Olbers’ Paradox, also known as the dark sky paradox, is an apparent contradiction about why the night sky is dark instead of being completely illuminated by the light of an infinite number of stars.

It is believed that the first to raise the problem was Lord Kelvin, in an essay published in 1901, and that even Edgar Allan Poe anticipated some aspects of the argument in his famous scientific poem “Eureka”:

If the succession of stars were endless, then the background of the sky would present us with a uniform luminosity, like that shown by the Galaxy – since there could be absolutely no point, in all that background, in which there was not a star. The only way, therefore, in which, under such a state of things, we could understand the voids that our telescopes find in countless directions, would be to suppose the distance of the invisible background so immense that no ray from it has been able to reach us yet.

Edgar Allan Poe, Eureka

However, the formulation of the paradox is attributed to the German astronomer Heinrich Wilhelm Olbers in 1823. Olbers reasoned that if the universe is static, infinite, and full of stars, then no matter which direction we look, our line of sight would end up hitting the surface and light of a star. If that were true, the night sky should be completely illuminated all the time, with no dark areas.

Monument to Olbers in Bremen, Germany
Monument to Olbers in Bremen, Germany. Credit:Florean Fortescue / Wikimedia Commons

However, clearly, the night sky is not completely illuminated. In fact, on a clear night away from city lights, the night sky is mostly dark, only illuminated by a finite number of visible stars. This apparently contradicts Olbers’ idea of an infinite universe full of stars.

Over the centuries, astronomers and cosmologists have proposed various possible solutions to resolve this paradox. One of the first was suggested in 1826 by Michael Faraday, who argued that interstellar dust must be absorbing much of the light from more distant stars. Another early proposed solution was that the universe is not infinitely old, so the light from distant stars simply has not had enough time to reach us.

A significant advance in resolving the paradox came with the Big Bang theory. According to the Big Bang model, the universe originated 13.8 billion years ago in a great explosion and has been expanding ever since. In an expanding universe, the density of matter decreases over time. This means that the farther we look into space, we are also looking back in time to a period when the universe was denser and hotter but contained fewer burning stars.

The expansion of the universe also means that the light from distant galaxies is redshifted, losing energy along the way. The combination of the expansion of space, the lower density of stars in the distant past, and the redshift of distant light all help resolve Olbers’ paradox within the framework of the Big Bang.

The image on the left contains 10 bright stars, the one on the right contains 40 stars each 4 times dimmer than those in the left image. The total light received is the same
The image on the left contains 10 bright stars, the one on the right contains 40 stars each 4 times dimmer than those in the left image. The total light received is the same. Credit: Ze-aksent / Wikimedia Commons

Another piece of the puzzle came with the discovery in the 1990s that the universe is not only expanding but that the expansion is accelerating. Dark energy seems to be the culprit, pushing distant galaxies even faster apart.

The more the universe expands acceleratively, the less light from distant galaxies reaches Earth. This leaves the night sky darker than expected. Without the accelerated expansion of the universe and dark energy, Olbers’ Paradox would still lack a solution within the Big Bang model.

Therefore, the existence of dark energy is essential to resolving the paradox about the darkness of the night sky.

The paradox inspired generations of astronomers to seek a deeper understanding of the origin and structure of the universe. The proposed solutions, including the Big Bang theory, accelerated expansion, and redshift, as well as the existence of a finite number of stars in a non-uniform universe, eventually answered the question of why the night sky is dark.

We live within a spherical shell of the “observable universe” that has a radius equal to the age of the universe. Objects more than 13.7 billion years old are too far away for their light to reach us.

This article was first published on our Spanish Edition on October 17, 2023: La Paradoja de Olbers, ¿por qué el cielo nocturno es oscuro si hay miles de millones de estrellas?

Sources

Scott Chase, Olber’s Paradox | Paul S. Wesson, Olbers’s Paradox and the Spectral Intensity of the Extragalactic Background Light. Astrophysical Journal v.367, p.399. doi:10.1086/169638 | Peter Zamarovský, Why Is It Dark at Night?: Story of Dark Night Sky Paradox | J.M Overduin, P.S Wesson, Dark Sky, Dark Matter | Wikipedia

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