Strong Gravity

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.

We present a plausible counterexample to the weak cosmic censorship conjecture in four-dimensional Einstein-Scalar theory with asymptotically flat boundary conditions. Our setup stems from the analysis of the massive Klein-Gordon equation on a fixed Kerr black hole background. In particular, we construct the quasinormal spectrum numerically, and analytically in the WKB approximation, then go on to compute its backreation on the Kerr geometry. In the regime of parameters where the analytic and numerical techniques overlap we find perfect agreement. We give strong evidence for the existence of a nonlinear instability at late times.

Simulations that numerically solve Einstein's equations are the only means to accurately predict the outcome of the merger of two black holes. The most important outputs from these simulations are the gravitational waveforms, and the mass and spin of the final black hole formed after the merger. The waveforms are used in extracting astrophysical information from detections, while the final mass and spin are used in testing general relativity. Unfortunately, these simulations are too expensive for direct use in data analysis; each simulation can take a month on a supercomputer. Surrogate modeling is a data-driven approach to modeling, that uses machine learning like techniques to interpolate between hundreds of existing simulations. In this talk, I will discuss some recent developments in surrogate modeling, including a new 7-dimensional model that fully captures the effects of precession; the wobbling of the orbit caused when the black holes’ spin axes aren’t perpendicular to the plane of the orbit. This model reproduces the waveform as well as the final black hole's mass and spin as accurately as the simulations themselves, while taking only a fraction of a second to evaluate on a laptop.

Black holes in the background of the AdS soliton are, according to the gauge/gravity correspondence, dual to droplets of deconfined plasma surrounded by a confining vacuum. In this talk I will present, for the first time, the real time dynamics of finite energy black holes in these backgrounds. We consider horizonless initial data sourced by a massless scalar field. Upon time evolution, prompt scalar field collapse produces an excited black hole that eventually settles down to equilibrium at the bottom of the AdS soliton. Radiation in these backgrounds is carried by a set of massive states that, in the dual field theory, can be interpreted as particles. Thus, our results are relevant to describe the out-of-equilibrium dynamics in strongly coupled gauge theories that exhibit confinement. This is work in progress in collaboration with Hans Bantilan and David Mateos.

The detection of coalescing neutron stars via gravitational waves (GW170817) has revolutionized our understanding of the equation of state at supranuclear densities. The equation of state determines how neutron stars interact in a variety of astrophysical contexts, from rapidly rotating millisecond pulsars, to accreting X-ray sources, and, of course, coalescing binaries radiating gravitational waves. I will review the state of the field, including commonly adopted parametrizations of the neutron star equation of state in the context of theoretical expectations, before introducing a nonparametric inference scheme based on Gaussian processes. Nonparametric inference provides much greater functional freedom than parametrized analyses, allowing for the direct inference of neutron star composition and the existence of possible phase transtions above nuclear density. Additionally, I will review the search for predicted secular fluid instabilities within neutron star cores and their possible impact on gravitational-wave signals. This instability couples pressure-supported (p-mode) and gravity supported (g-mode) oscillations within the star, and I will show how GW170817 rules out only the most extreme theoretical predictions for how the instability could saturate. As the advanced LIGO and Virgo detectors gear-up for the second half of their third observing run, we will conclude by discussing the outlook for these measurements with future detections and the implications for broader astrophysical populations.

Advanced LIGO and Advanced Virgo are currently in the middle of their third observing run, and releasing open public event alerts for the first time. The LIGO-Virgo collaboration has issued 29 un-retracted candidate event alerts as of September 20th, 2019, potentially adding dozens more known compact binary object mergers to the eleven confident LIGO-Virgo detections from the first two Advanced-era observing runs. I’ll review novel LIGO-Virgo results to date, and discuss the challenges of extracting interesting new physics from noisy detector data. Finally, I'll summarize future prospects for astrophysics, cosmology, and tests of general relativity with gravitational waves, and the roadmap to future gravitational wave detectors on Earth and in space.

The event horizon and the Cauchy horizon of an extremal black hole admit conserved charges associated with scalar perturbations. We will see that these charges are externally measurable from null infinity. This suggests that these charges have the potential to serve as an observational signature for extremal black holes. The proof of this result is based on obtaining precise late-time asymptotics for the radiation field of outgoing perturbations.

I shall  analyze three specific general-relativistic  problems in which gravitomagnetism plays important role: the dragging of  magnetic fields  around  rotating black holes, dragging inside a collapsing slowly rotating spherical shell of dust, compared with the dragging by rotating gravitational waves (CQG 34, 205006 (2017), Phys. Rev. D 85 124003, (2012) etc). I shall also briefly show how "instantaneous Machian gauges“ can be useful in the cosmological perturbation theory (Phys. Rev. D 76, 063501 (2007)).