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No 57 - September 2008

Why is it dark at night?

Good heavens (sorry)! What a question to ask, especially in an astronomical journal? Most people would answer: 'It's dark because the Sun goes down.' Well, that's not quite true in Scotland in summer, where the Sun is rather reluctant to go away and, instead, makes darkness rather elusive. In winter, it certainly is dark at night (moonlight permitting), if you can get away from artificial lighting.

Nevertheless, the title poses an important question, more precisely expressed as: 'Why, apart from some luminous astronomical objects, is the night sky dark?' This question was not even asked until Thomas Digges wrote about it in 1576. Kepler thought about it in 1610, and Edmund Halley and Jean Philippe de Cheseaux discussed it in the 1720s.

The question arises because, assuming that the universe is infinitely large and contains an infinite number of uniformly distributed stars, each with a surface as bright as the Sun, the night sky should be blazing with light from these stars. Even if the farther stars are fainter, their number increases with distance and there should be an enormous amount of the starlight reaching Earth. The result should be a 'wall of light' about as bright and as hot as the Sun. Instead the night sky is practically black. To put the question another way then: 'Why is the night sky not as bright as the Sun?' Of course, in that case, there would be no 'night' - hence my title.

Kepler's explanation was that, because the universe of stars extends only out to a finite distance, once the line of sight passes that boundary, it encounters only empty space. This explanation seemed reasonable, but should not have been accepted after 1687 when Newton published his law of gravitation. For stability, his universe needed to be infinite. Kepler's limited universe would collapse.

Later astronomers proposed various solutions to the problem, which came to be called Olbers' Paradox because the German astronomer Heinrich Olbers stated the problem very clearly.

In 1826, Olbers concluded that the night sky is dark because intervening clouds of dust and gas absorb the starlight from distant sources. However, these clouds would heat up from the collection of light energy and glow brightly. In fact they would radiate as much energy as they absorbed and the problem remains. Even if one objects that, nowadays, we know that stars are collected into galaxies, in an infinite universe every line of sight would end in a galaxy and the sky should be as bright due to them as it would be to just stars.

The first scientifically reasonable answer was given in 1848 by the American poet and writer Edgar Allan Poe. He suggested that the universe is not old enough to fill the sky with light. The universe may be infinite in size, he thought, but there hasn't been enough time since the universe began for starlight, travelling at the speed of light, to reach us from the farthest reaches of space [1]. I find that an astonishing insight.

I write about this now because Horst Meyerdierks referred to the matter in a recent article. He wrote: 'it is dark because the universe is quite empty and not infinitely old' [2]. I queried this, explaining that my understanding was that the night sky is dark because the universe is expanding and that, as a consequence, light from distant, receding galaxies has its light red shifted by the Doppler effect so that it becomes invisible infra-red or radio waves [3].

Horst disagreed, claiming that the paradox is about a lack of stars and a lack of starlight in the night sky. He pointed out that there are few stars (he meant stars in other galaxies) at high red shift; consequently the expansion red shift does little to reduce the amount of starlight we have in the night sky. The paradox is solved, he claimed, by there being no stars more than 13 Gyr ago.

I also quoted a claim by someone in a website that, from this paradox, Olbers could have predicted the expansion of the universe. Horst rejected that idea, claiming that, if the one and only universe we are talking about here has all of the properties of being infinite, eternal, homogeneous and static, then the night sky must be as bright as the Sun. Since the night sky is dark, it follows that one or more of these properties does not apply to the actual universe. However, Horst allowed that the expansion red shift does help with the dark sky 'after all'; the faintness of the cosmic background radiation is due to red shift.

Strangely the paradox has not been discussed before in the pages of this Journal.

Confused, I looked for opinions on the web. Wikipedia, whose page on this matter contains a graphic illustration of the sky filling with light, discusses what it calls 'the mainstream explanation' together with alternatives [4]. The former is based on Edgar Allan Poe's idea that not enough time has elapsed for all the starlight to have reached us (since the Big Bang), modified by adding the effect of red shift. The website goes on to claim (confusingly) that the Big Bang model would by itself explain the darkness of the night sky, even if the universe were infinitely old. This is a reference to the Steady State model of the universe, which it points out, cannot explain the detailed behaviour of distant starlight and the microwave background (CBR). The CBR 'requires a continuous transformation of the former into the latter at decreasing frequencies; this transformation is not observed'. So an alternative explanation for the paradox is the finite lives of stars. There is also reference to Charlier's idea of fractal star distribution (1908), which allows for a dark sky even if there had been no Big Bang.

I also looked at several websites dealing with the problem. Explanations varied greatly, as follows:

Evidently some of these explanations are wrong. Yet it is surprising that this should be so and that there should be so much disagreement.

Surprisingly, some dictionaries carry an entry for the paradox. Chambers says that it is 'now explained by postulating a finite expanding universe'. Encyclopedia Britannica's Micropedia claims that:

'if the assumptions [infinite universe] are accepted, then the simplest resolution is that the average luminous lifetime of stars is far too short for light to have yet reached the Earth from very distant stars. In the context of an expanding universe, it can be argued similarly: the universe is too young for light to have reached the Earth from very distant regions.'

However, in EB's Macropedia, Prof. Frank H Shu, noting that it used to be believed that the solution lay in red shifts, reports that the consensus now is that the finite age of the universe is far more important. He points out that there are too few galaxies within the spherical 'cosmic event horizon' which surrounds us to make the night sky bright.

Lastly, I looked for Stephen Hawking's explanation. He states that 'the only way of avoiding ... [the paradox] would be to assume that the stars had not been shining forever but had turned on at some finite time in the past. In that case, the absorbing matter might not have heated up yet or the light from distant stars might not yet have reached us' [12]. Evidently, he does not consider red shift to be relevant.

It is clear that my opinion was influenced by the out-of-date notion that the explanation lies in red shifts. I have had to revise my view.

It appears that Edgar Allan Poe was right. It's dark at night because all the light in the universe has not yet reached us. In fact, much of it will never reach us because it is outside the 'cosmic event horizon', the limit of the observable universe; its sources are receding faster than the speed of light. More fundamentally, it is dark at night because the universe is expanding from an origin some 13.7 billion years ago.

In a way, Kepler was right: the observable universe reaches a limit, beyond which there are no more stars (galaxies), because it is also a limit in time, the time before any stars shone. Horst's notion that the explanation is partly because the universe is relatively empty seems irrelevant. If the universe were infinite, even a relatively empty one would be a blaze of light.

In short, it is dark at night because the universe is finite in size and began a finite time ago. This solution to Olbers' paradox only emerged a century after he outlined it. In 1927, Georges Lemaitre, trying to explain the recession of the galaxies discovered by Edwin Hubble, proposed that the cosmos originated from what he called 'a primeval atom', as Edgar Allan Poe had suggested. Only in 1948 did Ralph Alpher and George Gamow develop the Hot Big Bang Theory.

Notes and references

  1. Eureka: A Prose Poem (Subtitled: 'An Essay on the Material and Spiritual Universe'). It is based on a lecture he gave in New York and includes a cosmological theory that anticipated the Big Bang theory by 80 years. He contended that the universe filled with matter after a single, high-energy particle exploded and that, since the energy of the explosion is pushing matter outward, the universe must be expanding (see Eureka, an annotated edition by Stuart and Susan F Levine, University of Illinois Press, 2004).
  2. 'Atmospheric Extinction' in Journal 56, June 2008.
  3. Isaac Asimov in The Universe (1966), Ritchie Calder in Man and the Cosmos (1968), Iain Nicolson in Astronomy - a dictionary of space and the universe (1977) and Nigel Henbest in The Exploding Universe (1979).
  12. A Brief History of Time (1988).

Steuart Campbell


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