Cosmic microwave background

The oldest light there is, released 380,000 years after the Big Bang when the universe first became transparent. Its source has since been carried 45 billion light-years away by the expansion. The map is Planck's, the shell around the Sun is where that light comes from; nothing beyond it can be seen. Seen from outside, the sphere is a picture of our own horizon, not a place.

Distance now

45.3 billion ly

Light left

13.8 billion years ago

Temperature

2.725 K

Map

ESA Planck, SMICA

The cosmic microwave background, or relic radiation, is microwave radiation that fills all space in the observable universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dark. However, a sufficiently sensitive radio telescope detects a faint background glow that is almost uniform and is not associated with any star, galaxy, or other object.

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