Amplitude of Fluctuations (σ8)

A measure of how clumpy the universe is — the amplitude of matter-density fluctuations averaged over spheres of 8 megaparsecs. It links the smooth early universe to today's web of galaxies, and a mild disagreement between its early-universe and weak-lensing determinations — usually quantified through the related combination S8 — is known as the S8 tension.