Astronomers have found when and how the cosmic fog was lifted

Astronomers have found when and how the cosmic fog was lifted was a Bad Astronomy article from a month ago (yes I'm catching up on old saved links). I know that when the universe was about 376,000 years old, it became transparent and that produced the Cosmic Microwave Background radiation which we can detect today and which is one of the significant pieces of evidence of the Big Bang.

The "became transparent" was always a little vague to me. I thought it was because the universe expanded enough that it cooled from a plasma to normal matter (mostly Hydrogen gas). Well that's true but this article, unusually, goes one step further and talks about photons interacting with free (ionized) electrons and how we measure the ionization that long ago. And it's still accessible. Good stuff.