Olbers’ paradox questions why the night sky is dark despite an infinite number of stars. It was first posed in a time when the universe was thought to be static, infinite, and infinitely old. Various attempts were made to solve the puzzle, but it was only with Edwin Hubble’s discovery in 1929 that the universe is expanding that a solution presented itself. The universe is finite in age, and the expansion causes a Doppler shift in starlight that increases with distance, making any star beyond a certain distance invisible.
Olbers’ paradox, also known as the night sky paradox, is the name given to the problem of explaining why the sky is dark at night. The puzzle was first posed at a time when the universe was assumed to be static, infinite in extent, and infinitely old. Based on these assumptions, it would appear that the sky should be bright, as there would be an infinite number of stars covering every point in the sky. The paradox is named after Heinrich Olbers, who, in 1826, stated that every line of sight would end on a star, making the sky bright. The problem, however, had arisen several times earlier in the history of astronomy, starting in the sixteenth century.
The first known reference to the paradox comes from astronomer Thomas Digges in 1576, who encountered the problem in his description of an infinite universe with a random distribution of stars. In 1610, Johannes Kepler cited what would become known as Olbers’ paradox to show that the universe must be finite. There seemed to be a problem with a finite universe, however, that it would collapse in on itself due to the gravitational pull of the stars and planets within it. Most astronomers therefore assumed that the universe was infinite and thus the paradox remained.
Initial suggestions that most of the stars were too far away to see were quickly dismissed. If the universe were infinitely old, starlight would have an infinite amount of time to reach us, so even the most distant stars would contribute to a bright sky. It can be shown mathematically that for an infinite universe with evenly distributed stars, the entire sky must be as bright as an average star. The brightness of stars decreases with distance, but the number of stars increases with distance, without limit in an infinite universe. The effects cancel out, leaving a bright sky.
Various further attempts were made to solve the puzzle over the next few hundred years. An early attempt at an explanation was that most of the starlight was obscured by dust. While it is true that dust clouds in our galaxy block large areas of it from view, if there were an infinite number of stars, eventually all the dust would heat up and glow, just like stars.
Another suggestion was that the stars were not randomly distributed, but were arranged in clusters with large gaps in between. Now we know that’s right: stars are grouped into galaxies, which are grouped into clusters and superclusters. On the largest scale, however, the universe is homogeneous, and Olbers’ paradox, as described by Olbers himself, states that every line of sight must end in a star. Put this way, it is clear that a non-random grouping of stars could only explain the dark sky if the stars were aligned behind each other, blocking each other’s light – a scenario that no one could take seriously. serious.
It was only with Edwin Hubble’s discovery in 1929 that the universe is expanding that a solution to Olbers’ paradox presented itself. It is now known that the observable universe is expanding at a rate that increases with distance, and looking back in time we arrive at a point of tiny volume and enormous density. This gives two reasons why the sky is dark. The first and foremost reason is that the universe is finite in age, so there would have been no time for light from stars beyond a certain distance to reach us. A second reason is that the expansion of the universe causes a Doppler shift in starlight that increases with distance; beyond a certain distance, all light would move beyond the visible spectrum, making any star invisible.
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