Three Clues to a Hot, Expanding Universe

Hi, I'm Lumi, and tonight we're playing detective with the whole universe. Scientists don't have a video of the cosmos beginning, so they reason from clues we can still measure today. Clue one: when we study distant galaxies, their light is stretched toward the red end of the spectrum — a phenomenon called redshift. The farther a galaxy is, the faster it appears to recede. This pattern, known as the Hubble-Lemaître law (named after Georges Lemaître, who derived the relationship in 1927, and Edwin Hubble, who confirmed it observationally in 1929), means space itself is expanding, like dots on a balloon spreading apart as it inflates. Rewind that expansion and everything was once packed tightly together. Clue two is the cosmic microwave background, or CMB. If the early universe was extremely hot and dense, it should have left behind faint heat now stretched into microwaves, glowing softly in every direction. We detect exactly that — at about 2.7 kelvin (kelvin is a temperature scale that starts at absolute zero, the coldest anything can get; 2.7 kelvin is just barely above that), almost perfectly uniform across the sky. It is the cooled-down afterglow of a hot beginning. Clue three is primordial nucleosynthesis. In the first few minutes, the universe was hot enough to fuse light nuclei. The hot-origin model predicts roughly 75 percent hydrogen and 25 percent helium by mass, plus traces of lithium — and that is exactly what we observe throughout the cosmos. Three independent clues, all pointing to one story: an expanding universe with a hot, dense origin. If any clue feels fuzzy, try the sorting activity next, then check yourself with the questions.