Perhaps the greatest revelation in the past hundred years of studying the universe is that our home changes and evolves with time. And not just in minor, insignificant ways like stars moving about, gas clouds compressing and massive stars dying in cataclysmic explosions.
By Paul Sutter | SPACE.com
No, our entire cosmos has changed its fundamental character more than once in the distant past, completely altering its internal state at a global — that is, universal — scale.
Take, for instance, the fact that at one time in the foggy, ill-remembered past, there were no stars.
Before the first light
We know this simple fact because of the existence of the cosmic microwave background (CMB), a bath of weak but persistent radiation that soaks the entire universe. If you encounter a random photon (a bit of light), there’s a good chance it’s from the CMB — that light takes up more than 99.99 percent of all the radiation in the universe. It’s a leftover relic from when the universe was just 270,000 years old, and transitioned from a hot, roiling plasma into a neutral soup (with no positive or negative charge). That transition released white-hot radiation that, over the course of 13.8 billion years, cooled and stretched down into the microwaves, giving us the background light that we can detect today. [Cosmic Microwave Background: Big Bang Relic Explained (Infographic)]
At the time of the release of the CMB, the universe was about one-millionth its present volume and thousands of degrees hotter. It was also almost entirely uniform, with density differences no bigger than 1 part in 100,000.
So, not exactly a state where stars could happily exist.