GRAVITATIONAL THEORY WITH A DYNAMICAL SPACE–TIME

Abstract

A gravitational theory involving a vector field χμ, which has the properties of a dynamical space–time, is studied. The variation of the action with respect to χμ gives the covariant conservation of an energy–momentum tensor [Formula: see text]. Studying the theory in a background that has Killing vectors and Killing tensors, we find appropriate shift symmetries of the field χμ which lead to conservation laws. The energy–momentum that is the source of gravity [Formula: see text] is different but related to [Formula: see text] and the covariant conservation of [Formula: see text] determines in general the vector field χμ. When [Formula: see text] is chosen to be proportional to the metric, the theory coincides with the Two Measures Theory, which has been studied before in relation to the Cosmological Constant Problem. When the matter model consists of point particles, or strings, the form of [Formula: see text], solutions for χμ are found. In a locally inertial frame, the vector field corresponds to the locally flat coordinates. For the case of a string gas cosmology, we find that the Milne universe can be a solution, where the gas of strings does not curve the space–time since although [Formula: see text], [Formula: see text], as a model for the early universe, this solution is also free of the horizon problem. There may also be an application to the "time problem" of quantum cosmology.