cosmic variance

Cosmic variance is the variation in density, mass, and populations on the largest scale, i.e., the large scale structure, which is a contributer to the statistics of surveys on the largest scale. Variation is presumed to occur on every observable scale (and presumably on scales beyond) so deep surveys of portions of the sky are presumed to be unrepresentative of the entire sky: since there are density variations on every scale, the survey field may be viewing a portion of the universe which happens to be more dense (or happens to be less). Statistics derived from such a survey must admit a factor regarding this effect, the cosmic variance. The entire observable universe (out to the surface of last scattering or particle horizon) might well be affected, more dense or less dense than the "universe" beyond, but attempts to accommodate them are unlikely to be totally convincing. On smaller scales the density variations can be determined using all-sky surveys and/or by surveying strategic samples scattered over the whole sky.