Cosmic light horizon
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In astronomy, the cosmic light horizon is a "horizon" which marks the edge of the visible universe. This horizon is the edge of a sphere, centered upon the Earth, which is approximately 13.7 billion light years or 130 Ym in radius; this figure is sometimes referred to as the "lookback distance" because astronomers, who view distant objects, are "looking back" into the history of the universe since it takes 13.7 billion years for the light to reach Earth. The general homogeneity of temperatures, at the edge of the cosmic horizon, is part of the evidence for the "Big Bang".