Table of Contents
What Is Olbers' Paradox?
Olbers' paradox asks: if the universe is infinite, eternal, and uniformly filled with stars, why is the night sky dark? In such a universe, every line of sight should eventually hit a star's surface, making the entire sky as bright as the surface of an average star. This paradox, posed by Heinrich Olbers in 1823, highlights fundamental issues with our assumptions about the universe.
The paradox is resolved by modern cosmology: the universe has a finite age (about 13.8 billion years), so light from distant stars hasn't had time to reach us. Additionally, the expansion of the universe redshifts light from distant sources, further reducing the background brightness. The cosmic microwave background is actually the remnant of this universal glow.
Resolution
- Finite age: The universe is about 13.8 billion years old, limiting the observable universe.
- Expansion: The universe is expanding, redshifting distant light out of the visible spectrum.
- Finite star lifetimes: Stars burn out, so the universe isn't uniformly bright.
- Structure: Matter is not uniformly distributed but clumped into galaxies.
The Mathematics
The mean free path in the Milky Way is about 10²³ light-years, far exceeding the observable universe radius of 46 billion light-years. This quantitatively shows why the sky is dark.
Frequently Asked Questions
Is Olbers' paradox truly resolved?
Yes. The finite age of the universe is the primary resolution. Even without expansion, the limited speed of light and finite stellar lifetimes ensure the night sky is dark. The cosmic microwave background (2.7 K) is the actual "glow" predicted by the paradox, just redshifted to microwave frequencies.
What would an infinite static universe look like?
If the universe were truly infinite, static, and eternal with uniform star density, every point in the sky would be as bright as the surface of an average star (about 5800 K for Sun-like stars). The night sky would be blindingly bright, about as bright as the Sun's surface in every direction.