matt grant

why the sky is dark at night

When we look up at the night sky, we see stars scattered across a mostly dark background.

Why is this?

If the universe was infinite, eternal and static, then no matter what direction we looked in, we’d end up looking at a star. So the night sky would be completely illuminated. However this doesn’t match our observations.

These assumptions form the basis of Olbers’ Paradox.

The most common resolution of this paradox was proposed by Edgar Allen Poe and goes as follows:

  1. The universe is finitely old.
  2. The speed of light is finite.
  3. Therefore only a finite number of stars can be observed from Earth.

The fact that most of the sky is dark is down to the density of stars within this finite volume - it’s sufficiently low that each observable point in the sky is unlikely to intersect with a star.

Another paradox, concerning the temperature of the early universe and the Big Bang theory, is resolved by considering cosmic expansion. In short, all of the energy from Big Bang is now microwaves.

But what does this expansion mean for our chair?

Nothing. It’s the scale of space which is changing. Everything in it stays still.

The models defining this only apply if we’re talking about things the size of galaxy clusters and fortunately our chair is a little smaller than that. I’m also (relatively) close to the chair. If I was further away, the calculation gets more complicated.

So I’m confident I can find the chair again. Now what time will I arrive…