75 research outputs found
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
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