There are a lot of stars out there--an estimated 70 billion trillion. With so many stars beaming their light our way, it seems only logical that the night sky would be as bright as day.
This is the essence of the so-called dark night sky paradox, also known as Olbers' paradox after German astronomer Heinrich Wilhelm Matthias Olbers (1758-1840).
Farthest-ever view of the universe, taken by the Hubble Space telescope. Even at this extreme magnification, big gaps are still visible between galaxies.
Though Olbers was the first to formally describe the paradox, in 1823, credit for resolving it lies elsewhere. Some say it was first resolved in 1901 by British physicist Lord Kelvin (1824-1907). But according to the website of the American Museum of Natural History, the first plausible explanation for the paradox came in Eureka: A Prose Poem, an 1848 essay by Edgar Allen Poe.
As the museum's website has it, Poe suggested that the universe simply isn't old enough to fill the sky with light:
The universe may be infinite in size, he thought, but there hasn't been enough time since the universe began for starlight, traveling at the speed of light, to reach us from the farthest reaches of space.
That turns out to be surprisingly close to the explanation given by present-day astronomers.
"As we look out, we are seeing back in time, and by the time we see 12 billion years back, the universe is only a couple of billion years old and there isn't stuff there for us to see," Dr. Jay M. Pasachoff, professor of astronomy at Williams College in Williamstown, Massachusetts, and co-author of The Cosmos: Astronomy in the New Millennium, told The Huffington Post. "The main solution to Olbers's paradox is, then, that the universe isn't old enough for stars and galaxies to fill our view as we look outward."
Or, as Dr. Anil Chandra Seth, assistant professor of physics and astronomy at the University of Utah in Salt Lake City, told The Huffington Post in an email: "There are a couple reasons the sky isn't bright. First off, the universe is expanding and has a finite age; this means we can't see galaxies infinitely far away (even if the universe is infinite). Also, the expansion means that light loses energy as it travels."
In addition to these two reasons, Dr. Seth said some of the light emitted by all those stars is absorbed by interstellar dust in the Milky Way, the galaxy that contains our solar system.
"If it weren't for dust," he said, "the Milky Way would be much brighter in the sky."
Another reason the night sky is mostly dark has to do with our eyes' insensitivity to the wavelengths of light that reach the Earth from the most distant stars.
Because these stars are speeding away from us very rapidly, their light is "red-shifted" into longer wavelengths--in much the same way that the sound of an ambulance's siren is of a lower pitch when the ambulance is speeding away from us than when it's heading toward us.
Ultimately, as Pasachoff and his co-author put it in The Cosmos, questioning why the night sky is dark instead of light leads to "incredibly interesting possible conclusions regarding the nature of the Universe. So the next time your friends are in awe of the beauty of the stars, point out the profound implications of the darkness, too!"
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