Scale of homogeneity of the universe from WMAP

We review the physics of the Grishchuk-Zel’dovich effect which describes the impact of large amplitude, superhorizon gravitational field fluctuations on the cosmic microwave background anisotropy power spectrum. Using the latest determination of the spectrum by the Wilkinson Microwave Anisotropy Probe satellite, we infer a lower limit on the present length scale of such fluctuations of $3.9\times {10}^3$ times the cosmological particle horizon (at the 95% confidence level).