  We compute the modified friction coefficient controlling the propagation of
tensor metric perturbations in the context of a generalized cosmological
scenario based on a theory of gravity with quadratic curvature corrections. In
such a context we discuss the differences between gravitational and
electromagnetic luminosity distance, as well as the differences with the
standard results based on the Einstein equations. We present numerical
estimates of the modified luminosity distance on the cosmic redshift scale
typical of Supernovae and standard sirens.
