why is the night sky dark?

How I got here

Started with

If the universe is full of stars, why isn't every direction bright?

Ended at

Cosmic microwave background

The Trail

1. “why is the night sky dark if there are infinite stars” (google.com)
2. Olbers' paradox (en.wikipedia.org)
3. Eureka: A Prose Poem (en.wikipedia.org)

   Edgar Allan Poe basically guessed the modern answer in 1848??

4. Age of the universe (en.wikipedia.org)
5. Cosmic microwave background (en.wikipedia.org)

   the sky isn't actually dark, it glows at 2.7 K. just not in a color we can see

What I came away with

If the universe were infinite, eternal and static, every line of sight would end on a star and the sky would blaze. It's dark mainly because the universe has a finite age, so light from far enough away hasn't reached us yet, and expansion redshifts much of what does arrive.

Keep going

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