"We measured the distance to these galaxies, determining that the farther away they were, on average, the faster they appeared to be speeding away from us. And we calculated that a Universe that was uniformly filled with "stuff" - whether matter, radiation, a cosmological constant, or any other form of energy - would be unable to be static and stable; it must either expand or contract."
"From these revolutionary realizations, the notion of the expanding Universe was born. Over the past century, we've learned much more about the history and properties of our Universe. We know it's been 13.8 billion years since the hot Big Bang; we know our Universe is dominated by dark energy and dark matter; we know that the hot Big Bang was preceded and set up by a period of cosmic inflation. But what came before inflation, and could the inflationary period have lasted forever?"
In the 1920s astronomers identified spiral and elliptical nebulae as separate galaxies beyond the Milky Way and measured their distances, finding that more distant galaxies recede faster. A uniformly filled Universe containing matter, radiation, or a cosmological constant cannot be static and must expand or contract. The expanding Universe framework leads to a 13.8 billion-year-old hot Big Bang cosmology dominated by dark matter and dark energy, with cosmic inflation preceding the hot Big Bang. Fundamental questions remain about what preceded inflation and whether inflation could be eternal or require a beginning.
Read at Big Think
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