Strong Gravity

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)).

Following the advent of LIGO measurements, it has been recently observed that QFT amplitudes can be used to derive observables appearing in the scattering of two black holes, to very high orders in perturbation theory. Such framework easily fits into the Post-Newtonian and Post-Minkowskian expansions appearing in the treatment of the binary inspiral. In this talk we will review recent progress in this direction for the case of spinning black holes, focusing on radiation and the multipole expansion. From the QFT point of view these are in close relation to long-studied Soft Theorems.

Ground-based gravitational wave observatories have begun to uncover a large number of compact binary coalescences in the universe through gravitational wave signals. I will discuss novel and effective techniques we have developed recently to analyze the publicly available LIGO/Virgo bulk strain data from scratch. Built on simple ideas and easy to implement, those address the questions of template bank construction, signal processing, trigger ranking, and fast parameter estimation. Applying those techniques, we searched for compact binary mergers during the LIGO/Virgo O1 and O2 runs, and detected a few binary black hole mergers in addition to what have been reported in the literature.

The conservation law for the total (orbital plus spin) angular momentum of a Dirac particle in the presence of gravity requires that spacetime is not only curved, but also has a nonzero torsion. The coupling between the spin and torsion in the Einstein–Cartan theory of gravity generates gravitational repulsion at extremely high densities, which prevents a singularity in a black hole and may create there a new, closed, baby universe undergoing one or more nonsingular bounces. We show that quantum particle production caused by an extremely high curvature near a bounce creates enormous amounts of matter and can generate a finite period of inflation. Our scenario has only one parameter, does not depend significantly on the initial conditions, does not involve hypothetical scalar fields, avoids eternal inflation, and predicts plateau-like inflation that is supported by the Planck observations of the cosmic microwave background. This scenario suggests that our Universe may have originated from a nonsingular bounce in a black hole existing in another universe.

At the event horizon of a black hole, gravity reaches its most extreme behaviour. Studying the dynamics of event horizons is key to understand gravity in is ultra-strong field regime and investigate the most fundamental properties of black holes. Black hole collisions provide a unique scenario to observe event horizons in a highly distorted and violently changing regime, which leads to a vast collection of phenomena that has not yet been detected by Advanced LIGO and Virgo. In this talk I will discuss the imprint that these phenomena leave in the gravitational-wave emission of black hole collisions and what it can teach us about the properties of black hole horizons.

By applying a parabolic-hyperbolic formulation of the constraints and superposing Kerr-Schild black holes, a simple method is introduced to initialize time evolution of binary systems. As the input parameters are essentially the same as those used in the post-Newtonian (PN) setup the proposed method interrelates various physical expressions applied in PN and in fully relativistic formulations. The global ADM charges are also determined by the input parameters, and no use of boundary conditions in the strong field regime is made.

The source of about half of the heaviest elements in the Universe has been a mystery for a long time. Although the general picture of element formation is well understood, many questions about the nuclear physics processes and particularly the astrophysical details remain to be answered. Here I focus on recent advances in our understanding of the origin of the heaviest and rarest elements in the Universe.