Science Puzzle
Olbers’ Dark Sky
If the universe is infinite and has always existed, then in every direction you look, your line of sight must eventually hit the surface of a star. Every part of the sky should glow as brightly as the surface of the Sun.
But the night sky is dark. Why?
The Answer
Because the universe is not infinite in time. It has a finite age of about 13.8 billion years, and light from stars further than about 13.8 billion light years away has not had time to reach us yet. There is an observable horizon.
Even within that horizon, the universe is expanding. Light from very distant galaxies is redshifted, stretching its wavelengths toward the infrared and reducing its energy. The most distant light arrives as microwaves, not visible light.
Olbers' Paradox was recognised as early as the 1500s and seriously puzzled astronomers for centuries. Its resolution, requiring the universe to have a finite age and to be expanding, turned out to be one of the strongest early conceptual arguments that the universe began in a specific event: the Big Bang.
The darkness of the night sky is direct evidence of cosmology.
The principle: Olbers' Paradox. The dark night sky implies a universe with a finite age and a cosmic horizon. If the universe were infinite and eternal, every line of sight would reach a star and the sky would be blindingly bright.