14 research outputs found

### Examining gravitational collapse with test scalar fields

Numerical simulations are performed of a test scalar field in a spacetime
undergoing gravitational collapse. The behavior of the scalar field near the
singularity is examined and implications for generic singularities are
discussed. In particular, our example is the first confirmation of the BKL
conjecture for an asymptotically flat spacetime.Comment: 10 pages, 11 figures, references adde

### Gravitational collapse of k-essence

We perform numerical simulations of the gravitational collapse of a k-essence
scalar field. When the field is sufficiently strongly gravitating, a black hole
forms. However, the black hole has two horizons: a light horizon (the ordinary
black hole horizon) and a sound horizon that traps k-essence. In certain cases
the k-essence signals can travel faster than light and the sound horizon is
inside the light horizon. Under those circumstances, k-essence signals can
escape from the black hole. Eventually, the two horizons merge and the
k-essence signals can no longer escape.Comment: 14 pages, 8 figure

### Non-Stationary Dark Energy Around a Black Hole

Numerical simulations of the accretion of test scalar fields with
non-standard kinetic terms (of the k-essence type) onto a Schwarzschild black
hole are performed. We find a full dynamical solution for the spherical
accretion of a Dirac-Born-Infeld type scalar field. The simulations show that
the accretion eventually settles down to a well known stationary solution. This
particular analytical steady state solution maintains two separate horizons.
The standard horizon is for the usual particles propagating with the limiting
speed of light, while the other sonic horizon is for the k-essence
perturbations propagating with the speed of sound around this accreting
background. For the case where the k-essence perturbations propagate
superluminally, we show that one can send signals from within a black hole
during the approach to the stationary solution. We also find that a ghost
condensate model settles down to a stationary solution during the accretion
process.Comment: 8 pages, 10 figure

### Collinear and Soft Divergences in Perturbative Quantum Gravity

Collinear and soft divergences in perturbative quantum gravity are
investigated to arbitrary orders in amplitudes for wide-angle scattering, using
methods developed for gauge theories. We show that collinear singularities
cancel when all such divergent diagrams are summed over, by using the
gravitational Ward identity that decouples the unphysical polarizations from
the S-matrix. This analysis generalizes a result previously demonstrated in the
eikonal approximation. We also confirm that the only virtual graviton
corrections that give soft logarithmic divergences are of the ladder and
crossed ladder type.Comment: 10 pages, 12 figure

### Selected Studies in Classical and Quantum Gravity.

This thesis is composed of two parts, one corresponding to classical and the other to quantum gravitational phenomena. In the classical part, we focus on the behavior of various classical scalar fields in the presence of black holes. New fundamental results discussed include the first confirmation of the Belinskii, Khalatnikov, and Lifschitz (BKL) conjecture for an asymptotically flat spacetime, where we find that the dynamics of a canonical test scalar field near a black hole singularity are dominated by terms with time derivatives. We also perform a numerical simulation of the gravitational collapse of a non-canonical scalar field showing that signals can escape black holes in the k-essence dark energy model and find numerical confirmation that the accretion of various scalar fields onto a black hole from generic initial conditions is stationary.
In the second part, we focus on the long distance behavior of perturbative quantum gravity. New results discussed include a proof of the cancellation of collinear divergences to all orders in the amplitudes of the theory as well as a characterization of all infrared divergent diagrams. In particular, we find that the only diagrams that can have soft divergences are ladder and crossed ladder diagrams, and that the only collinearly divergent diagrams are those with only three point vertices and no internal jet loops.
Also presented is a construction of a double copy relation between gravity and gauge theory amplitudes similar to that conjectured by Bern, Carrasco, and Johansson for the case where there is no hard momentum exchange in the scattering, which we find implies a squaring relation between the classical shockwave solutions of the two theories as well. Finally, the first calculation of a gravitational scattering amplitude through the next-to-leading eikonal order is performed. We find that this correction to the scattering amplitude exponentiates, and that these power corrections probe smaller impact parameters compared to the leading eikonal case. This suggests that researching such corrections in a general setting may yield evidence of black hole formation from the quantum theory.PhDPhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/97818/1/rsaotome_1.pd