Olbers’ paradox

Why is the night sky dark?

If stars went on forever, every line of sight would hit a star and the night would be as bright as a surface of suns. It is not. The dark is a clue that the universe is not an infinite, unchanging hall of lights.

Each shell of stars at a greater distance has more stars, but each star looks smaller. In an endless steady universe those shells would add up to a wall of light. They do not.

The puzzle is older than the answer

People noticed the puzzle long before anyone had a Big Bang. Dust does not save you: dust would heat until it glowed too. The way out is that we cannot see forever in time. Light has a starting gun, and most lines of sight run out of time before they run into a star.

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Expansion makes it darker still

Space stretching while light is in flight also drops the energy of distant starlight. The dark is then two facts together: a finite age, and an expanding space. Either one would dim the sky; we have both.

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