Why is it dark at night?

You might think that this is a rather trivial, even silly, question to ask. After all, even a child 'knows' that this is because the Sun sets below the horizon, and since there is nothing else in the sky anywhere near as bright as the Sun we have to make do with the feeble reflected light from the Moon and even more feeble light from the distant stars. Well, guess what? It's not as simple as that!

We have good reason to believe that even if the Universe is not infinite in size, it is probably so enormous that, for all intents and purposes, it is infinite. If so, then we come up against something known as Olbers' paradox. Simply stated, this says that the night sky has no right being dark at all. It should be even brighter than it normally gets during the day. In fact, the sky should be so bright, all the time, that it should not even matter whether the Sun is up in the sky or not.

Now consider a simple model universe that is infinite, static and with stars evenly spread out. It does not matter where we look in the sky, if the Universe is infinite we should always see a star in our line or sight. So there would not be any gaps in the sky where we do not see a star and the whole sky should be as bright as the surface of the Sun, all the time!

So why then, is it then that it is dark at night? (To be continued..)

Note: These are extracts from the book "Black Holes, Wormholes & Time Machines" by Jim Al-Khalili.