Using the renormalization group method, we improved the first order solution
of the long-wavelength expansion of the Einstein equation. By assuming that the
renormalization group transformation has the property of Lie group, we can
regularize the secular divergence caused by the spatial gradient terms and
absorb it to the background seed metric. The solution of the renormalization
group equation shows that the renormalized metric describes the behavior of
gravitational collapse in the expanding universe qualitatively well.Comment: 12 pages, 1 figure, typeset by REVTe