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
not only light 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. 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.
Feel free to use them (with attribution) in your next presentation.
For more
details, check out the arxiv.
Explosion of Strings from Black Holes
What happens to boson clouds around black holes
as they grow larger and larger through superradiance?
Studying a dark version of the photon, I found that something dramatic can occur:
an explosive event where strings are
created, disrupting the growth of the cloud. These strings are smaller versions of the cosmic strings that have
been hypothesized to form in the early universe.
The gravitational waves from this stringy bosenova
may give rise to an observable signal.
Click on images to watch animations.
The energy in a boson cloud around a black hole is disrupted by string formation.
Strings form around a black hole, with the phase of complex scalar
shown in the equatorial plane.
What happens to a neutron star that develops a small black hole inside it? That's the
question we set out to answer in this project. If some of the dark matter in our Universe
is made of primordial black holes or certain types of "asymmetric" dark particles, these
could be captured by a neutron star's gravity and eventually settle to the star's center,
giving rise to such a scenario. What we found is that such endoparasitic black holes can efficiently
consume their host star from the inside, even when the star is rapidly rotating, leaving essentially
no matter behind.
Looking for signatures of this phenomenon in electromagnetic or gravitational wave observations could
help shed light on the nature of dark matter.
Click on images to watch animations.
A neutron star being consumed from the inside by a small black hole.
A very rapidly rotating neutron star being consumed from the inside by a small black hole.
Einstein's theory of gravity predicts that as an object collapses, it will
eventually warp spacetime in a way that is so severe that the theory itself
breaks down. Remarkably, however, it seems that the places where this happens
are "cosmically censored," which means that Einstein's description remains
valid for any observer lucky enough to avoid falling into a black hole. A
putative counterexample to this principle of cosmic censorship was raised
almost 20 years ago by considering football-shaped distributions of particles
that collapse in on themselves at near the speed of light. Despite the wide
attention that result has received (including a write-up in the
New York Times)
the question of what exactly happens in
the collapse of such objects has remain unanswered. In this work, I resolved
this quandary, using new calculations to
show that such objects do in fact form
black holes, "censoring" the final stages of collapse, while producing powerful
gravitational waves. Thus, Einstein's General Relativity is rescued from
predicting its own demise.
Click on images to watch animations.
A sequence of successively larger distributions of particles collapsing
and forming black holes and gravitational radiation.
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 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.
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.
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).
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.
Two neutron stars merge and create a hypermassive star that is susceptible to the one-arm instability.
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 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.
Out-of-page component of a 2D force-free magnetic field undergoing
an inverse cascade. Credit: J. Zrake.
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.
A star like our sun falling into black hole a million times as massive. Visualization by Ralf Kähler
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.
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.
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.
An ultrarelativistic collision with lorentz factor γ=8.An ultrarelativistic collision with lorentz factor γ=10 that results in black hole formation.