Olbers’ Paradox
If the universe is infinite, eternal, and uniformly filled with stars, every line of sight should eventually hit a stellar surface. The night sky should blaze as bright as the Sun. So why is space dark?
Why Is the Night Sky Dark?
Olbers’ paradox, named after the German astronomer Heinrich Wilhelm Olbers who formulated it in 1823 (though Kepler and others had considered the puzzle earlier), poses a deceptively simple question: if the universe is infinite, eternal, and uniformly populated with stars, why isn’t the night sky uniformly bright? In such a universe, every line of sight would eventually terminate on the surface of a star. While distant stars are dimmer (brightness falls as 1/r²), the number of stars in each spherical shell increases as r², and their apparent sizes decrease as 1/r² — meaning each shell contributes equal total brightness. An infinite number of shells means infinite total brightness.
Even if closer stars occlude more distant ones, a simple calculation shows that in an infinite static universe, every point on the sky would eventually be covered by a star. The sky would reach the average surface brightness of a star — roughly 5,800 K, the temperature of the Sun’s photosphere. This is clearly not what we observe: the cosmic background temperature is only 2.7 K.
The resolution involves several key features of the real universe. First, the universe has a finite age (approximately 13.8 billion years), so light from stars beyond a certain distance has not yet reached us — there is an observable horizon. Second, the universe is expanding, which means light from distant galaxies is redshifted to longer wavelengths and lower energies (Hubble’s law: v = H₀d). Extremely distant objects are redshifted entirely out of the visible spectrum. Third, stars have finite lifetimes; they are not eternal. The combination of these three effects — especially the finite age of the universe — is sufficient to resolve the paradox completely.
Use the controls above to explore: the “Paradox” mode shows the classical infinite static universe filling with stars as you increase depth. Switch to “Expanding” to see how redshift dims distant shells, “Finite age” to impose a light-travel-time horizon, or “Realistic” to combine all effects and see why the real night sky is dark. The brightness bar shows the cumulative sky coverage compared to a fully bright sky.