Ghost-free braneworld bigravity

Abstract

We consider a generalisation of the DGP model, by adding a second brane with localised curvature, and allowing for a bulk cosmological constant and brane tensions. We study radion and graviton fluctuations in detail, enabling us to check for ghosts and tachyons. By tuning our parameters accordingly, we find bigravity models that are free from ghosts and tachyons. These models will lead to large distance modifications of gravity that could be observable in the near future. * a.padilla1@physics.ox.ac.uk

Brane-induced gravity in warped backgrounds and the absence of the radion

“…As we will see later, if there is matter on the brane, there appears a scalar field due to the existence of the extra dimension, which cannot be identified with the 44-component of the metric fluctuations. It should be noted, that the elimination of the radion in the Randall-Sundrum model with brane-localized curvature terms was discussed earlier, for example, in [2] and [5] (it is evident that since there exists some range of parameters for which the coefficient in front of the kinetic term for the radion could be either positive or negative, there exist some values of the parameters for which the radion is absent at all). But in this paper the equations of motion for the model with warped background are treated much more thoroughly, a convenient gauge is used and these equation are solved exactly.…”

“…There were attempts to merge the DGP-proposal and models with warped backgrounds to get a long-distance modification of gravity. But it turnes out that such models must be rejected for some reasons (see, for example, [2,4,5]). For example, the Randall-Sundrum model with brane-localized curvature terms admits a long-distance modification of gravity, but in this case either the radion, the graviton or both fields become ghosts.…”

Linearized gravity on the Randall–Sundrum two-brane background with curvature terms in the action for the branes

“…Also note that this term is absent in the original Randall-Sundrum model which has m = 0. m * ). Several aspects of this setup were also studied in [8][9][10][11]. Our treatment in this and in the subsequent section is similar to that of [20].…”

“…When one adds curvature terms in the action for the branes, this assumption might be relaxed, and, in this paper, we allow for positive as well as negative bulk gravitational constant while keeping gravitational constants on the branes positive. More generally, one could consider various relations between the signs of the four-dimensional gravitational constants in the action for the branes, bulk gravitational constant, and conventional matter Lagrangians [8][9][10][11]. Although we are not aware of any fundamental multidimensional theory that can produce different signs of the gravitational couplings for the bulk and branes, we consider this possibility from the viewpoint of the effective action regardless of the unknown underlying theory.…”

Bi-galileon theory II: phenomenology

“…They considered a class of scalar tensor theories of gravity, where all modifications of General Relativity are encoded in the Lagrangian for a single scalar field propagating in Minkowski space. The scalar field Lagrangian L(π, ∂π, ∂∂π) is invariant under a Galilean symmetry, π → π + b µ x µ + c. The inspiration for the galileon description comes from co-dimension one brane world models exhibiting infra-red modifications of gravity [2][3][4][5]. In these models, gravity on the brane is mediated by the exchange of the graviton and an additional scalar, often corresponding to the strongly coupled brane bending mode [6].…”