Olbers' paradox

Imagine a universe that is infinite in extent, unchanging over time and old enough for light to have travelled from every star. In a sufficiently uniform distribution of stars, every direction through space would eventually encounter a stellar surface. Although individual distant stars appear fainter with distance, increasingly distant layers also contain more stars; in the idealized static model, their accumulated light would make the night sky bright rather than dark. This conflict between that prediction and the observed dark sky is called Olbers' paradox, after the German astronomer Heinrich Wilhelm Olbers, who discussed it in the nineteenth century. Earlier thinkers also considered versions of the question.

The idealized assumptions do not describe the real cosmos. The universe has a finite age, and light travels at a finite speed. Consequently, only light that has had time to reach Earth can contribute to what we observe; the observable region is not filled by the light of an infinite past of stars. The universe is also expanding. As light travels through expanding space, its wavelength is stretched, shifting some radiation from distant sources toward infrared, microwave or longer wavelengths rather than visible light. These effects mean the night sky does not receive the unlimited visible starlight predicted by the static, eternal model.

The cosmic microwave background (CMB) is a separate but related part of the broader picture of cosmic history. It is relic radiation released when the young universe became transparent; expansion has since stretched it into microwave wavelengths. It is not ordinary visible starlight filling the night sky, but it can be detected with appropriate instruments. The CMB and other evidence support a universe that has evolved over time, not the eternal, static arrangement assumed in the paradox. Olbers' paradox is therefore a useful illustration of how a simple observation can test assumptions about the universe's history and structure.

The paradox is a thought experiment based on idealized assumptions, not a claim that every real direction must literally end at a star. Dust absorption alone cannot resolve the classical problem: absorbed energy would be re-emitted, so the material would eventually contribute radiation of its own. The detailed observed brightness at different wavelengths depends on cosmic expansion and the history and distribution of luminous sources. The key point is that the actual universe is not both static and infinitely old, the conditions required for the classical prediction of an overwhelmingly bright sky.