Continuing the Big-Bang Timeline

30 min < t < 380,000 million years
- The universe is too hot for electrons and protons to form hydrogen atoms. It is ionized.
- Light cannot travel through ionized gas. The universe is opaque.
- Its blackbody radiation is trapped.
t ~ 380,000 years.
- The universe has cooled to about 3,000 K, hydrogen atoms form.
- The universe becomes transparent, the 3,000 K blackbody radiation travels freely.
- We see that black body radiation today, greatly red-shifted. It now looks like a 3K blackbody.
- It is the farthest thing we can see. It defines the edge of the observable universe.

30 min < t < 380,000 million years