  In this paper we revisit and extend a previous analysis where the possible
relevance of quantum gravity effects in a cosmological setup was studied. The
object of interest are non-local (logarithmic) terms generated in the effective
action of gravity due to the exchange in loops of massless modes (such as
photons or the gravitons themselves). We correct one mistake existing in the
previous work and discuss the issue in a more general setting in different
cosmological scenarios. We obtain the one-loop quantum-corrected evolution
equations for the cosmological scale factor up to a given order in a derivative
expansion in two particular cases: a matter dominated universe with vanishing
cosmological constant, and in a de Sitter universe. We show that the quantum
corrections, albeit tiny, may have a secular effect that eventually modifies
the expansion rate. For a de Sitter universe they tend to slow down the rate of
the expansion, while the effect seems to have the opposite sign in a matter
dominated universe. To partly understand these effects we provide a
complementary newtonian analysis.
