Cosmological Parameters

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.

The fraction of the universe's total energy density carried by dark energy — the component driving the accelerating expansion. In the standard model it is the cosmological constant Λ, and it makes up the majority of the present-day energy budget.

The fraction of the universe's total energy density that is in matter — dark matter plus ordinary baryonic matter — today. Together with the dark-energy density it sets the geometry and the fate of the universe; it is measured most precisely from the cosmic microwave background.

The 'tilt' of the spectrum of primordial density fluctuations laid down by inflation — whether fluctuations have slightly more or less power on large scales than on small. A value just below one is a key prediction of, and constraint on, inflationary models, and is measured from the CMB.