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9. Cosmic Background Radiation 19.1. Introduction

The observed cosmic microwave background (CMB) radiation provides strong evidence for the hot big bang. The success of primordial nucleosynthesis calculations (see Sec. 16, “Big-bang nucleosynthesis”) requires a cosmic background radiation (CBR) characterized by a temperature kT ∼ 1 MeV at a redshift of z ' 109. In their pioneering work, Gamow, Alpher, and Herman [1] realized this and predicted the existence of a faint residual relic, primordial radiation, with a present temperature of a few degrees. The observed CMB is interpreted as the current manifestation of the required 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 Fig. 19.1). A non-interacting Planckian distribution of temperature Ti at redshift zi transforms with the universal expansion to another Planckian distribution at redshift zr with temperature Tr/(1 + zr) = Ti/(1 + zi). 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 109 photons per nucleon, the transition from the ionized primordial plasma to neutral atoms at z ∼ 1000 does not significantly alter the CBR spectrum [3].