Lois & Clark Forums Forums General Discussion Off Topic The Big Bang and the Universe, Part 6: Why is it dark at night?

It's dark at night.



I googled "Why is it dark at night?" and got the following answer from WikiAnswers:

Hmmm, good answer. But, if you managed to contact German astronomer Heinrich Wilhelm Matthäus Olbers, who might not answer you since he has been dead since 1840, you would be told that the Earth's rotation on its axis has nothing to do with the darkness of the night.



Olbers.

The real question is not why it is dark on the Earth circa twelve hours out of twenty-four, Olbers would tell you if he could, but instead, the question is why the universe itself is in a state of more-or-less permanent darkness. It's dark out there! Why is it so dark?

In Olbers' days, most astronomers believed that the universe was infinite and unchanging, and that it had the same average "concentration of stars" that we observe in our own night sky. But if the universe is infinite and evenly filled with stars, then there is also an infinity of stars.

If there is an infinity of stars out there, then that means that when you look up at the sky, you will stare directly into the blinding disk of a star wherever you look. If you look in a certain direction, no matter which one, sooner or later your eye will encounter a star. Remember that stars are suns. If there is a star in the sky in whatever direction you are looking, then the entire sky will necessarily be as blindingly bright as the face of the Sun. There is simply no way of looking away from this terrible blinding brightness.



There are a few ways of solving this problem. One way of solving it is to think that the universe isn't infinite. And if the universe isn't infinite, then there can't be an infinity of stars. And if there isn't an infinity of stars, then there will be directions where you can look without ever encountering a star, and in those directions the universe will be dark.

Another possibility is to imagine that the universe is infinite, but the number of stars are not. It could be that the universe runs out of stars after a while, and then again there will be directions in which you can look without ever encountering a star.

A third way of solving this paradox, Olbers' paradox, which asks why the sky is dark at night, is to invoke the Big Bang and the expanding universe. Because if the universe is expanding, then then the "stellar density" of space will go down when space grows larger, if not a sufficient number of new stars are born to fill all the new emptiness. But in fact, astronomers are confident that the overall star formation in the universe is going down.

Perhaps even more importantly, if the universe is expanding and carrying the stars with it, then all the energy that is emitted by all the stars of this expanding universe will be "redshifted" and "stretched" and "diluted". The energy that eventually reaches us from these stars will be less energetic than t was when it originally left the stars. Stars which are sufficiently far away will have all their energy redshifted into the infrared part of the spectrum, and then their light will simply disappear from our point of view. When we look in the direction of such highly "redshifted stars", there is nothing there to see. The sky is dark there. Stars even farther away will have their visible light shifted all the way into radio part of the spectrum, so that their energy doesn't even reach us as heat.

Indeed, even though Einstein's theory of relativity tells us that nothing can move faster than light, Einstein's theory doesn't forbid space itself to expand faster than light. Therefore, there might be galaxies out there which are so far away from us, and thus are moving so fast (since velocity increases with distance) that they are indeed receding from us at speeds faster than light. Then the light from the stars of these galaxies can never reach us in any form whatsoever. We can gaze in the direction of these stars and galaxies and never detect anything oth there but darkness and emptiness, no matter how incredibly sophisticated and powerful our telescopes might become in the future. By moving faster than light, these stars and galaxies have literally "fallen off the edge of the universe", in the same way that people in the 15th century thought that Columbus might fall off the edge of the world when he was sailing west to find another route to India.



The Earth doesn't have an edge. But in an expanding universe where the recession velocity increases with distance you do reach an "edge of the universe", where things start moving away from you faster than light. Then those things have fallen off the edge of the universe that is observable to us.

The expansion of the universe makes the night sky darker. Indeed, the expansion of the universe makes the night sky dark.



There are many bright galaxies out there. But in our expanding universe, there is a lot of dark emptiness out there, too.

Have you ever wondered what it would be like if the universe started contracting, so that everything in the universe was basically falling toward us? The light of every star out there would be blueshifted and more energetic when it reaches us than it was when it left the star. The entire universe would be blindingly blue-white, far brighter and hotter than the face of the Sun. And I'm not kidding you.



In a contracting universe everything grows blindingly bright, much brighter than in this picture.

Have you heard of Asterix, the brave Gallic comic book character who lives in a little comic book village in France in the year 79 A.D. (or was it B.C.?). Asterix's village is ruled by Majestix, who fears only one thing in the world, namely, that the sky will fall down on his head! And Majestix, that is exactly what would happen if the universe stopped expanding and started contracting instead. First everything would become blindingly bright, and then the entire cosmos would indeed come crashing down upon your head!



Majestix, who realizes that there is no reason to fear the darkness of the night. Let's all be thankful that the night sky is dark and not blindingly bright. The serene darkness of the night sky is paying tribute to the fact that the universe is not in the process of crashing down on itself. Instead the universe is expanding, thinning out and spreading out, taking everything with it.

Ann