Cosmic Microwave Background

Looking out to a radius of 13.8 billion light-years in all directions, we see the cosmic microwave background (CMB, for short)—light that was emitted soon after the Big Bang beginning of the universe 13.8 billion years ago. It’s the most distant thing we can see. Earth is at the center of this spherical shell—illustrated here—and our visible universe lies within. Penzias and Wilson’s 1965 discovery of this radiation showed that the universe began with a hot Big Bang. The temperature of the CMB radiation is remarkably uniform (2.73° Kelvin above absolute zero). It has cooled as the universe expanded. Its small fluctuations in temperature (around one part in 100,000) are shown here ranging from red (slightly hotter) to blue (slightly cooler). These fluctuations agree with the theory of inflation, which says the Big Bang began with a hyperfast period of expansion, in which the universe doubled in size many times during its first 10⁻³⁵ seconds.