/sci/ - Science & Math

>>11650846

Let's undo our rewind a bit, fast forward again to 400,000 years after that crazy first fraction of a second. The universe is space sized. But it's still 1,000 times smaller than the modern universe. It's full of this hot glowing hydrogen plasma. But at 400,000 years, it's cooled down just enough to form the very first atoms, and in the process, release the cosmic background radiation. We now see this light as an almost perfectly smooth microwave buzz across the entire sky. That smoothness tells us that all of the material in the universe when the CMB was released was almost exactly the same temperature, around 3,000 Kelvin, varying from one patch to the next by at most one part in 100,000 across the entire observable universe. This is weird.

Why is this so weird? If you have a cup of coffee, and drop in some cold milk, it will all smooth out and become the same temperature after a bit of time. Well, the universe works in the same way. But based on the simplistic expansion you predict from general relativity, when the CMB was released, there just hadn't been enough time for this mixing to have occurred. See, in order for the most distant patch of the universe we can see in that direction to have the same temperature and density as the most distant patch in that direction, there needs to have been enough time for something to travel between those points to diffuse and even out that heat. And there just wasn't, not even for light, the fastest thing that there is.