4.5. Baryon Density Concordance

In Figure 12 are shown the likelihood distributions for the three baryon density determinations discussed above. It is clear that these disparate determinations, relying on completely different physics and from widely separated epochs in the evolution of the universe are in excellent agreement, providing strong support for the standard, hot big bang cosmological model and for the standard model of particle physics. Although it has been emphasized many times in these lectures that the errors are likely dominated by evolutionary and systematic uncertainties and, therefore, are almost certainly not normally distributed, it is hard to avoid the temptation to combine these three independent estimates. Succumbing to temptation: ₁₀ = 5.8^+0.4_-0.6 (_B h² = 0.021^+0.0015_-0.0020).

Figure 12. The likelihood distributions, normalized to equal areas under the curves, for the baryon-to-photon ratios (10) derived from BBN (~ 20 minutes), from the CMB (~ few hundred thousand years), and for the present universe (t0 ~ 10 Gyr; z 1).