Cosmic Microwave Background Temperature

Penzias and Wilson found it by accident in 1964, chasing an inexplicable hiss in a Bell Labs horn antenna that they could not remove by cooling the receiver or evicting the pigeons nesting inside it. A group at Princeton down the road was building an instrument to look for exactly that signal — the cooled afterglow of a hot early universe, predicted by Gamow, Alpher and Herman in the late 1940s. The detection ended the steady-state cosmology and won a Nobel Prize.

COBE's FIRAS instrument later showed the spectrum to be a blackbody to within 50 parts per million, a fit so good that the error bars are smaller than the width of the plotted curve. The temperature falls as 1/(1+z), so the same radiation was 3000 K at recombination 380 000 years after the Big Bang, when the universe first became transparent. The anisotropies, one part in 100 000, encode the seeds of every galaxy and are where the cosmological parameters in this shard come from.