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

Ernst equation and spheroidal coordinates with a cosmological constant term

We discuss solution generating techniques treating stationary and axially symmetric metrics in the presence of a cosmological constant. Using the recently found extended form of Ernst's complex equation, which takes into account the cosmological constant term, we propose an extension of spheroidal coordinates adapted to asymptotically de-Sitter and anti de-Sitter static spacetimes. In the absence of a cosmological constant we show in addition that any higher dimensional metric parametrised by a single angular momentum can be given by a 4 dimensional solution and Weyl potentials parametrising the extra Killing directions. We explicitly show how a stationary, and a static axially symmetric spacetime solution in 4 dimensions, can be {\it added} together to give a 5 dimensional stationary and axisymmetric solution.Comment: 9 pages, no figures, some additional results to gr-qc/0610091. Prepared for 12th Conference on Recent Developments in Gravity (NEB XII), Nafplio, Greece, 29 Jun - 2 Jul 200