Unseen Mass

Abstract

The arguments for “unseen mass”, and the evidence on its distribution, are still somewhat controversial as far as the details are concerned. The following four statements would, however, be widely accepted:

• (i) Baryons cannot contribute much less than 1% of the critical density — i.e. ^Ωbaryon ≳ 0.01. This limit comes from the “luminous” content of galaxies, and from the inferred amount of X-ray emitting gas in clusters.

• (ii) (M/L)_B values in the range around 200 h₁₀₀ are derived from studies of virialised clusters and from the cosmic virial theorem. These refer to length scales of (1 – 2)h ^-1₁₀₀ Mpc.

• (iii) (M/L)_B appears to be a non-decreasing function of length scale.

• (iv) If Ω ≃ 1 — the value favoured by some theorists, especially the advocates of “inflationary” cosmology — then (M/L)_B must continue to increase out to at least ∿ 10 h₁₀₀ ^-1 Mpc. Otherwise, a high density Universe would be incompatible with the low random velocities which lead to conclusion (ii) above.