In the early 1800's, Olber posed the question "why isn't the sky brilliantly white?" Actually it was presented earlier but this dilemma had been known as "Olbers' Paradox". The thought went something like this ....
Imagine you consider the space around us (the Earth) in equally spaced concentric spheres ... so that each sphere acts like a layer of an onion. Now each sphere contains stars which contribute light to our sky. It can be shown that each sphere will contribute the same amount of light to the Earth as any other sphere (the volume of each sphere is greater but also farther away ... a great calculus problem). If each sphere contributes a small amount of light, then as you add up the light from more and more spheres, it starts building, ... and building, until the sky is blinding us. But we all know that does not happen ... it is dark, ... hence, the paradox. Another way to state the problem is to consider looking in any (random) direction. Eventually you will hit the surface of a star, ... and as a result, the sky should be bright.
Your mission is to find a way out of this paradox (using your own words ... please). Actually there are two (common) explanations that resolve the issue. Please see if you can verbalize each solution.
In a way, the universe is aglow ... only not in visible light but. rather, microwaves. This is because when Olber posed this paradox, the universe was assumed to be static and our universe is NOT static. The universe is expanding and that expansion is stretching any radiation from the stars to longer, and longer wavelengths. This "reddening" effect can work itself to an extreme so that the light has shifted to wavelengths that are not even visible to the eye. You should have read in this unit about the cosmic background radiation. The early universe was aglow in gamma radiation. Cosmic expansion has stretched this radiation which we now observe in microwaves.
This brings us to the second part in explaining away Olbers' paradox, and that is the original assumption that the universe was infinite. Our universe is NOT infinite. Therefore, there are not an infinite number of shells to produce a blinding bright sky. There are, however, enough stars that this factor alone is not enough to completely explain the paradox. Cosmic expansion is the biggest factor. Now you can really go crazy on this by bringing up the concept of an "observable universe" which is covered in the very last section of this course. If you have read this far, you will see that the concentric shells showed above do have an "observable limit" .... limited by cosmic expansion. You see, it is possible for a galaxy to be so far away that cosmic expansion means it is going away from us faster than the speed of light (since cosmic expansion is NOT limited by the theory of relativity). In this case we could never see it since it is beyond our observable limit. Simply stated, there are a finite number of observable concentric shells. Talk about mind blowing!!!