Olbers' paradox, also known as the night sky paradox, is the question of why the night sky appears dark if the universe is infinite and homogeneous (uniform in all directions), and the number of stars is infinite. This paradox was first formulated by Heinrich Olbers, a German astronomer in 1823.
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An animation of Olbers' thoughts. Olbers believed that if the sky were dark, then one of the properties of the universe (infinite, static, and homogeneous) must be wrong.
The basic concept of this paradox is that if the universe consisted of an infinite number of stars, then the night sky should not be dark, but rather should appear as bright as the sun during the day. This is because, if the stars were evenly distributed throughout space, then every line of sight from Earth should pass through a star and bring its light to us. Therefore, the sky should not be dark at night.
Some proposed solutions to this paradox include the filtering of light by interstellar dust, the time limit of the universe, and the cosmic microwave background radiation. These solutions explain that light from distant stars does not reach us, either because it is absorbed by interstellar dust or because the universe is limited by time. The microwave background radiation also suggests another solution: that after the universe began, the temperature of the universe was about the same as the Sun, and the sky was completely bright. However, due to the expansion of the universe, the light was redshifted, leaving only microwave radiation remaining, explaining why the night sky now appears dark.