Why is the night sky dark?
Quick Answer
If the universe were infinite and unchanging, every line of sight would end on a star and the sky would blaze with light. This puzzle, Olbers' paradox, is resolved by the finite age of the universe.
The Full Story
Olbers’ paradox asks why, in a vast universe full of stars, the night sky is dark rather than uniformly bright. The resolution lies in cosmology: the universe is about 13.8 billion years old, so light from the most distant stars has not had time to reach us — we can only see objects within the observable universe. On top of that, the expansion of space stretches distant light into invisible infrared and microwave wavelengths (redshift), and stars have finite lifetimes. Together these facts mean most lines of sight end in darkness rather than on a glowing star.
Key Facts
YouTube Angle
Suggested video title for this topic:
"Why Is Space Dark? — Olbers’ Paradox Explained"