Why is the night sky dark? If the universe were infinite, eternal and evenly sprinkled with stars, every line of sight would end on the surface of a star, and the night would blaze like the face of the sun. It doesn’t. The puzzle is called Olbers’ paradox, and the first to sketch the answer was not an astronomer but a poet: in Eureka (1848), Edgar Allan Poe guessed that light from the farthest stars simply hasn’t had time to reach us.

Modern cosmology agrees, and adds a twist. Space itself is expanding, and old light is stretched on its way, a redshift, until it slides out of the visible range. It is still all around us as the cosmic microwave background, a faint, almost perfectly uniform hum at 2.7 kelvin. The night sky is not dark, only dark to our eyes.