Science Questions

Why does it get dark at night?

Sun, 11th Oct 2009

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Question

Gerjon de Vries asked:

Late last night when I was looking at the stars I was struck by a thought which I would like/expect to see/be proven wrong..

 

Since the consensus is that the universe is infinite, there should be a (close to) infinite number of stars, and quite a bit of these emit light. (Since quite a bit of infinite is .. ah well.. infinite, an infinite number of stars are throwing their light at us, 24/7 so to speak. Now is my question, why does it get dark at night?

Answer

Dave -   Itís actually a really quite a deep question.  If the universe is infinite, then any direction you look in, you ought to end up hitting a star.  And therefore surely, the whole sky ought to be bright white.  The simple answer is, because the universe is expanding, the universe is expanding really quite fast.  So, the further away you go, itís expanding as if you're blowing up a balloon.  So the further away you go, the faster things are moving away from us.  If you go a really long way, tens of billions of light years away, theyíre moving away so fast that all light coming from it has been really, really red-shifted.  So, itís been so red shifted,that we canít see it.  In fact, it has been so red shifted that itís in the microwave region in the spectrum.  So you can see light in every direction, but itís been so red shifted that our eyes canít see it.  So the sky looks dark.

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