  We investigate the backreaction of cosmological long wavelength perturbations
on the evolution of the Universe. By applying the renormalization group method
to a Friedmann-Robertson-Walker universe with long wavelength fluctuations, we
demonstrate that the renormalized solution with the backreaction effect is
equivalent to that of the separate universe. Then, using the effective
Friedmann equation, we show that only non-adiabatic mode of long wavelength
fluctuations affects the expansion law of the spatially averaged universe.
