Olbers' Paradox
Why is the night sky dark?
First published: H. W. M. Olbers, "Über die Durchsichtigkeit des Weltraums", *Astronomisches Jahrbuch für 1826*.
In an infinite, eternal, static universe uniformly filled with stars, every line of sight ends on a star. The sky should be ablaze.
Olbers' paradox: in an infinite, eternal, static universe with stars distributed uniformly, every line of sight should eventually intersect a stellar surface, making the sky as bright as the surface of an average star. It manifestly isn't. The paradox indirectly argued — long before Hubble's expansion — that one of the three premises must fail. Modern resolution: the universe is finite in age and expanding, so most lines of sight terminate at the cosmic microwave background (itself redshifted to the microwave range from the optical surface of last scattering). The paradox was historically suggestive but only fully resolved by 20th-century cosmology.
Formulation
Assume universe: (1) infinite spatial extent, (2) eternal in time, (3) uniform stellar density. Then sky brightness should equal stellar surface brightness. Observation: sky is dark at night. Hence at least one premise false. Modern resolution: (1) finite age (~14 Gyr) → causal horizon; (2) expansion → cosmological redshift; both effects darken the sky.
Dimensions Engaged
Time
Bears on Time · Extent: dark night sky is evidence of a finite past.
Space
Bears on Space · Extent: an infinite static universe is empirically untenable.
Energy
Engages Energy · Conservation: stellar luminosity in a static infinite universe would give infinite sky brightness.
Responses — How Schools Engage
Affirms / takes the bait 4
A canonical demonstration that "obvious" cosmological assumptions have testable consequences. Big Bang cosmology resolves the paradox.
The dark night sky is a real cosmological fact with deep theoretical content.
Compatible with creation cosmology: the universe has a finite past, consistent with creation in time.
Operationally exemplary: a quantitative prediction from clean premises is empirically falsified, forcing revision of one or more premises.
Reframes the question 2
The block universe accommodates the finite past required by the resolution; the paradox vindicates finite-time cosmologies.
Bears on multiverse readings: our observable universe is finite-age, but the multiverse may be infinite and eternal; the paradox applies only to causally accessible regions.
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Experiments engaged by an overlapping set of schools — likely to surface the same fault lines.
Further reading
- Olbers (1826), op. cit.
- Harrison, *Darkness at Night* (1987)
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