19 research outputs found
Gravitational Lensing as a Mechanism For Effective Cloaking
In light of the surge in popularity of electromagnetic cloaking devices, we
consider whether it is possible to use gravitational lensing to cloak a volume
of spacetime. A metric for such a spacetime geometry is presented, and its
geometric and physical implications are explained.Comment: 5 pages, 6 figure
Gravitational collapse of quantum matter
We describe a class of exactly soluble models for gravitational collapse in
spherical symmetry obtained by patching dynamical spherically symmetric
exterior spacetimes with cosmological interior spacetimes. These are
generalizations of the Oppenheimer-Snyder type models to include classical and
quantum scalar fields as sources for the interior metric, and null fluids with
pressure as sources for the exterior metric. In addition to dynamical
exteriors, the models exhibit other novel features such as evaporating
horizons, and singularity avoidance without quantum gravity.Comment: 8 pages, 4 figures, revtex 2co
Traversable acausal retrograde domains in spacetime
In this paper we present geometry which has been designed to fit a layperson's description of a 'time machine'. It is a box which allows those within it to travel backwards and forwards through time and space, as interpreted by an external observer. Timelike observers travel within the interior of a 'bubble' of geometry which moves along a circular, acausal trajectory through spacetime. If certain timelike observers inside the bubble maintain a persistent acceleration, their worldlines will close.Our analysis includes a description of the causal structure of our spacetime, as well as a discussion of its physicality. The inclusion of such a bubble in a spacetime will render the background spacetime non-orientable, generating additional consistency constraints for formulations of the initial value problem. The spacetime geometry is geodesically incomplete, contains naked singularities, and requires exotic matter
Tomimatsu-Sato geometries, holography and quantum gravity
We analyze the Tomimatsu-Sato spacetime in the context of the
proposed Kerr/CFT correspondence. This 4-dimensional vacuum spacetime is
asymptotically flat and has a well-defined ADM mass and angular momentum, but
also involves several exotic features including a naked ring singularity, and
two disjoint Killing horizons separated by a region with closed timelike curves
and a rod-like conical singularity. We demonstrate that the near horizon
geometry belongs to a general class of Ricci-flat metrics with
symmetry that includes both the extremal Kerr and
extremal Kerr-bolt geometries. We calculate the central charge and temperature
for the CFT dual to this spacetime and confirm the Cardy formula reproduces the
Bekenstein-Hawking entropy. We find that all of the basic parameters of the
dual CFT are most naturally expressed in terms of charges defined intrinsically
on the horizon, which are distinct from the ADM charges in this geometry.Comment: 20+1 pages, 3 figures, changed title, expanded discussion, matches
version published in CQ