Dear Cecil: I’m enclosing an article that poses a question that had never occurred to me before: Why is the night sky dark? According to the author of the article, Robert Cowen, “the traditional answer holds that the universe is expanding so fast that light from the distant stars is degraded and thinly spread.” Another theory suggests, “the darkness is better explained by the simple fact that the universe is of finite age. Galaxies have not had time to flood the sky with starlight.” Excuse me, but aren’t we overlooking the obvious here? Bill, Nanaimo, British Columbia
This is one of those questions so bizarre that only serious drugs or an astronomer could be behind it. But it’s not as nutty as it might seem. We can rule out one obvious answer right off the bat: the night sky isn’t dark merely because the sun goes down. The stars alone ought to be enough to make the night sky intensely bright.
Think about it this way. If we assume the universe contains an infinity of stars scattered in endless space, we should see a star in any direction we look. It’s like being in the middle of a forest — all you can see in any direction is tree trunks. The sky should be so completely filled with pinpoints of light that they should all merge into a uniform white glow.
Clearly it doesn’t work that way, a puzzle astronomers call “Olbers’s paradox.” Why not? We can nix a few possibilities: The light emitted by the most distant stars is so faint it’s below the threshold of vision. Forget it. You can’t see an individual glowing atom, but you can see zillions of them massed together in a candle flame. The same ought to hold true of a horde of distant stars. The most distant stars are obscured by interstellar dust. Won’t work either. The dust would absorb so much light it’d eventually start glowing itself.
So what does explain the paradox? After 400 years of debate on the question, there is now fairly wide agreement among astronomers: there just aren’t enough stars in the observable universe to fill up the night sky.
Your reaction to this may be: It took scientists 400 years to come up with that? No wonder we haven’t found a cure for the common cold. But I’m making the answer seem simpler than it is. We don’t really know how many stars there are. What we do know is that however many there are, we can see only a finite number of them.
The oldest stars are about 10 billion years old, meaning that the greatest distance starlight can have traveled is 10 billion light-years. So the only stars we could possibly see are those within a 10 billion light-year radius of us — the light from stars farther away has yet to reach us. The few jillion stars in our corner of the cosmos (AKA the “observable universe”) don’t have the collective candlepower to illuminate the night sky. (True, as time goes on, light from more distant stars does reach us, but meanwhile some close-in stars are dying out.)
So that’s why the night sky is dark. All right, it’s a complicated way of telling you what you might have guessed anyway. But sometimes the obvious ain’t.
Send questions to Cecil via firstname.lastname@example.org.