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Sunday, April 28, 2013
On the next clear, moonless night, head away from city lights and look up. You will notice a basic fact of the night sky: It is dark. It may be hard to believe, but the reason for the sky’s darkness eluded astronomers from when the issue was first raised in the early 1600s until its solution was proposed in the mid-1800s. In fact, the mere existence of the dark night sky went against the cosmological principle, one of the foundations of the reigning model of the universe. It predicted that the night sky must appear as bright as the sun. Clearly, some facet of it must be wrong.
The cosmological principle held that the universe is infinite and, on large scales, appears much the same throughout. Astronomers reasoned that, on average, stars are spread uniformly in space and that the more distant a star lies from Earth, the less brightly it shines. The closer stars may be brighter, but the more distant stars are much more numerous, contributing just as much light as the closer ones. When all the light from the nearby stars and all the light from the great many more distant stars is combined, their total brightness should equal the luminosity of the sun.
Astronomers also reasoned that, because space was infinite, no matter which direction you looked, your gaze would eventually fall on the surface of a star. Hence, the entire sky should be as bright as a star’s surface.
Today we know that stars aren’t uniformly distributed in space but are grouped together in immense star cities known as galaxies, each containing hundreds of billions of stars. The space between the stars isn’t empty, either. It contains vast but tenuous clouds of light-diminishing gas and dust.
These two facts still don’t explain the discrepancy between the sky’s predicted and actual appearances. Galaxies can be substituted for stars in this discussion because they are just huge glowing agglomerations of stars. The light from each one of the nearly 200 billion galaxies would add to the brightness of the night sky. Interstellar gas and dust within the galaxies, while initially blocking and absorbing starlight, eventually would release the light, causing that light to reach our eyes on Earth, brightening the night sky.
So why was the night sky dark? Astronomers were flummoxed.
This conundrum was popularly described in 1823 by the German amateur astronomer Heinrich Wilhelm Olbers and now bears his name — Olbers’ paradox. Quite fittingly, the paradox was solved in 1848 by a complete non scientist. Edgar Allan Poe wrote in “Eureka, A Prose Poem,”his musing on mankind’s discovery of the universe and God’s role within it:
“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.”
In other words, the universe is not old enough for the light from its most distant stars to have reached us yet. Astronomers in Poe’s day didn’t know that the universe is “only” 13.7 billion years old and that there aren’t enough stars in our finite, observable universe to light up the night sky.
Isn’t it incredible that some of the most profound cosmological concepts are confronted when you look up?
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