Cosmological implications of unimodular gravity

Abstract

We consider a model of gravity and matter fields which is invariantonly under unimodular general coordinate transformations (GCT). The determinant of the metric is treatedas a separate field which transforms as a scalar under unimodular GCT. Furthermore we also demand that the theory is invariant under a newglobal symmetry which we call generalized conformal invariance.We study the cosmological implications of the resulting theory. We show that this theory gives a fit to the high-z supernova data which is identical to the standard BigBang model. Hence we require some other cosmological observations to test the validity of this model. We also consider some models which do notobey the generalized conformal invariance. In these models we can fit thesupernova data without introducing the standard cosmological constant term.Furthermore these models introduce only one dark component and hencesolve the coincidence problem of dark matter and dark energy.