222 Dark Matter
With a large-scale bias of푏=1 to a precision of about 10%, the훽error dominates, so
that one obtains
훺m= 0. 25 ± 0. 07 , (9.19)in good agreement with other determinations.
Later than the above results the Planck collaboration [21] obtained a value for the
characteristic amplitude of velocity fluctuations within 8Mpcℎ−^1 spheres at푧=0,
휎 8 = 0. 83 ± 0. 01.To summarize one can state that on scales larger than a few Mpc the distribution
of DM in CDM models is essentially understood. Understanding the inner structure
of DM haloes and the mechanisms of galaxy formation has proved to be much more
difficult.
Problems
- Suppose that galaxies have flat rotation curves out to푅max. The total mass inside
푅maxis given by Equation (9.10), where푣may be taken to be 220kms−^1 .If
the galaxy number density is푛= 0. 01 ℎ^3 /Mpc^3 , show that훺=1when푅maxis
extended out to an average intergalactic distance of 2. 5 ℎ−^1 Mpc [18]. - Suppose that neutralinos have a mass of 100GeV and that they move with a
virial velocity of 200kms−^1. How much recoil energy would they impart to a
germanium nucleus?
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