621 research outputs found

### Covariant gravity with Lagrange multiplier constraint

We review on the models of gravity with a constraint by the Lagrange
multiplier field. The constraint breaks general covariance or Lorentz symmetry
in the ultraviolet region. We report on the $F(R)$ gravity model with the
constraint and the proposal of the covariant (power-counting) renormalized
gravity model by using the constraint and scalar projectors. We will show that
the model admits flat space solution, its gauge-fixing formulation is fully
developed, and the only propagating mode is (higher derivative) graviton, while
scalar and vector modes do not propagate. The preliminary study of FRW
cosmology indicates to the possibility of inflationary universe solution is
also given.Comment: 10 pages, to appear in the Proceedings of the QFEXT11 Benasque
Conferenc

### 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

### Gauss-Bonnet gravity renders negative tension braneworlds unstable

We show that the Gauss-Bonnet correction to Einstein gravity induces a
gravitational tachyon mode, namely an unstable spin 2 fluctuation, in the
Randall-Sundrum I model. We demonstrate that this instability is generically
related to the presence of a negative tension brane in the set-up, with or
without $Z_2$-symmetry across it. Indeed it is shown that the tachyon mode is a
bound state localised on any negative tension brane of co-dimension one,
embedded in anti-de Sitter background. We discuss the possible resolution of
this instability by the inclusion of induced gravity terms on the branes or by
an effective four-dimensional cosmological constant.Comment: published versio

### Dilaton Brane Cosmology with Second Order String Corrections and the Cosmological Constant

We consider, in five dimensions, the effective action from heterotic string
which includes quantum gravity corrections up to (a')^2. The expansion, in the
string frame, is in terms of |a'R|, where R is the scalar curvature and uses
the third order Euler density, next to the Gauss-Bonnet term. For a positive
tension brane and infinite extra dimension, the logarithmic class of solutions
is less dependent from fine-tuning problems than in previous formulations. More
importantly, the model suggests that in the full non-perturbative formulation,
the string scale can be much lower than the effective Planck mass, without the
string coupling to be vanishingly small. Also a less severe fine-tuning of the
brane tension in needed.Comment: 19 pages, 5 figures LaTeX. Accepted for publication in IJMP

### Classical Duals, Legendre Transforms and the Vainshtein Mechanism

We show how to generalize the classical duals found by Gabadadze {\it et al}
to a very large class of self-interacting theories. This enables one to adopt a
perturbative description beyond the scale at which classical perturbation
theory breaks down in the original theory. This is particularly relevant if we
want to test modified gravity scenarios that exhibit Vainshtein screening on
solar system scales. We recognise the duals as being related to the Legendre
transform of the original Lagrangian, and present a practical method for
finding the dual in general; our methods can also be applied to
self-interacting theories with a hierarchy of strong coupling scales, and with
multiple fields. We find the classical dual of the full quintic galileon theory
as an example.Comment: 16 page

### Charged C-metric with conformally coupled scalar field

We present a generalisation of the charged C-metric conformally coupled with
a scalar field in the presence of a cosmological constant. The solution is
asymptotically flat or a constant curvature spacetime. The spacetime metric has
the geometry of a usual charged C-metric with cosmological constant, where the
mass and charge are equal. When the cosmological constant is absent it is found
that the scalar field only blows up at the angular pole of the event horizon.
The presence of the cosmological constant can generically render the scalar
field regular where the metric is regular, pushing the singularity beyond the
event horizon. For certain cases of enhanced acceleration with a negative
cosmological constant, the conical singularity disappears all together and the
scalar field is everywhere regular. The black hole is then rather a black
string with its event horizon extending all the way to asymptotic infinity and
providing itself the necessary acceleration.Comment: regular article, no figures, typos corrected, to appear in Classical
and Quantum Gravit

### Radion and moduli stabilization from induced brane actions in higher-dimensional brane worlds

We consider a 4+N-dimensional brane world with 2 co-dimension 1 branes in an
empty bulk. The two branes have N-1 of their extra dimensions compactified on a
sphere S^(N-1), whereas the ordinary 4 spacetime directions are Poincare
invariant. An essential input are induced stress-energy tensors on the branes
providing different tensions for the spherical and flat part of the branes. The
junction conditions - notably through their extra dimensional components - fix
both the distance between the branes as well as the size of the sphere. As a
result, we demonstrate, that there are no scalar Kaluza-Klein states at all
(massless or massive), that would correspond to a radion or a modulus field of
S^(N-1). We also discuss the effect of induced Einstein terms on the branes and
show that their coefficients are bounded from above, otherwise they lead to a
graviton ghost.Comment: 23 pages, no figures, references added, typos correcte

### Birkhoff for Lovelock Redux

We show succinctly that all metric theories with second order field equations
obey Birkhoff's theorem: their spherically symmetric solutions are static.Comment: Submitted to CQ

### Searching for non-Fermi liquids under holographic light

This discussion is aimed at identifying physically relevant, albeit lying off
the beaten track, gravity models that may provide a holographic description of
some prototypical non-Fermi liquid states of strongly correlated condensed
matter systems.Comment: Latex, 4+ pages, no figure

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