Gravitational SelfForce and Extreme MassRatio Inspiral

An eccentric orbit of the Kerr spacetime.
Image credit: Steve
Drasco. 
The motivation for some
of my work comes from the
socalled
"extreme massratio inspiral" (EMRI). When a neutron star or
stellar
black hole is scattered into a close orbit about a supermassive black
hole, the orbit begins to slowly decay due to the emission of
gravitational radiation. These orbits can be quite intricate
(see figure to the right, or view movies
here),
and
the resultant gravitational radiation carries detailed
information
about the system, offering a unique glimpse into the astrophysics of
galactic centers as well as a precision test of strong field general
relativity. However, detecting such a signal in a noisy
interferometer like LISA will require waveform templates that track the
orbit with exquisite precision. Producing such templates will
require going beyond the testbody approximation for the compact object
to include the influence of its own gravitational field on its motion,
i.e., one must include
gravitational
selfforce effects. I have worked on
the
foundations
of
this problem, on the
role
of the central body, on
alternative
gauge
(coordinate) choices, and on
including
nexttoleading order effects. This research appeals to me because of the way it combines fundamental and astrophysical questions.
Exact ForceFree Magnetospheres
The undulating current sheet that appears in an exact solution to forcefree electrodynamics 

Many of the most dramatic of observed astrophysical phenomena
involve the combination of compact objects, magnetic fields, and
relativistic plasma. Examples include pulsars (rotating,
magnetized neutron stars), active galactic nuclei (likely powered by
supermassive black holes), and possibly even gammaray bursts (if
powered by binary neutron stars). When the magnetic fields are
very strong, one may use forcefree electrodynamics, a fascinating set
of equations that describes the plasma only through its effect on the
electromagnetic fields. Eschewing the traditional 3+1
decomposition and focusing on the (rich and beautiful) spacetime
structure of these equations, we have
found a large class of exact solutions
in black hole spacetimes. These include nonstationary,
nonaxisymmetric solutions in Kerr. The solutions reveal a new
type of nonlinear forcefree wave that propagates through black hole
spacetimes without scattering. Remarkably, the solution class
appears sufficiently broad to to describe the farfield magnetosphere
of a
generic (aligned,
inclined, glitching, younameit) magnetized neutron star, opening the
door to detailed analytical modeling of the outer pulsar magnetosphere.
Cooling a Black Hole with a Moon

Spacetime diagram for a Kerr black hole with a
corotating moon. 
The physical (Hawking) temperature of a stationary, isolated
black hole is given by its classical surface gravity. For
general, nonstationary or interacting black holes, the surface
gravity is not defined and there is no natural candidate for the
temperature.
We
studied a particular dynamical black hole spacetime for
which the surface gravity can in fact still be defined in
the
usual way: a Kerr black hole that corotates with an orbiting moon.
Here, the system is "stationary in a rotating frame", giving
sufficient symmetry. (The event horizon is a Killing horizon
of
the helical Killing field.) From the basic
elements of the Hawking calculation, it is clear that this surface
gravity must represent the semiclassical temperature of the tidally
perturbed black hole; we thus have a first example of an
interacting black hole whose Hawking temperature is welldefined.
We explicitly compute the change in surface
gravity/temperature caused by the orbiting particle, finding that it is
negative: the moon has a cooling effect on the black
hole. This work was
featured
in New Scientist.
Motion in Modified Gravity
In recent years many modified gravity theories have been
proposed to explain the acceleration of the universe without the need
for dark energy. To be viable, such theories must also pass
solarsystem tests, binary pulsar tests, and (eventually)
gravitationalwave tests, all of which involve motion of bodies.
I investigated the motion of
bodies in the very general context of an arbitrary theory following
from a covariant lagrangian in four spacetime dimensions and having
secondorder field equations. Remarkably, there is a universal
force law for the motion of bodies in such theories,
involving certain
effective charges for a body that entirely encode the predictions of
any specific
theory for the body. These charges may be computed using surfaceintegral formulae, giving rise to specific predictions.
Black Hole Bobbing and Kicks
A diagram illustrating a kinematical effect that
makes spinning bodies bob. 

Numerical simulations of binary black holes with spin
revealed some
surprising behavior: a
bobbing up and down motion prior to merger, leading to a large kick
velocity that has the appearance of an inertial continuation of the
bobbing motion.
We looked at whether similar effects could be found
in analogousbut simplersystems. We
found that the bobbing
effect is in fact ubiquitous, occuring whenever two spinning
bodies are held in orbit by any sort of force. For example,
two spinning balls connected by a string will display this
behavior! The kick, however, is more special and can only
occur for systems that possess field momentum which can be
radiated to infinity. After studying an electromagnetic
analog as
well as the gravitational case directly, we concluded that bobbing and
kicks are
basically unrelated phenomena, which can nevertheless appear
correlated for spinning black holes because the spin parameter
happens to control both the bobbing and the kick. We bolstered this conclusion by giving an
electromagnetic example in which large kicks can be obtained with no bobbing at all.