Think about all the luminous energy constantly bathing our planet. Space contains billions of galaxies, spread out uniformly through the sky. Journeying in any direction, the farther out you go, the more galaxies you’d encounter—passing one after another like mile markers on a Nebraska highway. And a typical galaxy pumps out light at a colossal rate. All these sources of illumination, added up, should be enough to rival sunlight and put an end to the dimness when the Sun isn’t even in view. Then why is endless space and, therefore, the night sky not ablaze with light?
The dark sky riddle slipped into astronomical folklore thanks to Heinrich Wilhelm Olbers, a German physician and amateur astronomer. In 1826 he argued that in a uniform, infinite universe populated by eternal and unchanging stars we could potentially see an arbitrarily large number of them. The farther away we looked, the more and more we’d see, because the number would increase as the cube of the distance.
This cubic relationship is like baking larger and larger blueberry muffins. While a thimble-sized pastry might be large enough to contain one blueberry, double each of its dimensions, and it might easily accommodate eight. Make it 10 times bigger in diameter and height and it might even pack in a thousand such morsels. Imagine all the juice that would leak out if all of these were to burst while baking. Similarly, picture all the light produced by greater and greater scopes of stars, each shining in all directions. The amount of illumination heading toward us would be tremendous.
Given a vast-enough cosmos, every single point on the sky should glow with the light of a brilliant star. Bombarded with the colossal radiation of myriad luminous objects, we should need to wear sunglasses night and day. The fact that this does not happen and that the nocturnal sky looks dark has become known as Olbers’ paradox.
Scientists wrestled for more than a century and a quarter with this dilemma before beginning to zero in on two conceivable solutions. One possibility, promoted in the 1950s and 1960s by