Why cannot the universe have net angular momentum?

[chill_factor]

http://physicsworld.com/cws/article/news/2011/jul/25/was-the-universe-born-spinning

http://arxiv.org/ftp/arxiv/papers/1111/1111.3873.pdf

Is there any particular reason why the universe cannot have a net angular momentum? If it did indeed have a net angular momentum are there any consequences of that?

 

[ImaLooser]

http://physicsworld.com/cws/article/news/2011/jul/25/was-the-universe-born-spinning

http://arxiv.org/ftp/arxiv/papers/1111/1111.3873.pdf

Is there any particular reason why the universe cannot have a net angular momentum? If it did indeed have a net angular momentum are there any consequences of that?

Kurt Godel found a solution of Einstein's GR equations in which the universe was rotating. Each point has an equal claim for being the center. In such a universe time travel is theoretically possible. Observations show that our Universe does not rotate in such a way.

 

[chill_factor]

thank you. then what of the results that show a tiny net angular momentum in terms of galaxy spin axes?

 

[ImaLooser]

thank you. then what of the results that show a tiny net angular momentum in terms of galaxy spin axes?

That is a sum of local rotations, not a global rotation. You could say is is a net angular momentum. It is not a terribly firm result though, and I'd forgotten about it.

 

[Chronos]

The question is what would you measure the angular momentum of the universe against, assuming it had any?

 

[chill_factor]

does angular momentum have to be relative to anything external though? it can be relative to itself. it can be measured by inertial forces such as the centrifugal force or Coriolis force and how they affect, say, the motion of gyroscopes within an object with angular momentum.

 

[Chronos]

If the universe is rotating, it does so at a very leisurely pace - re: Is the Universe Rotating http://arxiv.org/abs/0902.4575

 

[marty1]

The question is what would you measure the angular momentum of the universe against, assuming it had any?

I did laugh when I read that. That is funny.

I suppose what they would mean is that if you are stationary at the edge you would have an error between the total gravity you feel toward the center compared to measuring all the mass in the universe and doing the calculation on paper.

 

[bcrowell]

We have a FAQ about this: https://www.physicsforums.com/showthread.php?t=506988

It doesn't make sense to describe it as having a (zero or nonzero) angular momentum, because angular momentum isn't defined or conserved globally in GR. You can describe it as having a zero or nonzero angular velocity.

 

[Chronos]

There is also a thread on this - https://www.physicsforums.com/showthread.php?t=217956