The universe has a temperature. Not a very high one, mind you — only a few degrees above absolute zero. But it’s enough to reveal much about the universe’s birth and evolution. That temperature is called the CMB — the cosmic microwave background. It’s visible as a microwave glow that fills the entire universe. It looks almost identical in every direction. This background is the afterglow of the Big Bang. The early universe was an opaque “soup” of energy and particles. Light couldn’t travel freely inside that mixture — it simply bounced around like sunlight scattering inside a fog bank. The CMB is the view of the cosmic fog as the universe expanded enough for it to dissipate, about 380,000 years after the Big Bang. At the time, the temperature of the universe was still thousands of degrees. But in the 13.8 billion years since then, the universe has expanded a thousand fold. That has stretched the cosmic background radiation to longer wavelengths — in essence, lowering its temperature to
