**4.2. CDM models for structure formation**

The elements discussed so far assemble into the
CDM cosmological
model, which is the simplest possibility that is consistent with current
evidence. The overall matter power spectrum is written dimensionlessly
as the logarithmic contribution to the fractional density variance,
^{2}:

(91) |

which undergoes linear growth

(92) |

where the linear growth law is *D*(*a*) = *a*
*g*[(*a*)]
in the matter era, and the growth suppression factor for a
density parameter
_{m}
1 is

(93) |

The transfer function *T*_{k} depends on the dark-matter
content as discussed earlier, in particular displaying a horizon-scale break
at *k*
0.06(_{m}
*h*)^{-1} *h* Mpc^{-1}.
Weak oscillatory features are also expected if the universe has
a significant baryon content.
Eisenstein & Hu
(1998)
give an accurate fitting formula that describes these wiggles.
This detailed fit of the CDM spectrum is to be preferred to
the older notation in which the spectrum was described by
the zero-baryon form, but with an effective value of
_{m}
*h* that allowed for the main effects of the baryon content:

(94) |