Enlarging minimal-supergravity parameter space by decreasing pre-nucleosynthesis Hubble rate in scalar-tensor cosmologies

We determine under what conditions Scalar Tensor cosmologies predict an expansion rate which is reduced as compared to the standard General Relativity case. We show that ST theories with a single matter sector typically predict an enchanced Hubble rate in the past, as a consequence of the requirement of an attractive fixed point towards General Relativity at late times. Instead, when additional matter sectors with different conformal factors are added, the late time convergence to General Relativity is mantained and at the same time a reduced expansion rate in the past can be driven. For suitable choices of the parameters which govern the scalar field evolution, a sizeable reduction of the Hubble rate prior to Big Bang Nucleosynthesis can be obtained. We then discuss the impact of these cosmological models on the relic abundance of dark matter in minimal Supergravity models: we show that the cosmologically allowed regions in parameter space are significantly enlarged in this class of ST theories with reduced Hubble rate. The ensuing effect on the potential reach of LHC are biefly discussed. © 2008 SISSA.