Why is the night sky dark?

This is a question that many astronomers have asked themselves for more than 400 years. However, we owe the official formulation of this paradox to the German doctor Heinrich Wilhelm Olbers. Since then, it has been known as the Olbers paradox. Even if it is a simple question, the solution to why the night sky is black took a long time to arrive.

If we consider that the darkness of the sky is due to the absence of light, it isn’t comprehensible for the night sky to be dark. In fact, the space is full of countless stars that radiate light in all directions.

The distance to the stars

The light that the stars emit disperses in a uniform way through the distance, but, as the light gets away from the source from where it has been emitted, its intensity heavily goes down. This is the reason for which we see stars as small luminescent dots, as they are so distant from us that, when their light arrives, its intensity is very low.

Additionally, as an answer to its proposal for the darkness of the night, Olbers proposed that there has to be something in the space, as for example nebulae, that blocked most part of the light that had to arrive on the Earth. For a century, this proposal of Olbers wasn’t argued. Even so, scientists of our days have demonstrated that this is not a viable solution. We now know that this supposed spatial matter that blocks the light would warm up and, with time, would end by emitting as much or more light than the stars.

The age of the universe

Even if space is infinite in what concerns its size, it isn’t infinitely old. The current astrophysics estimates that the universe was formed about 13.8 billion years ago. Light is very fast, but not enough to cover all the universe during this time. This means that we can only see objects as distant as the light has been able to cover in 13.8 billion years. The light of stars further away can’t already be seen.

Another part of the problem is that, even if the universe has 13.8 billion years, the light sources that there are in it aren’t so long-lasting. The main part of the stars we know have between one and 10 billion years. This means that many stars die, for which we can’t see their light.

The expansion of the universe

Another reason for which the night sky is black is due to the expansion of the universe. As the universe expands, the distance between the galaxies and stars increases, and this movement affects the very same propagation of light. If an object gets away from another, its wavelength gets bigger until it arrives at the red colour. However, if it gets closer, the light tends to go to a blue colour. This is known as the Doppler effect.

The problem with this is that, being so distant from us and getting away so quick, the wavelength gets so big that, instead of stopping in the red colour, it goes beyond, until getting into the infrared spectrum, which is an invisible type of light for our eyes. For this reason, it is possible for us not to see stars because the light that arrives on our planet is invisible to us.

The best explanation for the Olbers paradox is that, since the formation of the universe, the light of all stars hasn’t had enough time to arrive to us, and, in addition, many stars have already died.

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