Astronomy

The Olbers Paradox

The Olbers Paradox

Olbers' paradox is the apparent contradiction that exists between the night sky being black and the Universe being infinite.

If it is, each line of sight from Earth should end in a star. Therefore, the sky should be completely bright. But everyone knows that during the night the sky between the stars is black.

A paradox occurs when two opposite results are reached using two apparently valid reasoning methods. The Olbers paradox is named after the German physicist and astronomer Wilhelm Olbers, who wrote about the paradox in the 1820s.

The paradox between a dark night and an infinite universe was known before it was discussed by Heinrich Wilhelm Matthäus Olbers. In the early 17th century, German astronomer Johannes Kepler used the paradox to support the idea that the Universe is infinite. In 1715, British astronomer Edmund Halley identified some bright areas in the sky and proposed that the sky does not shine evenly at night because, although the Universe is infinite, the stars are not evenly distributed.

Swiss astronomer Jean-Philippe Loys de Chéseaux began studying the paradox based on Halley's work. At the end of a book that dealt with the bright comet he studied in 1743, Chéseaux discussed the paradox explicitly. He suggested that either the sphere of the stars was not infinite or the intensity of the light diminished rapidly with distance, perhaps due to some absorbent material, still unknown, present in space.

In 1823 Olbers raised the solution that the sky was dark at night because something in space blocked most of the starlight that had to reach Earth. Current scientists have realized that Olbers' solution would not work, since the matter in the space that blocked the light would heat up over time and eventually radiate as brightly as the stars.

Translations of Olbers' articles into English and French made his work well known. For the next hundred years the paradox and its solution were not discussed.

In 1948, British astronomer Hermann Bondi referred to the Olbers paradox as a part of the steady state theory. Bondi's solution was that the expansion of the Universe caused the light perceived from the distance to be reddish and, therefore, with less energy in each photon or particle of light. This solution is equally valid for the Big Bang theory.

In the 1960s, American astronomer Edward Harrison came to the current understanding and solution of the Olbers paradox. Harrison showed that the sky is dark at night because we don't see the stars that are infinitely far away. Harrison's solution depends on the Universe having an infinite age. Since light takes some time to reach Earth, looking far into space is like looking in the past. Every line of sight from Earth does not end in a star because the light from the furthest stars needed to create the Olbers paradox has not yet reached Earth.

During the time of existence of the Universe, the stars have not emitted enough energy to make the night sky shine. The effect of redshift, whereby the energy of the farthest stars decreases, is a minor effect on this model.

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