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We investigate the backreaction of cosmological long wavelength perturbationson the evolution of the Universe. By applying the renormalization group methodto a Friedmann-Robertson-Walker universe with long wavelength fluctuations, wedemonstrate that the renormalized solution with the backreaction effect isequivalent to that of the separate universe. Then, using the effectiveFriedmann equation, we show that only non-adiabatic mode of long wavelengthfluctuations affects the expansion law of the spatially averaged universe.Comment: 10 pages. to be published in Phys. Rev.

Separate universe and the back reaction of long wavelength fluctuations