Asked by: Simon Lennox, London
There is a lot of variation in temperature throughout the Universe. The coldest naturally occurring temperature was discovered inside the Boomerang Nebula and is only one degree above absolute zero. The hottest temperatures (not including the Big Bang itself) are likely to be generated in the interactions that create so-called gamma-ray bursts.
But what about the average temperature of the Universe? Astronomers often regard the temperature of the Cosmic Microwave Background (CMB) as the temperature of the Universe. The CMB is a snapshot of the oldest light in the Universe, imprinted on the sky when the Universe was just 380,000 years old. It has a temperature of just 2.735 degrees above absolute zero. The Big Bang theory predicts that as the Universe expands this temperature should drop. This is what astronomers have found by deducing the temperature of the CMB at various distances across the Universe.