Here’s a question that undid the ancient idea of an eternal, infinite universe: if stars go on forever, then EVERY line of sight should eventually land on a star, and the whole night sky should blaze as bright as the Sun. It doesn’t — night is dark. The resolution: the universe has a finite AGE, so we only see stars within a finite horizon, and expansion redshifts the rest away. Slide the cosmic horizon and watch the sky fill — or stay dark.
Olbers’ paradox: in a static, infinite, eternal universe uniformly filled with stars, the number of stars in a shell at distance r grows as r² while each star’s brightness falls as 1/r² — the two cancel, so every shell contributes equally and the total sky brightness would DIVERGE to infinity; every sightline would end on a stellar surface. The dark night sky is therefore evidence AGAINST that model. The resolution is that the universe has a finite age (~13.8 Gyr): light from beyond the cosmic horizon (~one light-travel-age away) hasn’t reached us, capping the sum, and cosmological redshift dims the most distant light. A fail-loud self-check throws unless a finite horizon yields a finite (dark) sky while an ever-larger horizon keeps growing it. ◆ real cosmology, node-verified.
The classic shell argument (exact for the divergence); the DOMINANT real resolution is finite age (the horizon cutoff), with redshift a secondary dimming — the darkness-implies-not-infinite-and-eternal logic is exact.