrophysical models. Next, I will show how various uncertain aspects of binary stellar evolution and supernova physics can be constrained by phenomena attributed to merging neutron stars, such as enigmatic r-process enrichment in globular clusters and the association between short gamma-ray bursts and their host galaxies.

With the detection of GW170817 we have observed the first multi messenger signal from two merging neutron stars. This signal carried a multitude of information about the underlying equation of state (EOS) of nuclear matter, which so far is not known for densities above nuclear saturation. In particular it is not known if exotic states or even a phase transition to quark matter can occur at densities so extreme that they can't be probed by any current experiment.

I will show how the information carried in the gravitational wave signal of GW170817 can be used to constrain the EOS at densities above saturation and what we can learn about the possible existence of phase transitions. I will
also comment on how we can improve on those limits with upcoming observations of the NICER mission.In the second part of the talk I will focus on what we can learn about exotic states of matter from neutron star mergers.I will comment briefly on the impact of high spins in mergers and the importance of accurate numerical modelling in the context of these studies.Finally I will show how neutron star mergers can be used to study quark phase transitions and the phase diagram of QCD.