17,300 research outputs found

### Lorentz violation and Hawking radiation

Since the event horizon of a black hole is a surface of infinite redshift, it
might be thought that Hawking radiation would be highly sensitive to Lorentz
violation at high energies. In fact, the opposite is true for subluminal
dispersion. For superluminal dispersion, however, the outgoing black hole modes
emanate from the singularity in a state determined by unknown quantum gravity
processes.Comment: 5 pages, Talk presented at CPT01; the Second Meeting on CPT and
Lorentz Symmetry, Bloomington, Indiana, 15-18 Aug. 200

### A positive energy theorem for Einstein-aether and Ho\v{r}ava gravity

Energy positivity is established for a class of solutions to Einstein-aether
theory and the IR limit of Ho\v{r}ava gravity within a certain range of
coupling parameters. The class consists of solutions where the aether 4-vector
is divergence free on a spacelike surface to which it is orthogonal (which
implies that the surface is maximal). In particular, this result holds for
spherically symmetric solutions at a moment of time symmetry.Comment: 4 page

### Einstein-Aether Waves

Local Lorentz invariance violation can be realized by introducing extra
tensor fields in the action that couple to matter. If the Lorentz violation is
rotationally invariant in some frame, then it is characterized by an
``aether'', i.e. a unit timelike vector field. General covariance requires that
the aether field be dynamical. In this paper we study the linearized theory of
such an aether coupled to gravity and find the speeds and polarizations of all
the wave modes in terms of the four constants appearing in the most general
action at second order in derivatives. We find that in addition to the usual
two transverse traceless metric modes, there are three coupled aether-metric
modes.Comment: 5 pages; v2: Remarks added concerning gauge invariance of the waves
and hyperbolicity of the equations. Essentially the version published in PR

### Black holes and neutron stars in the generalized tensor-vector-scalar theory

Bekenstein's Tensor-Vector-Scalar (TeVeS) theory has had considerable success
as a relativistic theory of Modified Newtonian Dynamics (MoND). However, recent
work suggests that the dynamics of the theory are fundamentally flawed and
numerous authors have subsequently begun to consider a generalization of TeVeS
where the vector field is given by an Einstein-Aether action. Herein, I develop
strong-field solutions of the generalized TeVeS theory, in particular exploring
neutron stars as well as neutral and charged black holes. I find that the
solutions are identical to the neutron star and black hole solutions of the
original TeVeS theory, given a mapping between the parameters of the two
theories, and hence provide constraints on these values of the coupling
constants. I discuss the consequences of these results in detail including the
stability of such spacetimes as well as generalizations to more complicated
geometries.Comment: Accepted for publication in Physical Review

### Destroying black holes with test bodies

If a black hole can accrete a body whose spin or charge would send the black
hole parameters over the extremal limit, then a naked singularity would
presumably form, in violation of the cosmic censorship conjecture. We review
some previous results on testing cosmic censorship in this way using the test
body approximation, focusing mostly on the case of neutral black holes. Under
certain conditions a black hole can indeed be over-spun or over-charged in this
approximation, hence radiative and self-force effects must be taken into
account to further test cosmic censorship.Comment: Contribution to the proceedings of the First Mediterranean Conference
on Classical and Quantum Gravity (talk given by T. P. S.). Summarizes the
results of Phys. Rev. Lett. 103, 141101 (2009), arXiv:0907.4146 [gr-qc] and
considers further example

### Construction of N = 2 Chiral Supergravity Compatible with the Reality Condition

We construct N = 2 chiral supergravity (SUGRA) which leads to Ashtekar's
canonical formulation. The supersymmetry (SUSY) transformation parameters are
not constrained at all and auxiliary fields are not required in contrast with
the method of the two-form gravity. We also show that our formulation is
compatible with the reality condition, and that its real section is reduced to
the usual N = 2 SUGRA up to an imaginary boundary term.Comment: 16 pages, late

### Degenerate Metric Phase Boundaries

The structure of boundaries between degenerate and nondegenerate solutions of
Ashtekar's canonical reformulation of Einstein's equations is studied. Several
examples are given of such "phase boundaries" in which the metric is degenerate
on one side of a null hypersurface and non-degenerate on the other side. These
include portions of flat space, Schwarzschild, and plane wave solutions joined
to degenerate regions. In the last case, the wave collides with a planar phase
boundary and continues on with the same curvature but degenerate triad, while
the phase boundary continues in the opposite direction. We conjecture that
degenerate phase boundaries are always null.Comment: 16 pages, 2 figures; erratum included in separate file: errors in
section 4, degenerate phase boundary is null without imposing field equation

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