The Paradox of the Dark Night Sky

ByANDY FLEMING

Some of us in urban areas such as London, New York or here in Teesside suffer massive problems with sky glow and light pollution.  Infact, so much so that we’re robbed of some of nature’s true spectacles; deep sky objects such as such as galaxies and star clusters with low surface brightness.

You may never have thought it, but velvet black night skies upon which the planets, stars and galaxies are cast create rather more of a scientific problem. It’s named after the German astronomer who first defined it, Wilhelm Olbers (1758-1840). It’s the oldest cosmological paradox of them all, and it was raised as a result of the fact that for centuries philosophy and science was beginning to point in the direction of a cosmos that was both infinitely old and infinitely large.

If you think about it logically, an infinite, ageless universe dictates that the whole sky both by day and night shouldn’t actually be dark at all, but should glow with the brightness of an infinite number of stellar furnaces.  After all, light with its finite speed would have had time to travel everywhere in infinite time, and every single direction and path that you look at in the sky should end at a point source of light or in other words, a star.

German astronomer Wilhelm Olbers

Over the years many possible solutions to Olber’s Paradox have been offered, such as:

Light from the distant stars is blocked by dust.

There’s a finite number of stars in the Universe.

Stars are not distributed evenly. So, for example, there could be an infinity of stars, but they hide behind one another so that only a finite angular area is subtended by them.

The Universe is expanding, so the light from distant stars is red-shifted into obscurity.

The Universe is young. Distant light hasn't even reached us yet.

Scientific analysis shows the first explanation to be incorrect, the dust will absorb starlight and radiation and will heat up too. There are many examples of such glowing 'absorption nebulae'. It does act like a radiation shield, exponentially damping the distant starlight. But you can't put enough dust into the universe to get rid of enough starlight without also obscuring our own Sun.

The premise of the second explanation may technically be correct. But the number of stars, finite as it might be, is still large enough to light up the entire sky, that is the total amount of luminous matter in the Universe is too large to allow this escape. The number of stars is close enough to infinite for the purpose of lighting up the sky.

The third explanation might be partially correct. We just don't know. If the stars are distributed fractally, then there could be large patches of empty space, and the sky could appear dark except in small areas.

But the final two possibilities are indeed correct and each partly responsible. There are numerical arguments that suggest that the effect of the finite age of the Universe is the larger effect. We live inside a spherical shell of "Observable Universe" which has radius equal to the lifetime of the Universe. Objects more than about 13.7 billion years old are too far away for their light ever to reach us.

The resolution of Olber's paradox is found in the combined observation that the speed of light is finite (although a very high velocity), and the Universe has a finite age, i.e. we only see the light from parts of the Universe less than 13.7 billion light years away.