/sci/ - Science & Math

>>11650714

For times after that point, our understanding is good enough to make some pretty bold and testable predictions about what the universe must have looked like at various times. One such prediction is that the entire universe was once as hot and dense and opaque as the inside of a star. It was a searing ocean of protons and electrons. A plasma. As the universe expanded, this plasma cooled. And at a very particular moment when the universe was around 400,000 years old and about 1,000 times smaller than it is today, it hit a critical temperature of 3,000 degrees Kelvin, at which point the entire universe slipped from plasma to gas as the first hydrogen atoms formed. In the same moment, the infrared light that had previously been trapped in this fog was free to travel the width of the cosmos. It's still traveling today, carrying with it an image of that early time. And we see it. It's no longer infrared. Having been stretched into microwaves as it traveled through an expanding universe, it's the cosmic microwave background. This ubiquitous radiation is almost impossible to explain without a universe that was once much smaller, hotter, and denser.