It's a question raised by writers as diverse as Kepler and Edgar Allan Poe. And it's a serious question, as pointed out by astronomers from de Cheseaux in 1744 to Olbers in 1826. And all of the answers to the question that I have seen to date are wrong in detail, and most are wrong in fact! The question is pretty simple: why is it dark at night? Specifically, why is the night sky black, except for isolated stars and groups of stars seen as single objects because of distance?

Let's look at the argument as posed by de Cheseaux and Olbers. Assume for the sake of the argument that the universe is infinite in time and space. That is, it has always existed and always looked pretty much as it does today. Then, in any direction we look, sooner or later our line of sight will intersect the surface of a star. There would be no “between” for the stars, and thus the entire sky should be as bright as the surface of an average star, say 4000 Kelvin! This is not a trivial “philosophical issue,” since if the argument were correct, it would mean that solid objects could not exist in the universe. The earth and similar bodies would never have formed. Imagine the earth plunged miles below the surface of the sun... that would be the equivalent radiation-environment for every object in the universe!

NO!

YES!

The actual answer to the question may occur to you at once if you recall in detail the contents of a previous page. True, the universe is neither infinite in time nor in space. Beyond a certain distance in both time and space, there are no stars to be seen. But this is not at all the solution to the Olbers Paradox, as it is sometimes called. The reason is that in any direction we look, between the stars, we are looking at the early universe at the moment it became transparent to visible light. That entire universe was then at an average temperature of about 3000 K. So, big deal, instead of the night sky appearing to have a temperature and brightness corresponding to 4000 K, it should have a temperature and brightness corresponding to 3000 K. So sue me! That isn't dark! The entire night sky should appear like the surface of a fairly cool red star, and the earth should environmentally be the equivalent of miles down inside such a cool red star. Why not? Keep reading!

So why is the sky dark at night? [And the earth not at 3000 Kelvin?] Well, the natural night sky isn't really dark, but it's pretty close to black, because it's the color of an incandescent object with a temperature of only 2.7 Kelvin, not 3000 Kelvin. So why is the sky this dark? Because of the  expansion of the universe, which leads to an effect on light that is analogous to a Doppler shift in the frequency of the light we see today from the early universe, the light from the Big Flash. The space between us today and  objects 13 billion light years away has expanded by such a tremendous amount that the light seems to have been emitted by a 2.7 Kelvin object, rather than a 3000 Kelvin object. And that is why the sky is dark at night! And why the surface of the earth is not orange-hot!

The sad addendum to all of this is that in fact very few people today have ever seen the actual dark night sky! Only a small fraction of the US population lives in an area where limited light pollution allows views of the Milky Way and similar night-sky wonders. And this is just as true worldwide. Human population centers have generally surrounded themselves with hideously blazing artificial lights which completely wash out the night sky. When I consider how many people I know who went into science because of a breath-taking view of the dark night sky, during some magic moment of childhood, I wonder what, if anything, would point kids towards science today?