As you begin to understand more about our Universe and learn of the millions of stars that are brighter than our Sun, you may begin to wonder, if stars are so bright and there are so many, why is space and our night sky dark? This contradiction is known as Olbers’ paradox.
Olbers’ Paradox: if the Universe is full of stars, why doesn’t the light from all the stars add up and make the sky bright?
This question comes from the thinking that if the Universe was infinitely old and infinitely big, then we would expect the sky to be bright from the light of all the stars at night how it is bright during the day. But all we have to do is wait until the Sun sets to know this is not true. So why is the night sky dark?
Solution to Olbers’ Paradox
There have been many proposed solutions to this paradox. The current best solution is breaking the assumption that the Universe is infinitely old. Astronomers find the Universe to only be 15 billion years old, and while this is a very long time to us, it is a mere fraction on the scale of infinity. How does the Universe having a finite age solve this paradox?
While light is one of the fastest things in the Universe it still takes time for it to get from point A to point B. With an infinitely large Universe there are stars so far away that even after 15 billion years their light has not yet reached us. If it has not reached us yet, then it cannot contribute to the brightness of our sky.
An additional explanation as to why space is dark is that the Universe is expanding, making stars and galaxies move away from each other. As the Universe expands, the light from galaxies gets stretched into different wavelengths. Some galaxies’ visible light may be stretched into wavelengths of nonvisible light such as radio waves, infrared, or microwaves.
While this is still light it is not visible light and therefore does not contribute to the brightness of our sky, and the sky remains dark. However, if you look at space in a different wavelength of light such as microwaves, space does glow. This is called the cosmic microwave background, which is the radiation left over from the Big Bang.
All sky image of the cosmic microwave background. Image Credit: ESA/LFI & HFI Consortia
Another contributing factor as to why space is dark is that space is nearly a perfect vacuum. If you recall our article on why the sky is blue you will remember that the Earth’s sky is blue due to the scattering of light with the molecules that make up our atmosphere. This scattering directs the light in all directions, including to our eyes.
However, if there is nothing for light to scatter or interact with, it will travel in a straight line from its source to its receiver. Space is nearly a perfect vacuum which means it has extremely few particles in the space between planets, stars, and galaxies. There is almost nothing to scatter light to our eyes. So, unless there is a light source directed right at us, we do not receive the light. With no light reaching our eyes we see black.
If you are still a bit confused that is okay! Scientist have been studying this paradox for centuries. To help your understanding here is a great video explanation.