Observational constraints on Rastall's cosmology

Rastall's theory is a modification of General Relativity, based on the non-conservation of the stress-energy tensor. The latter is encoded in a parameter gamma such that gamma = 1 restores the usual del T-nu(mu nu) = 0 law. We test Rastall's theory in cosmology, on a flat Robertson-Walker metric, investigating a two-fluid model and using the type Ia supernovae Constitution dataset. One of the fluids is pressure-less and obeys the usual conservation law, whereas the other is described by an equation of state p(x) = w(x)rho(x), with w(x) constant. The Bayesian analysis of the Constitution set does not strictly constrain the parameter gamma and prefers values of w(x) close to -1. We then address the evolution of small perturbations and show that they are dramatically unstable if w(x) not equal = -1 and gamma not equal = 1, i.e. General Relativity is the favored configuration. The only alternative is w(x) = -1, for which the dynamics becomes independent from gamma.