• Quick note - the problem with Youtube videos not embedding on the forum appears to have been fixed, thanks to ZiprHead. If you do still see problems let me know.

Why isn't the universe spinning?

Roboramma

Penultimate Amazing
R
Joined
Feb 22, 2005
Messages
16,747
Location
Shanghai
This question may be poorly posed, but what I'm looking for is an understanding based on some deeper underlying principle that would lead to it.

As far as I understand the universe as a whole isn't spinning. If it were that would seem to mean that it's not isotropic, right? And it is isotropic so, it must not be spinning. That seems odd to me. It seems like a big coincidence that all the matter in the universe somehow has as total angular momentum of... zero.

How does that happen? The only thing I can think of is the idea that the universe is some sort of quantum fluctuation, and given that momentum is conserved, if you start with zero angular momentum you should still have zero angular momentum.
 
Roboramma said:
The only thing I can think of is the idea that the universe is some sort of quantum fluctuation, and given that momentum is conserved, if you start with zero angular momentum you should still have zero angular momentum.
Click to expand...


Also, if the universe is a self-contained piece of space-time there is nothing for it to "spin against" and spin is meaningless in this context.
 
You can't spin except in relation to something. There's nothing for the universe to relate to.

Also, since the actual fabric of the universe is expanding, there's no center point. So, we can't measure spin against that.
 
Roboramma said:
This question may be poorly posed, but what I'm looking for is an understanding based on some deeper underlying principle that would lead to it.

As far as I understand the universe as a whole isn't spinning.
Click to expand...

As far as we can tell (our precision of measurement isn't perfect), yes.

If it were that would seem to mean that it's not isotropic, right?
Click to expand...

Correct.

And it is isotropic so, it must not be spinning.
Click to expand...

It is isotropic to within some limits. It may not be perfectly isotropic.

That seems odd to me. It seems like a big coincidence that all the matter in the universe somehow has as total angular momentum of... zero.
Click to expand...

It is within some limits of zero. It may not be exactly zero.

How does that happen?
Click to expand...

I doubt anyone really knows (your speculation may be as good as any other). But the converse is also true: we don't know how to make a universe with net angular momentum either.
 
Last edited:
Ziggurat said:
I doubt anyone really knows (your speculation may be as good as any other). But the converse is also true: we don't know how to make a universe with net angular momentum either.
Click to expand...

Yeah, we're having enough trouble just figuring out how to make a universe.
 
Loss Leader said:
You can't spin except in relation to something. There's nothing for the universe to relate to.

Also, since the actual fabric of the universe is expanding, there's no center point. So, we can't measure spin against that.
Click to expand...

Neither of these statements is correct. See, for example, the Gödel metric, which is a GR solution for a rotating universe.
 
gabeygoat said:
I'm glad you posted that, because you said it better than I. But that was my gut intuition.
Click to expand...

Gut intuition is unreliable. Rotation is not relative, rotating coordinate systems are not equivalent to non-rotating ones, and as Ziggurat pointed out, there are models where the universe rotates.
 
KAJ said:
Mach's principle may be relevant. I can't post links yet, but Wikipedia has an article.
Click to expand...

Feel free to post edited links and someone will quote it and fix the edit. Such as:

en.wikipedia(dot)org/wiki/Mach%27s_principle

And someone will replace (dot) with an actual . to fix up the link. You won't get in trouble as the rule only exists to prevent bots from spamming links to promote a website.

https://en.wikipedia.org/wiki/Mach's_principle
 
KAJ said:
Mach's principle may be relevant. I can't post links yet, but Wikipedia has an article.
Click to expand...

Mach's principle is little more than the vague suggestion that inertia has something to do with the rest of the mass in the universe, and General Relativity doesn't even adhere to it. It could very easily turn out to be another instance of unreliable gut intuition.
 
Ziggurat said:
It is isotropic to within some limits. It may not be perfectly isotropic.



It is within some limits of zero. It may not be exactly zero.
Click to expand...
That's a good point. I'm feeling some bias toward the idea that it is exactly zero, it just seems to make more sense, but then I'm reminded of the cosmological constant and I guess we have to be open to the possibility of a very low but non-zero value.



I doubt anyone really knows (your speculation may be as good as any other). But the converse is also true: we don't know how to make a universe with net angular momentum either.
Click to expand...

Cool, thanks for that, that makes sense. I guess I was hoping there might be some big idea that would make it just make perfect sense that of course the universe isn't spinning.
 
RecoveringYuppy said:
I think I'm going to go with the "not well posed question" option. Are you asking if the entire universe might be rotating around some point (several posters seem to have assumed that)? Or, are you asking does the angular momentum of all objects in the universe sum to zero?
Click to expand...

I think they are actually the same question, with the caveat that "rotating around some point" doesn't mean rotating around some point in space in the universe.

To make it clear, I can draw an analogy to the 2D surface of a sphere, that sphere can be spinning but it's not rotating around a point on it's surface. It's still a rotating coordinate system, and, if we have such a surface with points on it moving relative to each other with some net angular momentum that net angular momentum can be seen as rotation of the sphere rather than the stuff on it's surface.
 
Roboramma said:
To make it clear, I can draw an analogy to the 2D surface of a sphere, that sphere can be spinning but it's not rotating around a point on it's surface. It's still a rotating coordinate system, and, if we have such a surface with points on it moving relative to each other with some net angular momentum that net angular momentum can be seen as rotation of the sphere rather than the stuff on it's surface.
Click to expand...


But what about the case where the surface with points aren't moving relative to each other with some net angular momentum but the individual points possess their own (local) angular momentum? They can net out to zero or non-zero also. Is that distinction relevant to you?

Would their be a Hubble constant for distribution of angular momentum across space?
 
Last edited:
You must log in or register to reply here.

Back
Top Bottom