Technically Speaking: Why 2.7 K?

Why should the spectrum of light left over from the big bang correspond to such a cold blackbody temperature? Because the light has been redshifting since the beginning of time owing to the expansion of the Universe. The redshift lowers the energy of the light, and this in turn lowers the temperature. The animation shown below summarizes.

When radiation decoupled from matter the temperature of the Universe was about 3000 K, the hydrogen atom recombination temperature. But the decoupling transition took place at redshifts of around 1000. Thus, the photons in the Universe when light and matter decoupled had their wavelengths increased by about a factor of 1000 in the following evolution of the Universe. This corresponds to shifting the blackbody temperature from 3000 K to about 3 K.

Discovery: Serendipity and Science

The most important discovery in the history of cosmology other than the expansion of the Universe occurred by accident. In the mid-1960s a group from Princeton University led by Robert Dicke was getting ready to look for evidence of radiation left over from the big bang that had been predicted earlier by Gamow, Alpher, and Herman (see the right panel). But before they could start their experiments, someone else found by accident the evidence that they sought.

"Noise" in an Antenna
Two Bell Laboratory scientists, Arno Penzias and Robert Wilson, detected an annoying noise in the signal of a special low-noise radio antenna that they were using to track the earliest communication satellites, Telstar and Echo-1. After many attempts to get rid of the noise (including cleaning the antenna of pigeon droppings from birds that had taken up residence in it), they concluded that the signal was real. They further concluded that the signal was not coming from anything on the Earth but appeared instead to be coming uniformly from all directions in the sky.
The Whisper of the Big Bang
When Penzias and Wilson heard of the Princeton work through a side remark in a telephone conversation, they contacted the Princeton group and it was quickly concluded that Penzias and Wilson might have found the cosmic microwave background predicted by Gamow and collaborators. Subsequent observations confirmed that the Penzias and Wilson "noise" was in fact a blackbody spectrum corresponding to a temperature of near 3 K (thus it was initially often termed the "3-degree background"). This radiation was none other than the cooled, faint whisper of the big bang itself. Modern measurements with more precise instruments above the atmosphere give a value of 2.725 K with an uncertainty of only 0.002 K for the blackbody temperature of the "3-degree background". The animation shown below summarizes the effect of the expanding Universe on the background radiation spectrum.