Glossary
Olbers' Paradox

A number of early astronomers (e.g. Johaness Kepler, Edmond Halley) asked the "simple" question
Why is the night sky dark ?
Simple consideration of this problem leads to an apparent paradox, usually assigned to Heinrich Wilhelm Matthaeus Olbers (German, 1823)
  • if the universe was infinite, then in no matter what direction (up) from Earth one looked, one's line-of-sight would sooner or later hit a star.
    --Thus, the sky should be uniformly bright, and as bright as the Sun.
    ----This is clearly not the case


Infrared & visible HST image of the Hubble Deep Field South

It is important to note that there are a number of (linked) implicit assumptions within the initial question:
  • That we are dealing with a uniform, static, Euclidean (or Open) 3D space
    • ie. none of the parameters (e.g. stellar density, luminosity etc) vary with time
  • That the 1/R2 law holds, and there is no "loss" of photon while en route from the star to ourselves.
[Additional Notes for 2002/PHY316 students]. Olbers' explanation was that the assumption that there is no "loss" of photon while en route was incorrect, and that
  • the radiation from distant stars is absorbed by a tenuous absorbing medium
    This cannot be correct... over the (infinite) history of the universe, this intervening matter would be heated until it achieved thermal equilibrium (radiated as much energy as it received). As a result there would be no net reduction in the brightness of the radiation field.

Other Solutions Proposed for Olbers' Paradox

  • the universe is infinitely old, but is not infinite in (3-D) extent,
    This cannot be correct... if the universe is finite in 3-D extent, then there must be a center. An unsupported, finite, static universe is unstable to gravitational collapse.
    (This fact led Newton to propose an infinite universe.)
  • the universe may be infinite in (3-D) extent, but not infinitely old:
    OK, but... the light from the more distant stars would then not had time to reach us.
    Formally this solves the paradox, but the instanteous "birth" of an infinitely large universe raises more hypothetical problems than it solves.
  • the universe is expanding (& not infinitely old)
    Yes,... the finite age of the universe & the finite age of the stars within it is the most straightforward solution.

Some Other Notes on Olbers' Paradox

NOTE Not all of the arguments in all these links are neccessarily correct !!!