Why is the sky dark if there are billions of stars in the Universe?

The dark night sky paradox, which is often called the Olbers’ paradox, is a peculiar phenomenon that happens at night time. It is a known fact that there are billions of stars in the universe, and each of these stars shines brightly in the vast openness of space.  Because the number of stars in the sky is assumed to be essentially infinite, the sky on Earth during night time should be filled with the light coming from these stars and not shrouded in darkness. However, the latter is the one occurring on our planet, but why is that so?

If the logic where billions of stars shine brightly at equal intensity were correct, no matter where we looked up in the sky, the canopy of infinite stars would be continuous, and there will be no dark gaps between any two stars. The night sky would seem like daytime if that were the case. In addition, the sky would be a heavenly vault of light that is dazzlingly as bright and as hot as the Sun’s surface.

The paradox arises from the complicated conflict between logic and the observable darkness of the night sky. The solution for the paradox may have been discovered thanks to the landmark cosmological discovery made by an American astronomer named Edwin Hubble. In his demonstration, he proposed that the universe is continuously expanding, and the light coming from the receding galaxies is often shifting towards the red end of the spectrum, making them invisible in our night sky. To get a clearer picture of Hubble’s demonstration, let us imagine a lamp post that light up a corner of the street where you are standing. As you get further away from the lamppost, you would notice that its light is becoming dimmer and dimmer until it’s gone from your sight. The same thing happens with the brightness of stars in the galaxy whenever they move far away from the solar system, although the distance is much farther than the distance between you and the lamppost.

Furthermore, if the same set of stars follows Hubble’s theory, any star or galaxy that moves further away from us so rapidly will have its light shifted out of the visible spectrum and into the infrared or radio wave spectrum. The radiation we receive from a celestial body is therefore no longer visible to our eyes, hence the reason why we cannot see the light from certain stars at night. The phenomenon greatly reduces the number of stars and galaxies that are visible in the sky, and it could explain why the nighttime is so dark. Many scientists agree that the infrared shifting must contribute to the darkness of the night sky.

There is a supportive explanation as to why night time is dark, and it is based on the fact that the speed of light would always remain constant, so it cannot be boosted any further. One drawback of the fixed speed of light is that we cannot observe an object farther than about 15 billion light-years from Earth. Even if there are other galaxies beyond that distance, the light coming from them would not even reach us.

Where did the name “Olbers’ paradox” come from?

The Olbers’ paradox is named after Heinrich Wilhelm Olbers, a German amateur physicist from the early 19th century, who is believed to be the first one to hypothesize the paradox in 1823.

However, according to British astronomer Edward Robert Harrison, specifically in his book “Darkness at Night: A Riddle of the Universe,” published in 1987, the paradox has already been thought about by English Mathematician Thomas Digges between 1576 and 1580, which was years before Olbers proposed it. Harrison also stated that German astronomer and mathematician Johannes Kepler also came up with a similar paradox in 1610, but it was only during the 18th century when the paradox was taken for further research by an English geophysicist and astronomer Edmond Halley. The paradox was fully realized in a 1901 paper written by Lord Kelvin (William Thomson, 1st Baron Kelvin), a British physicist and engineer who is also notable for publishing formulations for the first and second laws of thermodynamics and as several analyses on electricity.