Let’s start off with a conundrum that began perhaps 400 years ago with Johannes Kepler. To put it simply: if the universe is infinite, then why isn’t the night sky white? That is to say, if the universe is infinite, then no matter which point of the night sky you look at, then there should be a star at a determinable distance, and irrespective of that distance and the effect on its luminence, the fact that the universe is infinite would mean that there would be no night and the black of night should in fact by white. Of course, it isn’t.
This phenomenon is better known as Olbers’ paradox, after questions posited by the astronomer Heinrich Wilhelm Olbers in 1823, although the principle had been considered earlier. The theory makes some basic assumptions that are obviously debatable, such as that every point of view should eventually terminate on the surface of a star, and that the light would be as bright as the surface of our sun, but arguments aside the allusion to looking at a forest and being unable to see the horizon beyond it (for always being interrupted by the bark or leaves of a tree) makes a decent analogy.
There are a number of assumptions that can be made as a result of this theory. The universe might not be infinite, that much seems clear. Indeed, one theory assumes that since the universe is not infinite, and the life of a star is decidedly finite, there need be no cause for concern that the night sky is not as bright as the surface of our sun, since the space between stars is sufficient that the limited lifespan of a star and its reduced luminance over such extreme distances combine to make our speckled night sky the only logical outcome. Another theory, posited as early as 1908 though rediscovered by Benoît Mandelbrot in 1974, is that whilst the universe might be infinite, the distribution of stars need not be random. Olbers’ paradox relies on a relatively uniform distribution of stars to hold true, whilst Mandelbrot (and others) assumed a fractal distribution for stars in the universe, that whilst not necessarily accurate, certainly provides a possible explanation to Olbers’ paradox.
Another logical conclusion to the paradox is that whilst the universe is infinite, and the night sky has the potential to be as bright as the light of day, the light from distant stars does not reach us because it is instead absorbed. However, this theory appears to contravene the laws of thermodynamics, since a body which absorbs energy cannot maintain energy above those of its surroundings without giving off radiation uniformly around its body (albeit possible different to the energy absorbed). Whilst the theory does sufficiently answer the question as to why our night sky is not bright, it is not able to answer why we are not uniformly bombarded with any other form of radiation that might be given off by the intervening bodies which absorbed the light in the first place. There may of course be extensions to this theory that I am neither aware of nor qualified to discuss.
Nevertheless, the most popular theory which satisfies Olbers’ paradox is better known to us from elsewhere as the famous Big Bang Theory. The theory provides us with three very basic premises which offer to explain the paradox in full. Firstly, and most fundamentally, the speed of light is finite. Whilst this is not exactly a basis for the Big Bang Theory, its principle is often employed throughout our scientific framework, and the paradox cannot be explained without. The second premise is that the universe has a finite age. Combining these two assumptions, one can determine that it is not possible to view anything beyond a certain distance (i.e. the distance light can have travelled since the dawn of the universe—or better light within the universe). Finally, the additional factor of universal expansion, and the concomitant Redshift or Doppler effect, which results in a perceived lengthening of wavelengths compared to those emitted at the source, combines with the former two factors to explain the paradox.
So whilst there are a number of possible explanations for Olbers’ paradox, the most widely accepted theory relies on a universe that is neither infinite, nor steady, both factors inherent in the Bang Bang cosmological model.
