What is Olbers' Paradox?

Here is a picture of the Virgo Cluster courtesy of Matt BenDaniel. Check out his webpage at http://www.starmatt.com.
(Selected as a NASA Astronomy Picture of the Day)

Look how the sky between the stars appears dark. If the universe were infinitely big and infinitely old, then the whole sky should appear to glow.

Why isn't the night sky uniformly bright?

In the 19th century, an astronomer named Heinrich Olbers stumbled on a contradiction that could not be easily explained: why doesn't the night sky look uniformly bright? A sky that is uniformly bright would appear to glow, yet our night sky appears black with a scattering of stars, planets, and galaxies dotting the observable sky.

Why do we expect the night sky to appear uniformly bright in a Steady State Universe?

To illustrate, Steady State theory is based on three previously-mentioned assumptions: the universe is

  1. uniform
  2. not expanding
  3. of infinite proportions
Let's examine this a little more closely.

Seeing a star in any direction...

If the universe extends infinitely, then eventually if we look out into the night sky, we should be able to see a star in any direction, even if the star is really far away.
Like we said, steady state theory is based on the assumption that the universe is not expanding, so that means that light waves travelling to us from that star would stay at the same wavelengths as they reached us. Moreover, if the universe was infinitely old, then the light from stars at extremely far distances would have already reached us, even if they were 40 billion light years away. Remember, in this scenario, the distance between us and the stars does not matter because we are assuming the universe is infinitely old.

So, what does all this mean? Well, we just proved that according to Steady State Theory and a little common sense, we should be able to see a star anywhere in the night sky, and so the sky should have the same brightness everywhere. But as you all know, if you look at the sky at night, it's dark and speckled with bright points of light called stars! How can this be explained? Something seemed to be amiss….

for more info, go to Olbers' Paradox

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