Why is the night sky dark? The answer reaches back to the Big Bang.
If the universe were infinite, eternal and full of stars, every line of sight would end on a star, and the night sky would blaze like the surface of the Sun. It doesn't. The darkness between the stars is a clue that the universe is not infinite, eternal and unchanging.
The puzzle is named after the German astronomer Heinrich Olbers, who described it in 1823, though he was far from first. Thomas Digges, an early English champion of Copernicus who imagined an infinite universe full of stars, came close; Johannes Kepler posed the problem in 1610, and Edmond Halley and Jean-Philippe de Chéseaux sharpened it in the 18th century. The historian of cosmology Edward Harrison argued that Olbers's own contribution was not especially valuable.
The logic is simple. Imagine the universe as a series of shells around Earth, each the same thickness. A shell twice as far away contains four times as many stars, but each looks a quarter as bright, so every shell sends us the same amount of light. Add infinitely many shells and the sky should be infinitely bright. Even if dust blocked the light, it would heat up until it glowed like the stars themselves.
The first published scientific solution came from Johann von Mädler in 1858, who suggested the universe has a finite age. Harrison credits Lord Kelvin with the first satisfactory mathematical resolution, in a little-known 1901 paper. Strikingly, the poet Edgar Allan Poe had anticipated part of the answer in his 1848 essay Eureka, suggesting the most distant light had not yet had time to reach us.
The modern answer combines two effects. Because the universe has a finite age, we can only see stars whose light has had time to arrive, so the number of visible stars is limited. And because the universe is expanding, light from the most distant regions is stretched to longer wavelengths. Look far enough in any direction and your line of sight does end on a glowing surface, the hot early universe itself, but its light has been redshifted all the way into microwaves at 2.7 kelvin. The sky is full of light after all. Our eyes just cannot see it.
Source: Wikipedia — Olbers' paradox · Text summarised from Wikipedia (CC BY-SA 4.0)
