Olbers’ paradox – Dark night sky paradox
Olbers’ paradox (Link)
The Universe as Newton saw it gave rise to a paradox, known as Olbers’ paradox after Heinrich Olbers, who raised the issue in 1823. He deduced that in an infinite Universe of infinite age, there would be an infinite number of stars. If you were, therefore, to look in any direction in the sky, your line of sight would eventually hit on a star’s surface. Since every direction would lead to a star, and given the absolute luminosity of a star and the inverse square law for the dimming of light with distance, the night sky would be infinitely bright. Olbers’ paradox asks the question “why is the night sky dark?”
Poet Edgar Allan Poe suggested that the finite size of the observable universe resolves the apparent paradox. More specifically, because the universe is finitely old and the speed of light is finite, only finitely many stars can be observed within a given volume of space visible from Earth (although the whole universe can be infinite in space). The density of stars within this finite volume is sufficiently low that any line of sight from Earth is unlikely to reach a star.
However, the Big Bang theory introduces a new paradox: it states that the sky was much brighter in the past, especially at the end of the recombination era, when it first became transparent. All points of the local sky at that era were comparable in brightness to the surface of the Sun, due to the high temperature of the universe in that era; and most light rays will terminate not in a star but in the relic of the Big Bang.
This paradox is explained by the fact that the Big Bang theory also involves the expansion of space which can cause the energy of emitted light to be reduced via redshift. More specifically, the extreme levels of radiation from the Big Bang have been redshifted to microwave wavelengths (1100 times longer than its original wavelength) as a result of the cosmic expansion, and thus form the cosmic microwave background radiation. This explains the relatively low light densities present in most of our sky despite the assumed bright nature of the Big Bang. The redshift also affects light from distant stars and quasars, but the diminution is minor, since the most distant galaxies and quasars have redshifts of only around 5 to 8.6.
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