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### Why is the Universe Black?

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Date: 04/11/2001 at 08:25:31
From: Shane Kenyon
Subject: Light in the universe and black hole

I was wondering...why is the universe black?

I also wanted to know if there is a such thing as a black hole that
sucks everything into it including light. If there is, wouldn't space
be void?

Koron
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Date: 04/11/2001 at 09:02:14
From: Doctor Mitteldorf
Subject: Re: Light in the universe and black hole

Dear Koron,

The first question you ask has a history dating back to 1744, when
Jean Philippe Olber first posed "Olber's paradox." I've done a Web
search on that topic, and here are a few places you can read about it.
One short explanation is that it is the stars very, very far from us
that would make the night sky bright as a star; but the expansion of
the universe weakens the light that comes from these stars, so that
the total amount of light from them ends up being quite small.

Olber's Paradox: Why is the Night Sky Dark?
- Cornell University Astronomy

http://pages.prodigy.net/jessieann/Space/olbrpara.html

Olber's Paradox - National Schools' Observatory, U.K.
http://www.schoolsobservatory.org.uk/study/sci/cosmo/internal/olbers.htm

Olber's Paradox - Eric Weisstein, Astronomy

these objects and think of them as "sucking up" everything around
them. One thing to remember about a black hole is that from far away
they have the same gravity as anything else. For example: the Earth
revolves around the sun, pulled by its gravity. If at this very moment
the sun suddenly implodes and becomes a black hole, with the same mass
that it had before it imploded, then the earth would continue around
in exactly the same orbit that it's in now.

The thing that makes black holes special is just their tiny size.
Take the sun again: now, you can't get within 100,000 miles of the sun
because you'd be inside it. Even if you were inside it, you wouldn't
be pulled by the sun's whole gravity, because some of the sun would be
"outside of you." This keeps the sun's gravity from ever getting
really, really strong. But if the sun collapsed into a black hole, it
would be less than 10 miles across. All that mass, concentrated in a
tiny space! Now you could get really close to it, and the gravity
would be terrifically strong, so strong it could "suck up" light. But
you see that in a sense the gravity didn't get stronger, only more
concentrated. The strange properties of black holes come from the fact
that they're small, so you can get close in.

- Doctor Mitteldorf, The Math Forum
http://mathforum.org/dr.math/
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