Cosmic Microwave Background Radiation (CMB) - In the 1940 's, scientists decided that there should be left-over energy from the Big Bang in the far-off edges of the.universe. - It would have cooled and spread out very evenly. Why would this energy likely be cool and spread out?

After the Big Bang, the universe was an incredibly hot and dense soup of particles. As it expanded, it cooled rapidly, much like how a hot air balloon cools as it rises. By the time the CMB radiation was released around 380,000 years after the Big Bang, it had also spread out across an ever-expanding universe, resulting in a uniform temperature of approximately 2.7 Kelvin, which is just above absolute zero! This coolness and uniformity can be attributed to the phenomenon of cosmic expansion. As the universe stretches, light and energy lose their wavelength, becoming stretched into the microwave range we observe today. This process results not only in a drop in temperature but also the remarkable homogeneity of the CMB, providing essential clues about the universe's infancy.