While observational tests on the details of cosmology proceed apace, astronomers are focusing on the mechanics of how matter came together in the early universe.
The fundamental question is: Did galaxies, stars, or black holes come first? The infant universe was a relatively uniform sea of several-thousand-degree gas and dark matter — the unseen, mysterious, and predominant form of matter that is indirectly known to exist because of its gravitational influence. But how galaxies, stars, and black holes came together is the key to understanding the puzzle of the early universe.
Based on cosmic microwave background data, astronomers think matter coalesced when the universe cooled and became “transparent” 380,000 years after the Big Bang. And according to recent studies, structures like stars and galaxies formed as early as 200 million years after the Big Bang. But exactly how matter clumped is open to future research.
Deciphering galaxy formation goes back to Walter Baade, who studied stars in galaxies and tried to interpret how the galaxies formed. One of the premier researchers at California’s Mount Wilson Observatory in the 1950s, Baade discovered a group of stars around the Milky Way with few metals (elements heavier than hydrogen and helium). These stars are ancient, probably 13 billion years old. Metals thrown out into interstellar space by supernovae and other processes were eventually incorporated into a younger generation of stars in our galaxy.
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