Part 5/11:
This expansion underpins the Big Bang theory, describing our universe’s origin from an extremely hot, dense state. Complementing Hubble’s work was the discovery of cosmic microwave background (CMB) radiation—faint microwave signals permeating space, representing the universe approximately 380,000 years after the Big Bang. This relic radiation, detected in the 1960s and mapped in detail by missions like WMAP and Planck, allows scientists to "rewind" the universe’s history, predicting how the expansion should have progressed.