Isotropic Observers and the Inflationary Backreaction Problem

In an inflationary regime driven by a free massive inflaton we derive within a genuinely gauge invariant approach the backreaction effects due to long wavelength scalar fluctuations on the effective Hubble factor and equation of state with respect to a class of observers which sees an inhomogeneous and isotropic Universe. We find that, for such so-called isotropic observers, contrary to what happens for the observables defined by free-falling observers, there is an effect to leading order in the slow-roll parameter in the direction of slowing down the measured rate of expansion and of having an effective equation of state less de Sitter like. From a general point of view the isotropic observers result has to be considered complementary to other cases (observers) in helping to characterize the physical properties of the models under investigation.