Cosmic background fluctuations 103Figure 2.22. Angular power spectraT^2 (")= "("+ 1 )C"/ 2 πfor the CMB, plotted
against angular wavenumber"in rad−^1. The experimental data are an updated version
of the compilation described in Whiteet al(1994), communicated by M White; see
also Hancocket al(1997) and Jaffeet al(2000). Various model predictions for
adiabatic scale-invariant CDM fluctuations are shown. The two full curves correspond
to(,B,h) = ( 1 , 0. 05 , 0. 5 )and (1,0.1,0.5), with the higherBincreasing power
by about 20% at the peak. The dotted line shows a flat-dominated model with
(,B,h)=( 0. 3 , 0. 05 , 0. 65 ); the broken curve shows an open model with the same
parameters. Note the very similar shapes of all the curves. The normalization has been set
to the large-scale amplitude, and so any dependence onis quite modest. The main effects
are that open models shift the peak to the right, and that the height of the peak increases
withBandh.
degeneracy). Now that we have a direct detection of a peak at low", this argument
for a flat universe is even stronger.
If the basic adiabatic CDM paradigm is adopted, then we can move
beyond generic statements about flatness to attempt to use the CMB to measure
cosmological parameters. In a recent analysis (Jaffeet al2000), the following
best-fitting values for the densities in collisionless matter (c), baryons (b) and
vacuum (v) were obtained, together with tight constraints on the power-spectrum
index:
c+b+v= 1. 11 ± 0. 07
ch^2 = 0. 14 ± 0. 06
bh^2 = 0. 032 ± 0. 005