Why is the sky dark at night?

by Richard Hall

The answer to this question is not as obvious as it might first seem.  In fact, the answer to this question is the key to understanding the nature of the Universe.  

Olbers’ Paradox

The 19th century mathematician Henrich Olbers was contemplating the grand structure of the Universe when he grappled with this very question.  It is now known as Olbers’ Paradox. Let’s see how it pans out.

On a clear dark night when we look up we see thousands of stars.  How many stars should we see? Well, if the Universe is infinite, we should see the light from an infinite number of stars. Now, the further the distance of a star the fainter it would be. But, Olbers realised that this would be more than compensated by the greater number of stars.  The further we look into space more stars would be visible.  

Is the Universe infinite?

We now know that the stars that we can see are part of the Milky Way galaxy and, while huge, it is finite in size.  However, beyond our galaxy are countless other galaxies.  Our equation simple replaces stars with the light from an infinite number of galaxies.  Olbers soon realised that if the Universe were infinite the entire sky would be as bright as the Sun.  We would be incinerated. Clearly then, the Universe is not infinite.  

However, there is a major problem in this line of thought.  An infinite universe would be stable because gravity would pull equally in all directions. But, in a finite universe it would collectively pull inwards.  The universe would collapse. 

At this point in time the universe has obviously not collapsed.  You could therefore argue that this supports the ‘creationist theory’ that the Universe is only a few thousand years old.  

However, the bulk of stellar and geological science tells us that the Earth, let alone the rest of the Universe, is billions of years old.  This being so, if the Universe were finite, it would have collapsed long ago and we would not be around to ask these questions.  Hence the paradox – finite or infinite Universe, we should not exist.

The Universe is expanding

The paradox was resolved in the early 20th century when it was discovered from the spectra of galaxies that the Universe was expanding.  Further, the more distant a galaxy the faster it was receding from us. Consequently, even if the Universe were infinite, there was a point in space, a distance where galaxies were receding so fast their light would never reach us. They would have passed over the cosmic horizon.  

While this does not tell us whether the universe is finite or infinite it does give us an answer to our original question. The sky is dark at night because… the Universe is expanding.