Step outside on a clear night and notice something so obvious it feels like a non-question: the sky between the stars is black. It has no business being black.

A child's question with a monstrous answer

Suppose the universe is infinitely big, unchanging, and sprinkled evenly with stars in every direction, forever. Now look out along any line — straight up, off to the side, anywhere at all. Keep going. In an endless field of stars, that line, sooner or later, has to land on the surface of some star. Every single line of sight must.

If that were true, the whole sky would be a seamless wall of starlight, as bright as the face of the Sun, glaring down from every direction at once. There would be no night. There would arguably be no life, in an oven like that.

But the sky is dark. So one of those innocent-sounding assumptions — infinite, unchanging, evenly filled — has to be false.

The puzzle that outlived its name

This is Olbers' paradox, named for a German astronomer who wrote it up in 1823, though he was far from the first to be nagged by it. Kepler had worried at it two centuries earlier and took the darkness as proof the universe must be finite. Others tried to hide the far stars behind clouds of dust — but dust in an eternal, blazing universe would simply heat up until it glowed as brightly as everything else.

The most beautiful early guess came from an unexpected quarter. In 1848 the poet Edgar Allan Poe, in a strange book-length prose poem called Eureka, suggested that the more distant reaches of space simply had not had time to send their light to us yet. He had no equations. He was essentially right.

The darkness overhead is not empty space. It is a record of the fact that the universe had a beginning.

What the black is telling you

The resolution has two parts, and both are triumphs of twentieth-century physics.

First, the universe is not ageless. It is about 13.8 billion years old, so light has only had that long to travel. Look far enough out and you are looking at regions whose light is still on its way — the wall of stars is there, but the light from the most distant ones has not arrived yet. Our night sky is small because our universe is young.

Second, space itself is stretching. The most distant light that does reach us is stretched thin on the journey, its waves pulled toward the red and beyond, until the faint uniform glow of the infant cosmos has been redshifted clean out of the visible — into the microwave hum that is one of our best reasons to trust the Big Bang.

So the wall of light is real, and it is out there. We have even found it. It is just no longer bright, and no longer visible — cooled and stretched by 13.8 billion years into a whisper. Look up at the black between the stars and you are seeing, quite literally, the edge of time.