Georges Lemaître’s theory of an expanding Universe, which has become known as the Big Bang, was supported by Hubble’s observations. The expanding Universe idea was challenged by influential scientists who believed the Universe was both infinite (and therefore not expanding) and steady state (unchanging). Supporters of the Big Bang idea needed to find other evidence that could confirm their model was correct.
The cosmic microwave background radiation (CMBR) is radiation left over from the big bang. When the universe was very young, only 380,000 years old, just as space became transparent to light, electromagnetic energy would have propagated through space for the very first time. At this stage in its development, the temperature of the Universe would have been about 3000K. Nowadays, the temperature of space has fallen to approximately 2.7 K (that’s 2.7 K above absolute zero!) and, using Wien’s Law, we can confirm that the peak wavelength of the electromagnetic radiation is so long that the background radiation lies in the microwave portion of the em spectrum.
The CMBR was first detected in 1964 by Richard Woodrow Wilson and Arno Allan Penzias, who worked at Bell Laboratories in the USA.