If there’s one lesson that humanity should have learned from the 20th century, it’s this: the Universe rarely behaves the way our intuition leads us to suspect. At the start of the 1900s, we thought the Universe was governed by Newtonian gravity. We thought that the Universe was static, stationary, and infinitely old, with no beginning and no end. And we couldn’t even be sure whether the Milky Way was one of many galaxies, or whether it encompassed everything there was.
Of course, developments in both theory and observation changed all of this. Newtonian gravity was superseded by General Relativity, which demonstrated that a static Universe would be unstable. Spirals (and later ellipticals) were determined to be their own “island Universes” far outside of the Milky Way, each with billions of stars of their own. And instead of an infinitely old Universe, we live in one that got its start 13.8 billion years ago during the hot Big Bang. This picture itself was revolutionary, but led to a brand new question: how did the Universe grow up?