Dumb questions with smart answers #23: Why is space dark?

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Save those little specks of light up there, space is awfully dark. It's such a fundamental assumption, I never thought to ask why. But as someone on the sci.astro Google group recently wondered, given that there are billions upon billions of stars in the night sky, most brighter than our own sun, shouldn't they light up the night like a Fourth of July fireworks display? Actually, there's already a name for this question -- Olbers' Paradox, after German astronomer Heinrich Wilhelm Olbers who described it in 1823 -- but it was also asked, perhaps most elegantly, by Edgar Allen Poe (of all people) in his 1848 essay Eureka:

"Were the succession of stars endless, then the background of the sky would present us a uniform luminosity, like that displayed by the Galaxy "“since there could be absolutely no point, in all that background, at which would not exist a star. The only mode, therefore, in which, under such a state of affairs, we could comprehend the voids which our telescopes find in innumerable directions, would be by supposing the distance of the invisible background so immense that no ray from it has yet been able to reach us at all."

Well, Edgar, it seems that the complete answer involves quite a bit of math ("Nevermore!" quoth I after completing pre-calculus in the 11th grade), but the short answer is mathless, fascinating, and a two-parter:

1) The universe is not infinite, and neither is the speed at which light travels. If they were, then the night sky would be as bright as the surface of the sun. As it happens (according to prevailing theories, at least), the universe Banged itself into existence about 13.7 billion years ago (give or take a few hundred million years), so that's all the time light has had to travel from distant stars to our neck of the woods. It would hold, then, that while finite, the universe is considerably more than 13.7 billion light years across, which would help explain why the night sky isn't blinding, and why even our most powerful telescopes can still find patches of nothingness in the Heavens.

2) The universe is expanding, and with that expansion comes a phenomenon known as redshift -- kind of like a visual Doppler effect. Starlight is moving away from us, which shifts its frequency higher, sometimes right out of the visual spectrum and into the microwave spectrum. So huge portions of what would otherwise be potentially blinding starlight has become a steady, cosmic microwave background radiation.

Pretty cool, huh?

September 10, 2007 - 2:00am
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