The oldest and simplest astronomical observation tells us something profound about the universe. The sky is dark at night. It isn’t obvious why this should be so. If you stand in a small grove of trees and look toward the horizon, you can see patches of sky in the distance between the tree trunks. But if you stand in a large forest, your view is everywhere blocked by a “solid wall” of tree trunks. Extending the analogy to three dimensions, if the universe of stars is large enough, your line of sight should be blocked in every direction by a “solid wall” of stars. If you could magnify that view sufficiently, the sky would everywhere look something like the image on the left.
The entire sky would be about as bright, and as hot, as the surface of the Sun. The immense distance to the stars making up the “wall of light” would have no effect on the total amount of energy reaching us. We should be surrounded by a blazing oven of light. Instead the night sky is practically black. So where does the argument go wrong?
The German astronomer Johannes Kepler first posed this problem in 1610. He also suggested a solution: the universe of stars, he believed, extends only out to a finite distance; once your line of sight passes that boundary, it encounters only empty space. But how far is that boundary? Why is it there? And what lies beyond it?
Astronomers after Kepler proposed various solutions to the problem of the dark night sky, which came to be called Olbers’ Paradox. In 1823, the German astronomer Heinrich Olbers suggested that starlight is gradually absorbed while traveling through space, and this cuts off the light from any stars beyond a sufficiently great distance. But that doesn’t solve the problem, either. Any absorbing interstellar gas or dust would simply heat up until it reradiated all the starlight it absorbed, and the energy reaching us would be the same. By analogy, sprinkling the air in a hot oven with absorbing dust won’t cool it for very long.
So why is the night sky dark? The first scientifically reasonable answer was given in 1848 by the American poet and writer Edgar Allan Poe! He suggested that the universe is not 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.
Astronomers have concluded that the universe began some 12 to 15 billion years ago. That means we can only see the part of it that lies within 12 to 15 billion light-years from us. There may be an infinite number of stars beyond that cosmic horizon but we can’t see them because their light has not yet arrived. And the observable part of the universe contains too few stars to fill up the sky with light.
But that is not the whole solution to the paradox. Most stars, like the Sun, shine for a few billion years or so before they consume their nuclear fuel and grow dark. Dying stars spew gas and dust back into space, and this material gives birth to new generations of stars. But after enough generations, all the nuclear fuel in the universe is eventually exhausted, and the formation of luminous stars must come to an end. So even if the universe were infinitely old as well as infinitely large, it would not contain enough fuel to keep the stars shining forever and to fill up all of space with starlight. And so the night sky is dark.