Why is the Sky Dark at Night ?

Having been brought up in a world where the natural state of the night sky is darkness (unless we live north of the Arctic Circle), most people would respond to the question by asking: “Why shouldn’t the night sky be dark? Without an obvious source of brightness – the sun – it is hardly surprising that after sunset we look out into a black void punctuated only by the pinpricks of stars, and of course the moon from time to time.” However this is one of those simple questions that scientists have asked in the past which lead to surprising and quite deep answers

When this question was first asked, the universe was believed to be infinitely large, with an infinite number of stars. If that were the case, wherever you looked in the night sky your line of sight would eventually interact with the surface of a star. It would be like looking around you in a forest with an infinite number of randomly scattered trees. Wherever you looked your view would eventually be blocked by a tree trunk.

So if wherever you looked in the sky your line of sight met a star, you would expect the entire night sky to be as bright as the surface of a star.

Edgar Allan Poe (THE Edgar Allan Poe) reasoned out an explanation and include it in what he called a prose poem:

What is wrong with this reasoning? …nothing actually, but it’s not the answer.

Another explanation might be … : Perhaps light from some of the distant stars is dimmed by dust in the way. But in fact, the dust would not make the starlight disappear. There is interstellar dust but it is actually heated up by starlight and re-radiates it, so the total amount of light is still the same.

Yet another explanation might be … : We now believe that the universe began with a Big Bang about 13-15 billion years ago. Starting from a single point the universe expanded, very rapidly at first and then more slowly, and today it is still expanding with a ‘frontier’ that is about 13-15 billion light years away. So perhaps the darkness of the night sky is because, beyond that frontier, there are no stars to contribute to the brightness of the night sky, just as if the trees in the forest, we mentioned earlier, stopped 15 miles away and we could see chinks of light beyond the edge of the forest. Again it’s another nice try, but a British mathematician has shown that even in the limited universe we inhabit, there are enough stars scattered over the sky out to the current frontier to produce a night sky that would be blazingly bright.

So we need another explanation…

The fact that the universe is expanding was not accepted until early in the twentieth century. One implication of this is that stars moving away from us seem less bright because of the Doppler effect . As with the change in the frequency of sound from a moving source, the colour of light changes depending on the speed of travel, and light from a receding star becomes redder. Since the mammalian eye has evolved most sensitivity to the spectrum of colours that make up white light, as the starlight shifts towards the red, some of those colours drop out and the light seems less bright. So one reason the night sky is not as bright as we would expect is probably that the fast-receding stars that should be as bright as the slower, nearer ones are much dimmer as a result of the Doppler shift.

This explanation might be sufficient, although new knowedge about the Big Bang and the expansion of the universe has introduced another factor. The Big Bang must have been very bright, because of the energy we know was concentrated in one spot. Shouldn’t we therefore still be aware of that brightness in the night sky? As it happens, it is possible to detect the bright ‘echo’ of the Big Bang , but as with starlight, the Doppler effect due to the rapid expansion of the universe has shifted the colour of that early flash of light far beyond the red end of the spectrum and it’s now detectable only as microwave radiation.

More Information : The Doppler Effect

Heard an ambulance go by recently? Remember how the siren’s pitch changed as the vehicle raced towards, then away from you? First the pitch became higher, then lower. Originally discovered by the Austrian mathematician and physicist, Christian Doppler (1803-53), this change in pitch results from a shift in the frequency of the sound waves, as illustrated in the following picture.

As the ambulance approaches, the sound waves from its siren are compressed towards the observer. The intervals between waves diminish, which translates into an increase in frequency or pitch. As the ambulance recedes, the sound waves are stretched relative to the observer, causing the siren’s pitch to decrease. By the change in pitch of the siren, you can determine if the ambulance is coming nearer or speeding away. If you could measure the rate of change of pitch, you could also estimate the ambulance’s speed.

By analogy, the electromagnetic radiation emitted by a moving object also exhibits the Doppler effect. The radiation emitted by an object moving toward an observer is squeezed; its frequency appears to increase and is therefore said to be blueshifted. In contrast, the radiation emitted by an object moving away is stretched or redshifted. As in the ambulance analogy, blueshifts and redshifts exhibited by stars, galaxies and gas clouds also indicate their motions with respect to the observer.