POWER-LAWS f(R) THEORIES ARE COSMOLOGICALLY UNACCEPTABLE

Abstract

In a recent paper,1 we have shown that f(R) = R + μRn modified gravity dark energy models are not cosmologically viable because during the matter era that precedes the accelerated stage the cosmic expansion is given by a ~ t1/2 rather than a ~ t2/3, where a is a scale factor and t is the cosmic time. A recent work by Capozziello et al.2 criticized our results presenting some apparent counter-examples to our claim in f(R) = μRn models. We show here that those particular Rn models can produce an expansion as a ~ t2/3 but this does not connect to a late-time acceleration. Hence, though acceptable f(R) dark energy models might exist, the Rn models presented in Capozziello et al. are not cosmologically viable, confirming our previous results in Ref. 1.