Researchers believe that there is at least a septillion star in the visible universe (one with 24 zeros). It is logical to assume that with so many bright objects the night sky should glow. However, it remains dark. Why?
Initially, this question was still interested in the astronomer Jean-Philippe Louis de Shezo in 1744, but in history he was remembered as the Olbers paradox (photometric paradox). He was named after Heinrich Olbers, who in 1823 drew attention to this topic.
This paradox consists of a condition: if the Universe were large enough so that each line of sight would end with a star, be infinitely old, static and not expand, then the whole night sky should be as bright as the solar surface. But this does not happen for a simple reason - the Universe does not meet all conditions. This is a short answer to the question.
It is believed that in 1901 the paradox was decided by William Thompson (Lord Kelvin), who was based on the universal age and the finiteness of the speed of light. But they say that the first clue was presented by the famous writer Edgar Poe in his poem “Eureka”, published in 1848. In fact, Poe suggested that the Universe is not yet old enough to fill the sky with light. He believed that outer space can be infinite in size, but the light moves at a certain speed and has not yet managed to reach us from all corners of the Universe.
His poem is a literary heritage, so it was not taken into account by scientists. However, the findings are close to scientific. For example, Professor Jay M. Pasachoff says:
“When we look deep into the universe, we see the past 12 billion years ago. When the cosmos was a couple of billion years old, we simply didn’t have anything to look at. Then the main solution to the Olbers paradox is that the Universe is not so old that the stars and galaxies can fill our review. ”
Anil Chandra Seth, an assistant professor of physics and astronomy at the University of Utah, adds:
“There are several reasons why the sky remains dark. The fact is that the Universe expands and has a finite age. This means that we are not able to see galaxies infinitely far (even if space is infinite). In addition, the expansion of the universe suggests that the light loses energy when moving through such a huge distance ".
Seth also says that part of the light emitted by all the stars is absorbed by interstellar dust in the Milky Way:
“If it were not for the dust, then our galaxy seemed much brighter than”.
But there is another reason. The fact is that our eyes are insensitive to the wavelengths of light that reach the planet from the most distant stars. Since these stars are rapidly moving away from us (due to the expansion of the Universe), their light “shifts to red” to longer waves. Interestingly, when observing space in special devices that capture long waves, the sky seems much brighter.