Abstract
Alternative cosmologies, based on extensions of General Relativity, predict modified thermal histories in the early universe during the pre Big Bang Nucleosynthesis (BBN) era. When the expansion rate is enhanced with respect to the standard case, thermal relics typically decouple with larger relic abundances. In this paper, we study the dynamical evolution of an f(R) model of gravity in a homogeneous and anisotropic background which is given by a Bianchi type-I model of the universe filled with dark matter, which is described by a perfect fluid with a barotropic equation of state. As an example of a consistent analysis of modified gravity, we apply the formalism to a simple background solution of R+βRn gravity. Our analysis shows that f(R) cosmology allows dark matter masses lesser than 100 GeV, in the regime ρc≲ρm. We finally discuss how these limits apply to some specific realizations of standard cosmologies: an f(R) gravity model, Einstein frame model.
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