Gravitation and Experiment

Abstract

The confrontation between Einstein's gravitation theory and experimental results, notably binary pulsar data, is summarized and its significance discussed. Experiment and theory agree at the 10 −3 level. All the basic structures of Einstein's theory (coupling of gravity to matter; propagation and self-interaction of the gravitational field, including in strong-field conditions) have been verified. However, some recent theoretical findings (cosmological relaxation toward zero scalar couplings) suggest that the present agreement between Einstein's theory and experiment might be naturally compatible with the existence of a long-range scalar contribution to gravity (such as the dilaton, or a moduli field of string theory). This provides a new theoretical paradigm, and new motivations for improving the experimental tests

Aspects of a supersymmetric Brans-Dicke theory

“…This is a non trivial property and has very important phenomenological implications. It can be shown, for instance, that in a theory where matter couples to gravity in a universal metric way the weak equivalence principle is satisfied by construction [4].…”

“…An example of this lack of knowledge is the difficulty to explain the extremely small value of its mass scale that, in a phenomenologically consistent model, should be of the order of the present value of the Hubble parameter [3]. As a consequence, direct couplings of Dark Energy to matter fields are strongly constrained by fifth force searches [4]. A possible way to avoid such constraints is to work in the framework of Scalar-Tensor theories [5].…”

“…Moreover, the energy density of this extra scalar degree of freedom can be easily identified with Dark Energy [6]. An interesting feature of such models is that the gravitational sector (that also includes the Dark Energy scalar) couples to the matter sector in a universal metric way so that fifth force bounds are satisfied by construction [4]. If Dark Energy has a scalar nature, Scalar-Tensor theories provide a natural framework to discuss its properties.…”

Cosmological implications of a supersymmetric extension of the Brans-Dicke theory

“…This is a non trivial property and has very important phenomenological implications. It can be shown, for instance, that in a theory where matter couples to gravity in a universal metric way the weak equivalence principle is satisfied by construction [5].…”

“…all matter fields feel the gravitational interaction only through one and the same metric. Because of this property, ST theories are by construction protected by any violation of the weak equivalence principle and even ultra-light degrees of freedom mediating long range forces are not in this framework phenomenologically dangerous [5]. Such a feature makes ST theories interesting arenas for dynamical Dark Energy model building [6,7].…”