4.3. CMB Baryon Density - The Baryon Density At A Few Hundred Thousand Years

As discussed in the first lecture, the early universe is hot and dominated by relativistic particles ("radiation"). As the universe expands and cools, nonrelativistic particles ("matter") come to dominate after a few hundred thousand years, and any preexisting density perturbations can begin to grow under the influence of gravity. On length scales determined by the density of baryons, oscillations ("sound waves") in the baryon-photon fluid develop. At a redshift of z ~ 1100 the electron-proton plasma combines (``recombination) to form neutral hydrogen which is transparent to the CMB photons. Free to travel throughout the post-recombination universe, these CMB photons preserve the record of the baryon-photon oscillations as small temperature fluctuations in the CMB spectrum. Utilizing recent CMB observations (Lee et al. 2001; Netterfield et al. 2002; Halverson et al. 2002), many groups have inferred the intermediate age universe baryon density. The work of our group at OSU (Kneller et al. 2001) is consistent with more detailed analyses and is the one I adopt for the purpose of comparison with the SBBN result: ₁₀ = 6.0 ± 0.6; _B h² = 0.022 ± 0.002.