Olber’s paradox asks “why the sky is dark at nights?”. Olber’s paradix is described by German astronomer Heinrich Olbers (1758-1840). Heinrich Olbers released a research paper on this problem in 1826, but we know that also Kepler been thinking about this problem already in 1610.

Olbers thought that if the universe was infinitely large and old, the universe would be filled by stars and the light of the stars would reach all the places in the universe. The basic idea is “where ever you look, you should see a star at some distance from you”. By this each direction provides the surface of a star. Now, Olber assumed that the sky would shine as right on day time as on night time, because the surface brightness of a star is not depending on distance.

One solution to this paradox is that the light is absorbed by the nebulae between the stars. During the time it would happen so that these elementary particles would start reflecting light as well. Another solution researchers have suggested is that the stars of the universe are not equally distributed. In fact they are strongly centralized. In this case most of the directions in the space would not contain stars.

The present understanding is that the universe is not everlasting, and has a beginning and it expands. Because it has a beginning, it is not infinitely old and by this the light from all the stars has not reached us. Because the stars are moving further and further from us, the redshift decreases the brightness of the light. Additionally we see the most distant stars like they used to be in the past, because the light has a finite speed. There were no stars in the beginning of the universe, and that’s why the most distant stars cannot be seen yet.

If you are interested in paradoxes, please read also what we wrote about The Twin Paradox.