746 research outputs found
Black Holes in Einstein-Aether Theory
We study black hole solutions in general relativity coupled to a unit
timelike vector field dubbed the "aether". To be causally isolated a black hole
interior must trap matter fields as well as all aether and metric modes. The
theory possesses spin-0, spin-1, and spin-2 modes whose speeds depend on four
coupling coefficients. We find that the full three-parameter family of local
spherically symmetric static solutions is always regular at a metric horizon,
but only a two-parameter subset is regular at a spin-0 horizon. Asymptotic
flatness imposes another condition, leaving a one-parameter family of regular
black holes. These solutions are compared to the Schwarzschild solution using
numerical integration for a special class of coupling coefficients. They are
very close to Schwarzschild outside the horizon for a wide range of couplings,
and have a spacelike singularity inside, but differ inside quantitatively. Some
quantities constructed from the metric and aether oscillate in the interior as
the singularity is approached. The aether is at rest at spatial infinity and
flows into the black hole, but differs significantly from the the 4-velocity of
freely-falling geodesics.Comment: 22 pages, 6 figures; v2: minor editing; v3: corrected overall sign in
twist formula and an error in the equation for the aether stress tensor.
Results unchanged since correct form was used in calculations; v4: corrected
minor typ
Numerical simulations of gravitational collapse in Einstein-aether theory
We study gravitational collapse of a spherically symmetric scalar field in
Einstein-aether theory (general relativity coupled to a dynamical unit timelike
vector field). The initial value formulation is developed, and numerical
simulations are performed. The collapse produces regular, stationary black
holes, as long as the aether coupling constants are not too large. For larger
couplings a finite area singularity occurs. These results are shown to be
consistent with the stationary solutions found previously.Comment: 9 pages, 7 figures; v2: corrected typos, added minor clarifying
remarks, improved discussion of results in conclusio
Two-dimensional gravity with a dynamical aether
We investigate the two-dimensional behavior of gravity coupled to a dynamical
unit timelike vector field, i.e. "Einstein-aether theory". The classical
solutions of this theory in two dimensions depend on one coupling constant.
When this coupling is positive the only solutions are (i) flat spacetime with
constant aether, (ii) de Sitter or anti-de Sitter spacetimes with a uniformly
accelerated unit vector invariant under a two-dimensional subgroup of SO(2,1)
generated by a boost and a null rotation, and (iii) a non-constant curvature
spacetime that has no Killing symmetries and contains singularities. In this
case the sign of the curvature is determined by whether the coupling is less or
greater than one. When instead the coupling is negative only solutions (i) and
(iii) are present. This classical study of the behavior of Einstein-aether
theory in 1+1 dimensions may provide a starting point for further
investigations into semiclassical and fully quantum toy models of quantum
gravity with a dynamical preferred frame.Comment: 11 pages, 4 figure
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
Higher Curvature Gravity and the Holographic fluid dual to flat spacetime
Recent works have demonstrated that one can construct a (d+2) dimensional
solution of the vacuum Einstein equations that is dual to a (d+1) dimensional
fluid satisfying the incompressible Navier-Stokes equations. In one important
example, the fluid lives on a fixed timelike surface in the flat Rindler
spacetime associated with an accelerated observer. In this paper, we show that
the shear viscosity to entropy density ratio of the fluid takes the universal
value 1/4\pi in a wide class of higher curvature generalizations to Einstein
gravity. Unlike the fluid dual to asymptotically anti-de Sitter spacetimes,
here the choice of gravitational dynamics only affects the second order
transport coefficients. We explicitly calculate these in five-dimensional
Einstein-Gauss-Bonnet gravity and discuss the implications of our results.Comment: 13 pages; v2: modified abstract, added references; v3: added
clarifying comments, modified discussio
Non-equilibrium Thermodynamics of Spacetime
It has previously been shown that the Einstein equation can be derived from
the requirement that the Clausius relation dS = dQ/T hold for all local
acceleration horizons through each spacetime point, where dS is one quarter the
horizon area change in Planck units, and dQ and T are the energy flux across
the horizon and Unruh temperature seen by an accelerating observer just inside
the horizon. Here we show that a curvature correction to the entropy that is
polynomial in the Ricci scalar requires a non-equilibrium treatment. The
corresponding field equation is derived from the entropy balance relation dS
=dQ/T+dS_i, where dS_i is a bulk viscosity entropy production term that we
determine by imposing energy-momentum conservation. Entropy production can also
be included in pure Einstein theory by allowing for shear viscosity of the
horizon.Comment: 4 pages. Dedicated to Rafael Sorkin on the occasion of his 60th
birthda
Viability of vector-tensor theories of gravity
We present a detailed study of the viability of general vector-tensor
theories of gravity in the presence of an arbitrary temporal background vector
field. We find that there are six different classes of theories which are
indistinguishable from General Relativity by means of local gravity
experiments. We study the propagation speeds of scalar, vector and tensor
perturbations and obtain the conditions for classical stability of those
models. We compute the energy density of the different modes and find the
conditions for the absence of ghosts in the quantum theory. We conclude that
the only theories which can pass all the viability conditions for arbitrary
values of the background vector field are not only those of the pure Maxwell
type, but also Maxwell theories supplemented with a (Lorentz type) gauge fixing
term.Comment: 13 pages, 2 figures, 1 table. Final version to appear in JCA
Vector field theories in cosmology
Recently proposed theories based on the cosmic presence of a vectorial field
are compared and contrasted. In particular the so called Einstein aether theory
is discussed in parallel with a recent proposal of a strained space-time theory
(Cosmic Defect theory). We show that the latter fits reasonably well the cosmic
observed data with only one, or at most two, adjustable parameters, whilst
other vector theories use much more. The Newtonian limits are also compared.
Finally we show that the CD theory may be considered as a special case of the
aether theories, corresponding to a more compact and consistent paradigm.Comment: 19 pages, 1 figure, to appear on Phys. Rev.
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