# I A new solution to Olbers Paradox?

Tags:
1. Apr 3, 2017

### John_RB

I read in a popular science magazine in the UK that "recent" observations of the universe indicate that the number of galaxies in the the universe is actually so vast that Olbers paradox is still a paradox. The short article postulated that they had solved the paradox by proving that light from distant objects is absorbed by hydrogen gas clouds thus causing it to be dark at night. I thought that Olbers paradox was resolved long ago - is this not so?

2. Apr 3, 2017

### phinds

That is NOT a solution, it's nonsense.
Yes, it is so.

3. Apr 3, 2017

### Staff: Mentor

Olber's paradox has been resolved for a long time.

From wiki: https://en.wikipedia.org/wiki/Olbers's_paradox
Olber's 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 dynamic 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."

Several things immediately solve this paradox.

The first is the known finite age of the universe as we know it. It means the universe is not eternal and light simply hasn't had time to travel from every point in the universe to us.

The second is that the expansion of the universe means that it is not static. Light from galaxies very far away is redshifted so much that we can't see any of it with our eyes. So even an eternal universe would not have a sky as bright as the surface of a star because most of our lines of sight would end on a heavily redshifted star and we wouldn't be able to see it.

In addition, stars themselves are dynamic. They are born, they shine for a while, and then they burn out and eventually stop glowing. I believe this conflicts with the "eternal" assumption since it would mean that even in a static, eternal universe, the universe should consist solely of dead stars and fundamental particles with no available energy for things like light and life. For there to be life and bright stars in such a universe, there would need to be some means of replenishing the energy tied up in these dead stars and preventing them from building up. But no such mechanism has ever been observed and any mechanism would violate a number of fundamental laws of physics.

I believe this would cause the hydrogen to glow, as the temperature eventually reaches a temperature equal to the surface of a star. But this isn't observed.

4. Apr 3, 2017

### Chalnoth

Nope. Olber's paradox is solved by the expansion of space.

1. The expansion of space makes it so that the observable universe is finite in extent. Olber's paradox relies upon the number of visible stars being infinite, but if the extent of the observable universe is finite, then it that can't be.
2. The expansion causes far-away objects to be redshifted. If you have a star sitting at $z=1$, then the incoming photons from that star will be at half the temperature they are at the source. Even if the observable universe were infinite (which isn't possible with a finite age of the universe, or in an expanding universe), the far-away stars just wouldn't contribute meaningfully to the temperature at any one location on the sky, because that temperature would have been reduced so far by the expansion.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted