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We calculate the back-reaction of long wavelength cosmological perturbationson a general relativistic measure of the local expansion rate of the Universe.Specifically, we consider a cosmological model in which matter is described bytwo scalar matter fields, one being the inflaton and the other one representinga matter field which is used as a clock. We analyze back-reaction in a phase ofinflaton-driven slow-roll inflation, and find that the leading infraredback-reaction terms contributing to the evolution of the expansion rate do notvanish when measured at a fixed value of the clock field. We also analyze theback-reaction of entropy modes in a specific cosmological model with negativesquare mass for the entropy field and find that back-reaction can becomesignificant. Our work provides evidence that, in general, the back-reaction ofinfrared fluctuations could be locally observable.Comment: 9 pages, no figure

Back-reaction of perturbations in two scalar field inflationary models