If the universe extended to infinity in all directions, and if stars and galaxies were distributed uniformly — all about the same distance apart everywhere, in space that is otherwise perfectly transparent — then there would be no difference between night and day. The sky would be equally bright day and night — about as bright as the sun — in all directions. We would be flooded in light, which we obviously are not.
Yet, contemplating a universe with an edge or boundary is pretty unsettling. All kinds of questions crop up. For example, if the universe has an edge, what is beyond the edge?
Our observations and our naive thoughts on this matter are so much at odds that the issue is known as Olbers’ Paradox, after the philosopher who first stated the situation in such simple terms.
Let’s call the infinite, homogeneous, isotropic (same in all directions) universe we’re thinking about Olbers’ Universe.
A good way to get your mind around Olbers’ Paradox is to consider where any line of sight you can imagine ends up pointing. Think about using a cardboard paper towel tube as a telescope without lenses.
If you looked through a paper towel tube out into Olbers’ Universe in any direction your line of sight would eventually terminate on the surfaces of stars. Some stars would be close, some would be far, but stars would completely fill the open end of the paper towel tube.
Let’s refine the experiment by using a drinking straw instead of a paper towel tube to better delineate a line of sight. Even still, eventually, at the end of that line of sight would be the bright surfaces of stars.
No matter how fine a line of sight you can imagine, and no matter where you aim it, in Olbers’ Universe every line of sight ends on the surface of an average star — a star like our sun — so the universe is equal in all directions.
It’s plain to see that our universe isn’t infinite, isotropic, homogeneous and transparent. Something has to give.
Maybe space in our universe isn’t perfectly transparent, and light from distant stars is absorbed by some intervening matter — like gas molecules or dust grains.
If our view of distant stars is obscured in this way, then the universe would surely be darker.
Does that do away with the paradox? Not quite, unfortunately.
When a tiny gas molecule or dust grain absorbs a ray of visible light, it has to do something with the energy it so gains. The only thing it can do is heat up, becoming warmer than the cold space in which it floats.
Eventually, a hot molecule or grain has to cool down, and the only way it can do that is by re-emitting one or more different rays of light: maybe visible light again, but most likely infrared light.
Of course, our eyes can’t see infrared light, but sensitive telescopes can.
If we used a sensitive infrared telescope instead of a paper towel tube to view Olbers’ Universe, it would still appear bright. But our universe isn’t bright in infrared light.
So we are still confronted with Olbers’ Paradox even if space isn’t perfectly transparent.
To be continued.
— By Doug Furton, a member of the physics faculty at GVSU. Send questions and suggestions to firstname.lastname@example.org. An archive of some of his “What’s Up” columns is available online at http://gegenschein.wordpress.com/.