Olbers' paradox: why is the night sky dark?
If the universe is full of stars, every direction should end on one. So why isn't the night sky blazing?
Heinrich Olbers, 1823; Kepler and Halley asked earlier
The forest of stars
Ananya was camping in the Western Ghats with her uncle, an astronomer. Lying on her back, she looked up at the black sky dotted with stars, and he asked her a question that sounded silly.
"Imagine you're standing in a huge forest," he said. "If it goes on far enough, every direction you look will end on a tree trunk. You'd see only bark, no gaps. Now imagine the universe goes on forever, filled evenly with stars. Every direction you look should end on a star."
"But far-away stars are dimmer," said Ananya.
"They are," he agreed. "But there are more of them, far away. Exactly enough more to make up for being dimmer. If the universe were endless and had always existed, the whole sky should be as bright as the surface of the Sun, day and night. So why can you see the dark?"
The puzzle
An endless, unchanging universe full of stars predicts a blazing sky, but the night sky is dark.
See it for yourself
What's really going on
The answer is that the universe has an age: about 13.8 billion years. Light travels at a fixed speed, so we can only see objects whose light has had time to reach us. Beyond that distance, the light simply hasn't arrived yet. There are only a limited number of stars in our view, and gaps between them.
The expansion of the universe helps too: light from distant galaxies is stretched to longer, redder, weaker wavelengths. Surprisingly, the first person to suggest the 'finite age' answer was the poet Edgar Allan Poe, in 1848. The dark sky is evidence that the universe had a beginning.
Think about it
The dark sky isn't completely dark: radio telescopes see a faint glow everywhere. Where do you think that glow comes from?