"When it comes to science in general, and physics in particular, we don't just want to know what's going to happen under a given set of circumstances. We also want to know the answer to the key question of, "how much," "how many," or "in what amount," when it comes to our answers. Being quantitative, and answering questions of amounts and timescales, not merely qualitative, is what separates a successful physical theory from one that must be discarded."
"Star formation makes qualitative sense: gas, gravity, time. In a static universe, it's clearly inevitable. But the quantitative side seems really surprising: with a density on the order of one proton per cubic meter, and accelerating expansion, I'd expect the universe to have been born at "escape velocity", i.e. that no star would ever form because everything was flying apart faster than gravity could pull it together... [so] how do we ever get enough matter density for a star to form?"
Quantitative predictions are essential in physics to determine amounts, timescales, and to decide which theories are successful. Star formation occurred within the first few hundred million years after the Big Bang. The accelerating expansion and the present mean cosmic density of order one proton per cubic meter raise a challenge to naive expectations that gravity could never overcome expansion to form stars. Local overdensities, gas physics, and the growth of gravitational perturbations allow matter to collapse and form stars despite the low average density and global expansion. On the largest scales the universe is smooth, but small initial fluctuations grow under gravity and produce the observed structure and star-forming regions.
Read at Big Think
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