In the post, “The Dark, Dark Sky (Part One),” Olber’s paradox was introduced. To recap, this is the problem that says if the universe is truly filled with stars spreading out to infinity then why is it that the sky is dark at night? Would not we instead expect for the light from an uncountable number of stars to make the sky piercingly bright at night?
We learn about distant objects in the universe most by receiving the light from stars. This light travels fast, mind you, but importantly not infinitely fast. We actually have to wait for light to reach us from some of the more distant places. For example, the light from the nearest large spiral galaxy that you can see through a pair of binoculars travels 2.2 million years to reach your eyes. The light from still more distant objects takes even longer to reach telescopes.
In fact, the universe is so big that some of the light from the farthest reaches is still journeying towards us but has not yet arrived (assuming we had the gigantic telescopes that could detect it). It is as if we are trying to assemble a puzzle on a coffee table and do not know how big of a coffee table we should use because we do not know the dimensions of the puzzle. This analogy breaks down a bit beyond this point as we do not think the universe has edges like a puzzle does. Still, it gives us the general idea.
Now then, one of the two parts needed to answer Olber’s Paradox is that the sky is not 100% covered in stars at night because we are still waiting for some light to reach us from the farthest reaches of space. The universe may be bigger than the part that we can see, called the ‘visible’ universe. In any event, the visible universe is not infinite.
The other part of the answer is that we know the universe is expanding away in all directions, similar to the surface of a balloon as you blow it up. The light that we receive at our telescopes from objects that are expanding away from us has lower energy that light emitted from a static (non-expanding) universe. In this way each star contributes less light to our night sky, further operating to make out night sky fainter.
In sum, the fact that the sky is dark at night is a profound observation which is explained by the fact that the universe is finite and is expanding with time.