Why the night sky is dark: Olbers' paradox and the light that hasn't arrived yet
30seconds
Go outside and look between the stars: it's dark. That's strange. In an endless, unchanging universe, every line of sight would end on a star, and the whole sky would blaze like the surface of the Sun.
Drag the horizon in a toy universe and watch the dark gaps fill in as light gets more time to arrive, then see why our real sky stays dark, and what the faint glow of microwaves has to do with it.
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