12.5. CMB
The baryon density can be obtained from the amplitude of the
fluctuations on the sky of the temperature of the CMB.
The baryons in the IGM at z
1300 scattered the CMB photons.
The amplitude of the fluctuations is a measure of
b h2, and other
parameters. Published data favor large
b , with large
errors, however dramatic improvements are imminent, and
future constraints may approach or exceed the accuracy of
b from
SBBN [192],
[193].