William East

Gravitational Physics


I investigate systems of compact objects, such as black hole-neutron star and neutron star-neutron star binaries, that are governed by Einstein gravity. The mergers of these systems are not only suspected of lighting up the sky with short gamma-ray bursts and other electromagnetic transients, but are also violent enough to cause ripples in spacetime — gravitational waves. There is currently an enormous effort underway to detect gravitational wave sources, a brand new type of astronomy inaugurated by the recent LIGO observations of binary black hole mergers. I also study the plasma dynamics underlying high-energy astrophysical phenomena, and explore fundamental gravity questions like black hole superradiance and how this can turn black holes into particle detectors through the superradiant instability, the outcome of ultrarelativistic collisions, and the Higgs instability. Much of my work is computationally intensive, and I'm interested in numerical methods and algorithms that allow us to expand the physical regimes we can simulate. On this page are some visualizations from a few of my research projects. For more details, check out the arxiv.

Superradiant Instability of Massive Bosons and Using Black Holes as Particle Detectors

Spinning black holes have a large reservoir of rotational energy that can be tapped into through superradiance. This property of black holes can actually be used to turn them into astrophysical particle detectors of sorts. Unlike the gravitational wave case below, where the wave interacted once and then went away, massive bosons can form bound clouds around black holes that continuously interact and grow unstably through superradiance. This would occur efficiently if there were an ultralight bosonic particle whose Compton wavelength was comparable to the black hole's radius. Though we haven't yet found any such bosons, particle physicists have theorized about a number of such dark particles which interact very weakly with ordinary matter. This includes so-called axions which arise in extensions of QCD or massive versions of photons. And observing black holes is a way to show evidence for, or to rule out their existence. In this work I studied the superradiant instability of massive vector bosons, finding that they can rapidly form clouds around black holes, efficiently spinning down the black holes in the process. The oscillating clouds produce gravitational radiation, meaning that gravitational wave detectors like LIGO could shed light on their existence.

Click on images to watch animations.
A massive vector boson cloud growing around a black hole through superradiance.
A massive vector boson cloud grows by tapping into the rotational energy of a black hole.
A massive vector boson cloud oscillates around a black hole and produces gravitational waves.
A massive vector boson cloud oscillates around a black hole and produces gravitational waves.

These animations are from the work described here, which builds on previous work detailed here and here. There are also a few science news articles about this research on Physics, PhysicsCentral, and PhysicsWorld.

The Higgs Instability and the Fate of the Early Universe

If the Standard Model of particle physics is extrapolated to very high energy scales (beyond where it's been experimentally tested) it tells us that that sufficiently large fluctuations of the Higgs field should be unstable. Though the electroweak vacuum (which we currently find our Universe to be in) is metastable on timescales that are long compared to the age of the Universe, during a period of inflation the Higgs field would have experienced large fluctuations which could have driven it towards its true vacuum at negative energy in some regions. In this project, we studied the dynamics and growth of such unstable Higgs fluctuations in an expanding spacetime, illustrating how they can halt inflation in the regions they develop, and quickly spread to engulf surrounding regions. By combining this with a detailed treatment of how such unstable Higgs fluctuations develop stochastically though quantum effects, we're able to use the existence of our current universe to place bounds on the energy scale of inflation relative to that of the Higgs instability.

Click on images to watch animations.
The Higgs field going unstable.
An unstable Higgs field fluctuation that becomes larger and larger until it locally ends inflation, creating a growing crunching region surrounded by a black hole horizon (white region).

Read more about this project at arxiv.org.

One-arm Instability in Hypermassive Neutron Stars

When two neutron stars merge together, they do not always immediately form a black hole. Instead they can form a hypermassive star that is temporarily supported against collapse by angular momentum and thermal energy. In this work, we found that in some cases such hypermassive neutron stars are a susceptible to an instability that causes the star to develop a lopsided "one-arm" shape that rotates. This persistent feature leaves a strong imprint on the gravitational wave signal coming from the hypermassive neutron star.

Click on images to watch animations.
A hypermassive neutron 
                                                       star developing a one-arm mode.
Two neutron stars merge and create a hypermassive star that is susceptible to the one-arm instability.

Read more about this project here and here.

Instability and Turbulence in Astrophysical Plasma

Observations of powerful gamma-ray flares from pulsar winds, black hole jets, and other extreme cosmic sources have revealed that these systems release energy on incredibly short timescales in ways that remain poorly understood. Motivated by that, in this work we revisited a ubiquitous class of plasma configurations called Taylor states, originally invoked to explain observations of plasma in fusion devices. We uncovered a previously missed instability causes these states to spontaneously decay, liberating magnetic energy at near the speed of light, and creating an ideal setting for accelerating particles.

Click on images to watch animations.
Magnetic field streamlines.
Magnetic field streamlines in a force-free electrodynamic simulation of an unstable Taylor state.

The transfer of magnetic energy to larger and larger length scales seen above is a process known as an inverse cascade, and is typical of turbulent magnetized plasma. In follow-up work, we considered similar configurations, but with all the energy initially at very small scales, and studied the properties of the ensuing turbulence.

Click on images to watch animations.
Force-free turbulence.
Out-of-page component of a 2D force-free magnetic field undergoing an inverse cascade. Credit: J. Zrake.

Read more about these projects here and here.

Black Holes Merging with Spinning Neutron Stars

There is a nice summary of some work I did studying how the outcome of a black hole merging with a neutron star changes when the neutron star is rapidly rotating on the AAS Nova site.

Stars Colliding with Black Holes

Supermassive black holes, millions or more times as massive as our sun, are thought to lurk at the centers of most galaxies. Stars that have the misfortune to pass too close to them can be torn apart by the black hole's tidal forces. In this project, I studied how the collision of such stars can cause the black hole to vibrate, much like a bell, producing gravitational waves. There's a short (non-technical) discussion of this work on the KIPAC blog.

Click on images to watch animations.
Visualization of star being tidally disrupted as it falls into a black hole.
A star like our sun falling into black hole a million times as massive. Visualization by Ralf Kähler

Read more about this project at arxiv.org.

Black Hole Superradiance

Once an object passes through the horizon of black hole, it can never reemerge. However, somewhat surprisingly, it is possible to extract energy from a rotating black hole. In fact, such processes are thought to power some of the most energetic astrophysical events. One way that a black hole's rotational energy can be extracted is through a phenomenon called superradiance where an incoming wave is scattered and amplified by the black hole. In this project, we used simulations to study the superradiant scattering of gravitational waves in the nonlinear regime, which has implications for black hole thermodynamics and cosmic censorship.

Click on images to watch animations.
Visualization of black hole superradiance
An incoming gravitational wave (blue) being superradiantly scattered resulting in an outgoing wave (red) that carries away some of the black hole's rotational energy. Visualization by the very talented Ralf Kähler.

Read more about this project at arxiv.org.

Ultrarelativistic Collisions

According to general relativity, kinetic energy — like other forms of energy — gravitates. This suggests that if particles traveling sufficiently close to the speed of light were to collide, packing enough kinetic energy in a small enough radius, they could form a black hole. Besides being an interesting theoretical topic, there has been speculation that this could happen at the Large Hadron Collider or in the collision of cosmic rays with the Earth's atmosphere if there are small or warped extra dimensions. In this project we used the tools of general-relativistic hydrodynamics to study collisions in the regime where approximately 90% of the energy of the spacetime was kinetic. We found that not only do black holes form at energies a factor of a few smaller than simple (hoop-conjecture) estimates predict, but just above the threshold for black hole formation, two separate apparent horizons initially appear. One way to understand this is in terms of a focusing effect, where a particle moving near the speed of light acts like a gravitational lens.

Click on images to watch animations.
Animated Model of a Sub-critical Ultrarelativistic Collision
An ultrarelativistic collision with lorentz factor γ=8.
Animated Model of Black Hole Formation in an Ultrarelativistic Collision
An ultrarelativistic collision with lorentz factor γ=10 that results in black hole formation.

Read more about this project at arxiv.org.

There are also a few (non-technical) science news articles about this work on Physics, ScienceNOW, LiveScience, and PhysicsWorld.

Copyright W. East, 2013-2018