F(R) Cosmology with torsion

Abstract

f(R)-gravity with geometric torsion (not related to any spin fluid) is considered in a cosmological context. We derive the field equations in vacuum and in the presence of perfect-fluid matter and discuss the related cosmological models. Torsion vanishes in vacuum for almost all arbitrary functions f(R) leading to standard general relativity. Only for f(R)=R2, torsion gives a contribution in the vacuum leading to accelerated behavior. When material sources are considered, we find that the torsion tensor is different from zero even with spinless material sources. This tensor is related to the logarithmic derivative of f′ (R), which can be expressed also as a nonlinear function of the trace of the matter energy–momentum tensor Σμν. We show that the resulting equations for the metric can always be arranged to yield effective Einstein equations. When the homogeneous and isotropic cosmological models are considered, terms originated by torsion can lead to accelerated expansion. This means that, in f(R)-gravity, torsion can be a geometric source for acceleration.