Cosmic microwave background

Space is filled with faint microwave light left over from the early universe. NASA's COBE satellite measured its temperature as 2.725 kelvin, only a little above absolute zero.

ESA's Planck mission rounds the same value to about 2.7 K.

Context

The cosmic microwave background, or CMB, is light released about 380,000 years after the Big Bang, when the universe cooled enough for atoms to form and radiation could travel freely. Expansion since then has stretched that light into the microwave range, which is why it now corresponds to a very low temperature.

It was discovered by accident in 1964 by Arno Penzias and Robert Wilson, who were trying to remove a persistent noise from a radio antenna in New Jersey. Their find became strong evidence for the Big Bang and earned them the 1978 Nobel Prize in Physics.

The COBE satellite, launched in 1989, measured the spectrum of this radiation with great precision and found it matches an ideal thermal glow. The reported temperature is 2.725 K with a small uncertainty. John Mather and George Smoot shared the 2006 Nobel Prize in Physics for COBE's results.

The CMB is not perfectly smooth. Tiny variations, about one part in 100,000, mark the seeds of later galaxies, and these are what missions like WMAP and Planck map in detail.