History of the Universe Project:
Cosmic Microwave Background
By the 1920's and 1930's, the work of Edwin Hubble and others showed that more distant galaxies recede from the Milky Way more quickly. This so called "Hubble Law" implies that the universe does not exist in the same state forever - the universe was more densely packed with matter in the past and is undergoing a process of expansion. As the twentieth century continued, scientists began to observe outer space using more and more parts of the electromagnetic spectrum:
In 1964, a team at Princeton using a new microwave detector discovered a 160GHz signal that persisted no matter what repairs they tried and appeared in every direction they turned the detector. It turned out to be "cosmic microwave background" (CMB) radiation and not a mistake. All throughout space there is a faint microwave afterglow from earlier in the history of the universe. This is strong evidence that the universe began in a very hot and very dense state in order to generate the original CMB signal.
Since that 1964 discovery scientists continue to study the CMB. Three satellites launched in the 1990's, 2000's, and 2010's were all used to view the CMB signal in finer detail.
Since that 1964 discovery scientists continue to study the CMB. Three satellites launched in the 1990's, 2000's, and 2010's were all used to view the CMB signal in finer detail.
The CMB was created when the universe was just 0.003% of its current age, so it remains one of our best observational sources for understanding the early universe.