The Parameterised Post-Newtonian Limit of Bimetric Theories of Gravity

We consider the post-Newtonian limit of a general class of bimetric theories of gravity, in which both metrics are dynamical. The established parameterised post-Newtonian approach is followed as closely as possible, although new potentials are found that do not exist within the standard frame-work. It is found that these theories can evade solar system tests of post-Newtonian gravity remarkably well. We show that perturbations about Minkowski space in these theories contain both massless and massive degrees of freedom, and that in general there are two different types of massive mode, each with a different mass parameter. If both of these masses are sufficiently large then the predictions of the most general class of theories we consider are indistinguishable from those of general relativity, up to post-Newtonian order in a weak field, low velocity expansion. In the limit that the massive modes become massless, we find that these general theories do not exhibit a van Dam-Veltman-Zakharov-like discontinuity in their gamma parameter, although there are discontinuities in other post-Newtonian parameters as the massless limit is approached. This smooth behaviour in gamma is due to the discontinuities from each of the two different massive modes cancelling each other out. Such cancellations cannot occur in special cases with only one massive mode, such as the Isham-Salam-Strathdee theory.Comment: 36 page

The Boulware-Deser mode in Zwei-Dreibein gravity

Massive gravity in three dimensions accepts several different formulations. Recently, the 3-dimensional bigravity dRGT model in first order form, Zwei-Dreibein gravity, was considered by Bergshoeff {\it et al.} and it was argued that the Boulware-Deser mode is killed by extra constraints. We revisit this assertion and conclude that there are sectors on the space of initial conditions, or subsets of the most general such model, where this mode is absent. But, generically, the theory does carry 3 degrees of freedom and thus the Boulware-Deser mode is still active. Our results also sheds light on the equivalence between metric and vierbein formulations of dRGT model.Comment: 4 page

Selfdual backgrounds in N=2 five-dimensional Chern-Simons Supergravity

We consider five-dimensional S(2,2|N) Chern-Simons supergravity on M_4 * R . By fine-tuning the Kaluza-Klein reduction to make the 4d cosmological constant equal zero, it is shown that selfdual curvatures on M_4 provide exact solutions to the equations of motion if N=2.Comment: Added comments on the Eguchi-Hanson metric as a solution to 5d Chern-Simons supergravity on M4*S

Charged Black Holes in Gauss-Bonnet Extended Gravity

Charged Black holes in Gauss-Bonnet extended gravity are studied. The electromagnetic field is coupled non-minimally, as in U(2,2) Chern-Simons theory. We find that the geometrical properties of the solution exhibit ``phase transitions" as one varies the mass and charge. The full phase diagram for all values of the ADM mass and charge is displayed.Comment: Added and corrected references. To appear in PLB. 3 eps figures, 15 page

Gravitons and gauge fields in 5d Chern-Simons supergravity

Despite the nice geometrical properties of higher dimensional Chern-Simons (CS) supergravity theories these actions suffer from one major drawback, namely, their connection with the real world. After some quick remarks on three-dimensional gravity, we consider five-dimensional CS supergravity and study to what extend this theory reproduces the standard low energy description of gravitons and gauge fields. We point out that if one deforms the CS action by changing the value of the cosmological constant by a small amount (thus breaking the CS symmetry), propagation around AdS becomes non-trivial, asymptotically (AdS) Schwarzschild solutions exist, and the gauge field acquires its standard quadratic propagator.Comment: Some improvements in the section on `backgrounds', and references added. 10 pages, latex, no figures, uses espcrc2.sty (twocolumn). Written version of an invited Lecture delivered at the QG99 meeting, held in Sardinia, Italy, on Sept. 199