William East

Gravitational Physics


I'm interested in investigating 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 directly detect gravitational waves, which could open up a brand new type of astronomy. In addition to astrophysics, I've spent some time investigating fundamental gravity questions like the outcome of ultrarelativistic collisions. Much of my work is computationally intensive, and I'm also interested in numerical methods and algorithms that allow us to expand the physical regimes we can simulate. On this page is a little bit about some of the projects I have been working on lately, along with animations. For more details, check out the arxiv.

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 collsion 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.

Dynamical Capture Binary Neutron Star Mergers

In dense stellar regions like the cores of globular clusters, neutron star-neutron star binaries can form dynamically when the stars undergo a close encounter, losing energy and angular momentum to gravitational waves. Some of these binaries will merge with large orbital eccentricities. In this project we found that, depending on the impact parameter, these binaries could either promptly collapse to a black hole upon merger or form a hypermassive neutron star that is temporarily supported against collapse by thermal energy and differential rotation. In some cases, these binaries will have large accretion disks and significant amounts of ejected material which could potentially source electromagnetic transients.

Click on images to watch animations.
Animated Model of Two Neutron Stars Merging into a Black Hole
Prompt collapse to a black hole.
Animated Model of Two Neutron Stars Merging into a Hypermassive Neutron Star
Formation of a hypermassive neutron star
Animated Model of Two Neutron Stars Having a Non-merger Close Encounter
Non-merger close encounter

Read more about this project at arxiv.org.

Copyright W. East, 2013