1 20. Cosmic background radiation 20. COSMIC BACKGROUND RADIATION

The observed cosmic microwave background (CMB) radiation provides strong evidence for the hot big bang. The success of pri-mordial nucleosynthesis calculations (see Sec. 16, " Big-bang nucle-osynthesis ") requires a cosmic background radiation (CBR) characterized by a temperature kT ∼ 1 MeV at a redshift of z ≃ 10 9. In their pioneering work, Gamow, Alpher, and Herman [1] realized this and predicted the existence of a faint residual relic, primor-dial radiation, with a present temperature of a few degrees. The observed CMB is interpreted as the current manifestation of the hypothesized CBR. The CMB was serendipitously discovered by Penzias and Wilson [2] in 1965. Its spectrum is well characterized by a 2.73 ± 0.01 K black-body (Planckian) spectrum over more than three decades in frequency (see Figs. 20.1 and 20.2). A non-interacting Planckian distribution of temperature T i at redshift z i transforms with the universal expansion to another Planckian distribution at redshift zr with temperature Tr/(1 + zr) = T i /(1 + z i). Hence thermal equilibrium, once established (e.g. at the nucleosynthesis epoch), is preserved by the expansion, in spite of the fact that photons decoupled from matter at early times. Because there are about 10 9 photons per nucleon, the transition from the ionized primordial plasma to neutral atoms at z ∼ 1000 does not significantly alter the CBR spectrum [3].