Newton's law on an Einstein "Gauss-Bonnet" brane

It is known that Newton's law of gravity holds asymptotically on a flat "brane" embedded in an anti-de Sitter "bulk" ; this was shown not only when gravity in the bulk is described by Einstein's theory but also in Einstein "Lanczos Lovelock Gauss-Bonnet"'s theory. We give here the expressions for the corrections to Newton's potential in both theories, in analytic form and valid for all distances. We find that in Einstein's theory the transition from the 1/r behaviour at small r to the 1/r^2 one at large r is quite slow. In the Einstein Gauss-Bonnet case on the other hand, we find that the correction to Newton's potential can be small for all r. Hence, Einstein Gauss-Bonnet equations in the bulk (rather than simply Einstein's) induce on the brane a better approximation to Newton's law.Comment: typos corrected, reference added, version to be published in Progress of Theoretical Physic

Conformal transformations and Nordstr\"om's scalar theory of gravity

As we shall briefly recall, Nordstr\"om's theory of gravity is observationally ruled out. It is however an interesting example of non-minimal coupling of matter to gravity and of the role of conformal transformations. We show in particular that they could be useful to extend manifolds through curvature singularities.Comment: 9 pages, no figure, prepared for the Proceedings of YKIS2010, to be published in Progress of Theoretical Physics Supplemen

On the mechanical stability of asymptotically flat black holes with minimally coupled scalar hair

We show that the asymptotically flat hairy black holes, solutions of the Einstein field equations minimally coupled to a scalar field, previously discovered by one of us, present mode instability against linear radial perturbations. It is also shown that the number of unstable modes is finite and their frequencies can be made arbitrarily small.Comment: 11 pages, 6 figures, one reference fixe