81 research outputs found
Tensor ghosts in the inflationary cosmology
Theories with curvature squared terms in the action are known to contain
ghost modes in general. However, if we regard curvature squared terms as
quantum corrections to the original theory, the emergence of ghosts may be
simply due to the perturbative truncation of a full non-perturbative theory. If
this is the case, there should be a way to live with ghosts. In this paper, we
take the Euclidean path integral approach, in which ghost degrees of freedom
can be, and are integrated out in the Euclideanized spacetime. We apply this
procedure to Einstein gravity with a Weyl curvature squared correction in the
inflationary background. We find that the amplitude of tensor perturbations is
modified by a term of O(alpha^2 H^2) where alpha^2 is a coupling constant in
front of the Weyl squared term and H is the Hubble parameter during inflation.Comment: 16 pages, no figure
Hoffmann-Infeld Black Hole Solutions in Lovelock Gravity
Five-dimensional black holes are studied in Lovelock gravity coupled to
Hoffmann-Infeld non-linear electrodynamics. It is shown that some of these
solutions present a double peak behavior of the temperature as a function of
the horizon radius. This feature implies that the evaporation process, though
drastic for a period, leads to an eternal black hole remnant. Moreover, the
form of the caloric curve corresponds to the existence of a plateau in the
evaporation rate, which implies that black holes of intermediate scales turn
out to be unstable. The geometrical aspects, such as the absence of conical
singularity, the structure of horizons, etc. are also discussed. In particular,
solutions that are asymptotically AdS arise for special choices of the
parameters, corresponding to charged solutions of five-dimensional Chern-Simons
gravity.Comment: 6 pages, 5 figures, Revtex4. References added and comments clarified;
version accepted for publicatio
Thermodynamics of third order Lovelock anti-de Sitter black holes revisited
We compute the mass and the temperature of third order Lovelock black holes
with negative Gauss-Bonnet coefficient in anti-de Sitter space and
perform the stability analysis of topological black holes. When , the
third order Lovelock black holes are thermodynamically stable for the whole
range . When , we found that the black hole has an intermediate
unstable phase for . In eight dimensional spacetimes, however, a new phase
of thermodynamically unstable small black holes appears if the coefficient
is under a critical value. For , black holes have
similar the distributions of thermodynamically stable regions to the case where
the coefficient is under a critical value for . It is
worth to mention that all the thermodynamic and conserved quantities of the
black holes with flat horizon don't depend on the Lovelock coefficients and are
the same as those of black holes in general gravity.Comment: 15 pages, 22 figure
Collider Production of TeV Scale Black Holes and Higher-Curvature Gravity
We examine how the production of TeV scale black holes at colliders is
influenced by the presence of Lovelock higher-curvature terms in the action of
models with large extra dimensions. Such terms are expected to arise on rather
general grounds, e.g., from string theory and are often used in the literature
to model modifications to the Einstein-Hilbert action arising from quantum
and/or stringy corrections. While adding the invariant which is quadratic in
the curvature leads to quantitative modifications in black hole properties,
cubic and higher invariants are found to produce significant qualitative
changes, e.g., classically stable black holes. We use these higher-order
curvature terms to construct a toy model of the black hole production cross
section threshold. For reasonable parameter values we demonstrate that detailed
measurements of the properties of black holes at future colliders will be
highly sensitive to the presence of the Lovelock higher-order curvature terms.Comment: 37 pages, 11 figures, references adde
Constraints on Gauss-Bonnet Gravity in Dark Energy Cosmologies
Models with a scalar field coupled to the Gauss-Bonnet Lagrangian appear
naturally from Kaluza-Klein compactifications of pure higher-dimensional
gravity. We study linear, cosmological perturbations in the limits of weak
coupling and slow-roll, and derive simple expressions for the main observable
sub-horizon quantities: the anisotropic stress factor, the time-dependent
gravitational constant, and the matter perturbation growth factor. Using
present observational data, and assuming slow-roll for the dark energy field,
we find that the fraction of energy density associated with the coupled
Gauss-Bonnet term cannot exceed 15%. The bound should be treated with caution,
as there are significant uncertainies in the data used to obtain it. Even so,
it indicates that the future prospects for constraining the coupled
Gauss-Bonnet term with cosmological observations are encouraging.Comment: 15 pages. v3: extended analysis, conclusions change
Simple compactifications and Black p-branes in Gauss-Bonnet and Lovelock Theories
We look for the existence of asymptotically flat simple compactifications of
the form in -dimensional gravity theories with higher
powers of the curvature. Assuming the manifold to be spherically
symmetric, it is shown that the Einstein-Gauss-Bonnet theory admits this class
of solutions only for the pure Einstein-Hilbert or Gauss-Bonnet Lagrangians,
but not for an arbitrary linear combination of them. Once these special cases
have been selected, the requirement of spherical symmetry is no longer relevant
since actually any solution of the pure Einstein or pure Gauss-Bonnet theories
can then be toroidally extended to higher dimensions. Depending on and the
spacetime dimension, the metric on may describe a black hole or a
spacetime with a conical singularity, so that the whole spacetime describes a
black or a cosmic -brane, respectively. For the purely Gauss-Bonnet theory
it is shown that, if is four-dimensional, a new exotic class of black
hole solutions exists, for which spherical symmetry can be relaxed.
Under the same assumptions, it is also shown that simple compactifications
acquire a similar structure for a wide class of theories among the Lovelock
family which accepts this toroidal extension.
The thermodynamics of black -branes is also discussed, and it is shown
that a thermodynamical analogue of the Gregory-Laflamme transition always
occurs regardless the spacetime dimension or the theory considered, hence not
only for General Relativity.
Relaxing the asymptotically flat behavior, it is also shown that exact black
brane solutions exist within a very special class of Lovelock theories.Comment: 30 pages, no figures, few typos fixed, references added, final
version for JHE
Inflation with a Weyl term, or ghosts at work
In order to assess the role of ghosts in cosmology, we study the evolution of
linear cosmological perturbations during inflation when a Weyl term is added to
the action. Our main result is that vector perturbations can no longer be
ignored and that scalar modes diverge in the newtonian gauge but remain bounded
in the comoving slicing.Comment: 14 pages, 4 figure
de Sitter thermodynamics and the braneworld
The de Sitter thermodynamics of cosmological models with a modified Friedmann
equation is considered, with particular reference to high-energy
Randall-Sundrum and Gauss-Bonnet braneworlds. The Friedmann equation can be
regarded as the first law of thermodynamics of an effective gravitational
theory in quasi de Sitter spacetime. The associated entropy provides some
selection rules for the range of the parameters of the models, and is proposed
for describing tunneling processes in the class of high-energy gravities under
consideration.Comment: 16 pages JHEP style, no figures. v2: references added; v3: typo
corrected in Eq.(3.1), supersedes published versio
Lorentz-violating vs ghost gravitons: the example of Weyl gravity
We show that the ghost degrees of freedom of Einstein gravity with a Weyl
term can be eliminated by a simple mechanism that invokes local Lorentz
symmetry breaking. We demonstrate how the mechanism works in a cosmological
setting. The presence of the Weyl term forces a redefinition of the quantum
vacuum state of the tensor perturbations. As a consequence the amplitude of
their spectrum blows up when the Lorentz-violating scale becomes comparable to
the Hubble radius. Such a behaviour is in sharp contrast to what happens in
standard Weyl gravity where the gravitational ghosts smoothly damp out the
spectrum of primordial gravitational waves.Comment: 14 pages, 3 figures, REVTeX 4.
Isolated horizons in higher-dimensional Einstein-Gauss-Bonnet gravity
The isolated horizon framework was introduced in order to provide a local
description of black holes that are in equilibrium with their (possibly
dynamic) environment. Over the past several years, the framework has been
extended to include matter fields (dilaton, Yang-Mills etc) in D=4 dimensions
and cosmological constant in dimensions. In this article we present a
further extension of the framework that includes black holes in
higher-dimensional Einstein-Gauss-Bonnet (EGB) gravity. In particular, we
construct a covariant phase space for EGB gravity in arbitrary dimensions which
allows us to derive the first law. We find that the entropy of a weakly
isolated and non-rotating horizon is given by
.
In this expression is the -dimensional cross section of the
horizon with area form and Ricci scalar ,
is the -dimensional Newton constant and is the Gauss-Bonnet
parameter. This expression for the horizon entropy is in agreement with those
predicted by the Euclidean and Noether charge methods. Thus we extend the
isolated horizon framework beyond Einstein gravity.Comment: 18 pages; 1 figure; v2: 19 pages; 2 references added; v3: 19 pages;
minor corrections; 1 reference added; to appear in Classical and Quantum
Gravit
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