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'''Olbers' paradox''' — the "dark night sky paradox" — is the observation that in a universe that is infinite, eternal, static and uniformly filled with stars, every line of sight would eventually terminate on a stellar surface, so the whole sky should blaze with the surface brightness of a star. It does not. Something in that list of assumptions must be wrong, and which one is wrong has been a central question of cosmology ever since.

==The paradox==

The argument is old. Thomas Digges raised the difficulty in the sixteenth century, Kepler used it against an infinite universe, Edmond Halley and Jean-Philippe de Chéseaux discussed it in the eighteenth, and Heinrich Wilhelm Olbers restated it in 1823, which is how it acquired his name.

Its geometric core is simple. Consider concentric shells of stars around the observer. A shell at distance ''r'' contains a number of stars proportional to ''r''², while each star's apparent brightness falls as 1⁄''r''². The two factors cancel, so every shell contributes the same amount of light, and an infinite series of shells gives an infinite — or, allowing for nearer stars blocking farther ones, a finite but enormous — sky brightness equal to the surface brightness of a typical star. The observed night sky is fainter than this by something like a factor of 10¹².

==The standard resolution==

Modern cosmology resolves the paradox chiefly by denying the assumption of eternity. If the universe has a finite age, light from beyond a finite horizon has not had time to arrive, so the summation over shells terminates. Equivalently, and as Edward Harrison emphasised, stars have finite lifetimes and there is simply not enough energy in the observable universe to fill the sky at stellar surface brightness — the shells beyond a certain distance contain stars that have not been shining long enough.

The expansion of the universe contributes as well, since redshift moves distant starlight out of the visible band and reduces the arriving energy, but in the standard treatment this is a secondary effect: a static universe of finite age would still have a dark night sky. It is worth noting what the standard account does ''not'' claim to resolve the paradox with: obscuration by dust or gas will not do it, because in an eternal universe any absorbing material would come into thermal equilibrium with the radiation and re-radiate it, restoring the bright sky. That objection is thermodynamic rather than optical, and it is the point on which most proposed resolutions turn.

The paradox is not merely historical. The night sky ''is'' bright at microwave wavelengths — the cosmic microwave background fills the whole sky at about 2.7 K — and mainstream cosmology reads that as the redshifted glow of a hot early universe rather than of stars.

==On this wiki==

Because so many contributors here hold the universe to be infinite and eternal (see [[Eternal Universe]] and [[Infinity]]), Olbers' paradox is a problem they must answer rather than one they can defer to a finite age. Nearly every non-expanding cosmology catalogued here addresses it explicitly, and the answers differ.

[[Lee Coe]] argued the obscuration case directly in ''[[Resolving Olbers' Paradox]]'' (1989): radiation from distant stars is intercepted by intervening bodies of matter and never reaches us, and once that is accounted for the paradox dissolves. This is the resolution the thermodynamic objection above is aimed at, and readers should weigh the two together.

[[Richard A Waldron]], in ''[[An Infinite Non-Expanding Universe in Dynamic Equilibrium]]'' (1991), set out the constraint in its sharpest form: any cosmological model must do two things at once — account for the redshift and answer Olbers' paradox — and a ballistic model of light, which forbids a Doppler reading of large redshifts, therefore owes a non-Doppler answer to both.

[[Henry P Dart]]'s ''[[A New Alternative to the Big Bang Theory]]'' (1993) proposes exponential photon decay in transit, the lost energy going into low-mass neutrinos which are eventually recycled into matter at galactic centres. Dart argues that this, combined with an infinite hierarchical universe, overcomes the objections of Olbers and Seeliger while also producing the frequency-independent cosmic redshift and the background radiation.

[[Thomas E Phipps]], in ''[[Steady-State Cosmology: Once More Unto the Breach?]]'' (1989), credits [[Paul Marmet]] with both a plausible non-Doppler redshift mechanism and what he calls a "brilliant new resolution of Olbers' paradox," and argues that the two together justify renewed interest in [[Steady State Theory|steady-state]] and related models.

More generally, the [[Tired Light|tired-light]] and plasma-redshift traditions documented here — [[Lyndon E Ashmore]], [[Ari Brynjolfsson]], [[John W Kierein]], [[Toivo Jaakkola]] — all supply a progressive energy loss over cosmological path lengths, which if real would cut off the shell summation without requiring a finite age. See [[Redshift]] for the mechanisms and the mainstream objections to them. Further material is indexed under [[:Category:Cosmology]] and [[:Category:Astronomy]].

==Open questions==

The status of the paradox in mainstream cosmology is not itself contested: finite age plus finite stellar lifetimes is the accepted answer, and the arithmetic works. What remains genuinely open, and is pressed hard by contributors here, is the converse question — whether the darkness of the night sky is strong independent evidence ''for'' a finite-age universe, or whether it merely rules out one particular kind of infinite one. Any eternal cosmology that removes energy from starlight in transit passes the test as well, which is why the argument keeps returning to the redshift mechanism rather than to the geometry.

==See also==

* [[Eternal Universe]]
* [[Redshift]]
* [[Tired Light]]
* [[Big Bang]]
* [[Infinity]]
* [[Steady State Theory]]

[[Category:Cosmology]]
[[Category:Astronomy]]
[[Category:Big Bang]]

Olbers' Paradox - Revision history

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