  We review the physics of the Grishchuck-Zel'dovich effect which describes the
impact of large amplitude, super-horizon gravitational field fluctuations on
the Cosmic Microwave Background anisotropy power spectrum. Using the latest
determination of the spectrum by WMAP, we infer a lower limit on the present
length-scale of such fluctuations of 3927 times the cosmological particle
horizon (at the 95% confidence level).
