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Why is the Universe Black?


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


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://astrosun.tn.cornell.edu/courses/astro201/olbers_paradox.htm   

  Olber's Paradox - Creation Moments
  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
  http://scienceworld.wolfram.com/astronomy/OlbersParadox.html   

Your second question is about black holes. Many people misunderstand 
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/   
    
Associated Topics:
High School Physics/Chemistry

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