olber's paradox

Olber’s Paradox, or why is the Sky Dark at Night?

In astrophysics and physical cosmology, Olbers’ paradox, named after the German astronomer Heinrich Wilhelm Olbers and also called the “dark night sky paradox”, is the argument that the darkness of the night sky conflicts with the assumption of an infinite and eternal static universe. The darkness of the night sky is one of the pieces of evidence for a non-static universe such as the Big Bang model. If the universe is static, homogeneous at a large scale, and populated by an infinite number of stars, any sight line from Earth must end at the (very bright) surface of a star, so the night sky should be completely bright. This contradicts the observed darkness of the night.

The paradox is that a static, infinitely old universe with an infinite number of stars distributed in an infinitely large space would be bright rather than dark. To show this, we divide the universe into a series of concentric shells, 1 light year thick. Thus, a certain number of stars will be in the shell 1,000,000,000 to 1,000,000,001 light years away. If the universe is homogeneous at a large scale, then there would be four times as many stars in a second shell between 2,000,000,000 to 2,000,000,001 light years away. However, the second shell is twice as far away, so each star in it would appear four times dimmer than the first shell. The total light received from the second shell is the same as the total light received from the first shell, so each shell of a given thickness will produce the same net amount of light regardless of how far away it is. That is, the light of each shell adds to the total amount. Thus the more shells, the more light. And with infinitely many shells there would be a bright night sky. Dark clouds could obstruct the light. But in that case the clouds would heat up, until they were as hot as stars, and then radiate the same amount of light. Kepler saw this as an argument for a finite observable universe, or at least for a finite number of stars. In general relativity theory, it is still possible for the paradox to hold in a finite universe: though the sky would not be infinitely bright, every point in the sky would still be like the surface of a star.

Let’s talk about the dark night. (NO! This post is not on Batman!).

Our common assumption is that the night sky is supposed to be dark with only few dots of light.

But then, aren't there supposed to billions upon billions of stars in the night sky emitting light. Yes, they are very far away, but, there is nothing stopping (like air or glass) the light from reaching us. So, shouldn't all those stars make the night sky (very) bright and not dark ?

This is actually called Olbers’ Paradox

Let’s look at the problem in another way. We can divide the universe into a series of concentric shells, being 5 light years thick. Thus, a certain number of stars will be in the shell 1,00,000 to 1,00,005 light years away. If the universe is homogeneous at a large scale (i.e., static), then there would be four times as many stars in a second shell between 2,00,000 to 2,00,005 light years away.

But, the second shell is twice as far away, so each star in it would appear four times dimmer than the first shell (intensity is inversely proportional to the square of distance). Thus the total light received from the second shell is the same as the total light received from the first shell.

Thus, the argument is that if the universe were static and filled infinitely with stars, the night sky should be much brighter than it is now.

I think you guessed the loop hole here. I said if the universe were static, which it clearly isn’t.

The Big Bang explains this paradox by saying that the universe started at a point, and expanded from that point. Thus, it is not static.

We know that the expansion is accelerating. So, two things happen. 

One is that, those stars in the night sky are moving away from us and the distance between them and us increases. This increase the time for to see them and eventually it takes millions of years for the light from those stars to reach us.

Second, which is the more important reason, is that these starts get redshifted away. Redshifting is when the wavelength of an object moving away from us goes towards the red side of the spectrum and eventually, it goes into the infra red, which we cannot see. It is like we listen to a honking truck passing by at great speed. As it moves away from us, the the sound becomes softer and softer and eventually it is inaudible.

So, because of these reasons, we never get to experience the real night sky light. But, it may be a good thing, as otherwise our eyes would be blinded by the light !

Hi there! As I understand it, Olber’s Paradox asks in its most basic form: “If the universe is static, infinitely old and filled with uniformly distributed stars, then why is the night sky dark?”

It was named after German astronomer Heinrich Olbers in 1823, but astronomers have been asking and posing solutions to this question for centuries. However, it’s not a paradox anymore because we understand more about the universe. Basically, the answer boils down to three facts:

  1. The universe is not infinitely old but instead has a finite age
  2. The universe is expanding and so stars are receding away from us
  3. Light has a speed limit, so light from distant stars hasn’t reached us yet

Bonus fact: Writer and poet Edgar Allan Poe gave the first scientifically viable answer in 1848 - he suggested that while the universe might be infinite in size, perhaps it’s not infinite in age, so there hasn’t been enough time for starlight to reach us from the distant reaches of space.

For a more in-depth answer, you should check out Minute Physics’ amazing and entertaining explanation

And then I thought about how for a long time scientists were puzzled by the fact that the sky is dark at night, even though there are billions of stars in the universe and there must be stars in every direction you look, so that the sky should be full of starlight because there is very little in the way to stop the light from reaching earth.

Then they worked out that the universe was expanding, that the stars were all rushing away from one another after the Big Bang, and the further the stars were away from us the faster they were moving, some of them nearly as fast as the speed of light, which was why their light never reached us.

I like this fact. It is something you can work out in your own mind just by looking at the sky above your head at night and thinking without having to ask anyone.

And when the universe has finished exploding, all the stars will slow down, like a ball that has been thrown into the air, and they will come to a halt and they will all begin to fall toward the center of the universe again. And then there will be nothing to stop us from seeing all the stars in the world because they will all be moving toward us, gradually faster and faster, and we will know that the world is going to end soon because when we look up into the sky at night there will be no darkness, just the blazing light of billions and billions of stars, all falling.

—  Mark Haddon, The Curious Incident of the Dog in the Night-time