One of the most important pieces of evidence for this limit is the cosmic microwave background radiation, faint heat left over from the Big Bang. It is seen everywhere in the sky at a temperature of just 2.7 degrees above absolute zero. This radiation comes from a time when the Universe was only 3,80,000 years old, and it marks the furthest point in time and space that light can reach us. Beyond this surface, the Universe was opaque to radiation, so the microwave background represents the horizon of observation.
