Why isn't the universe spinning?

[Roboramma]

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?

Oh that's interesting, I hadn't thought of that. I don't know the answer but would be interested if others do.

W.D.Clinger? Ziggurat?

 

[P.J. Denyer]

I tinkered around in my garage for a few weeks, but didn't make much progress.

I thought I'd start by making a world and then work up, apparently you need some brand name liquorice sweets.

 

[Roboramma]

No, we couldn't.

We are in inside our own "reference frame" (a technically bad choice of words but I couldn't think of anything better), with nothing outside to compare it with.

It's been pointed out a few times already in this thread that this is false.

 

[dann]

Could the centrifugal force explain the accelerated expansion of the universe?

 

[Steve]

I thought I'd start by making a world and then work up, apparently you need some brand name liquorice sweets.

Go to Magrathea. Ask for Slartibartfast. He can get you started.

 

[W.D.Clinger]

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.

Any generator of a rigid spin induces a generator for some family of continuous rigid transformations of its 2D surface, and the Brouwer fixed point theorem says there has to be at least one fixed point on that surface. It seems to me there will be two: the endpoints of the axis of spin. I'd say the sphere is rotating around both of those points, clockwise around one and counter-clockwise around the other.

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?

Oh that's interesting, I hadn't thought of that. I don't know the answer but would be interested if others do.

W.D.Clinger? Ziggurat?

I don't know.

Ziggurat mentioned the Gödel universe, which seems relevant:

Neither of these statements is correct. See, for example, the Gödel metric, which is a GR solution for a rotating universe.

I'm away from home at the moment, without access to my copy of Hawking and Ellis or other references, and I don't own copies of the Gödel papers anyway.

These quotations from the current Wikipedia article on the Gödel universe may be relevant to this thread if not directly to RecoveringYuppy's questions:

Following Gödel, we can interpret the dust particles as galaxies, so that the Gödel solution becomes a cosmological model of a rotating universe. Besides rotating, this model exhibits no Hubble expansion, so it is not a realistic model of the universe in which we live...Less well known solutions of Gödel's exhibit both rotation and Hubble expansion and have other qualities of his first model, but travelling into the past is not possible. According to S. W. Hawking, these models could well be a reasonable description of the universe that we observe, however observational data are compatible only with a very low rate of rotation.

That doesn't answer RecoveringYuppy's question because:

We have seen that observers lying on the y axis (in the original chart) see the rest of the universe rotating clockwise about that axis. However, the homogeneity of the spacetime shows that the direction but not the position of this "axis" is distinguished.

Someone mentioned Mach's principle. As cjameshuff explained:

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.

The vagueness of Mach's principle is illustrated by the following paragraphs:

Some have interpreted the Gödel universe as a counterexample to Einstein's hopes that general relativity should exhibit some kind of Mach's principle,^[4] citing the fact that the matter is rotating (world lines twisting about each other) in a manner sufficient to pick out a preferred direction, although with no distinguished axis of rotation.

Others^{[citation needed]} take Mach principle to mean some physical law tying the definition of nonspinning inertial frames at each event to the global distribution and motion of matter everywhere in the universe, and say that because the nonspinning inertial frames are precisely tied to the rotation of the dust in just the way such a Mach principle would suggest, this model does accord with Mach's ideas.

The Wikipedia article concludes by saying a book by Ryan and Shepley gives other examples of rotating universes, and that might be a good resource for those who want to pursue this further. (I myself have not seen that book.)

 

[phunk]

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?

Am I missing something or are the two hilights contradictory? If there's no overall net angular momentum, then the sum of all the local angular momentums is zero.

 

[phunk]

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.

What about the poles?

I think a better example would be a torus. It could rotate a couple different ways without the axis of rotation intersecting the surface.

 

[RecoveringYuppy]

Am I missing something or are the two hilights contradictory? If there's no overall net angular momentum, then the sum of all the local angular momentums is zero.

Imagine a square board that has gyroscopes bolted to it. Now think about these scenarios:

1. Gyros aren't spinning but you spin the entire board. System has net angular momentum.
2. Board stays still, start all gyros spinning the same way. System has net angular momentum.

 

[phunk]

Imagine a square board that has gyroscopes bolted to it. Now think about these scenarios:

1. Gyros aren't spinning but you spin the entire board. System has net angular momentum.
2. Board stays still, start all gyros spinning the same way. System has net angular momentum.

2 violates conservation of momentum if those aren't spun up in a way that adds up to zero.

 

[Ziggurat]

Am I missing something or are the two hilights contradictory? If there's no overall net angular momentum, then the sum of all the local angular momentums is zero.

Let's say you have two balls. You can give these two balls net angular momentum by having them orbit each other. You can also give these two balls net angular momentum by having each of them sit in a fixed spot but making them both spin in that spot.

Net angular momentum for the universe is similar. There's more than one way to do it. Elementary particles have spin, so you could imagine, for example, a universe where there is (or was) net spin moment to the particles. You could also imagine a universe where there's no net spin moment, but large-scale flows of matter that create angular momentum.

Or you could imagine the Gödel universe and go insane.

 

[RecoveringYuppy]

2 violates conservation of momentum if those aren't spun up in a way that adds up to zero.

??


Obviously not since you can actually do it.

 

[Ziggurat]

2 violates conservation of momentum if those aren't spun up in a way that adds up to zero.

Both violate conservation of angular momentum if they spontaneously transition from zero to net angular momentum. But that isn't the situation under consideration. It is merely an example to illustrate two different configurations with net angular momentum.

If the universe has net angular momentum, then it must have always had it, because of conservation. But saying it has net angular momentum doesn't uniquely determine how that net angular momentum is manifested.

 

[Ziggurat]

??


Obviously not since you can actually do it.

Sure, you can do it. You need to transfer some angular momentum to the ground, though (which won't be noticeable due to the size of the earth). But that's true in both cases, not just case 2. And it's not actually relevant to this thread (see my previous post).

 

[phunk]

??


Obviously not since you can actually do it.

If you actually do it, you're transferring momentum to other things. For example if you're holding a gyro and you spin it, your transferring the opposite momentum through yourself and into the Earth. For the board to represent the universe, you have to spin those gyros without external influence.

 

[theprestige]

I think the universe is probably spinning in its grave.

 

[RecoveringYuppy]

If you actually do it, you're transferring momentum to other things. For example if you're holding a gyro and you spin it, your transferring the opposite momentum through yourself and into the Earth. For the board to represent the universe, you have to spin those gyros without external influence.

Then you should have said both scenarios violate the conservation law because they both would have that same problem.

And, for scenario 2, you shouldn't have said that they have to add up to zero. You should have said they have to add to whatever value they started with.

Does that help clarify the scenarios?

 

[Buddha]

The word "spin" applies to closed systems only. The universe is an open system, at least in accordance with our current understanding of its structire; it boundaries, even if they exist, are unknown.

 

[Ziggurat]

The word "spin" applies to closed systems only.

Why would you say that?

Is an electron a closed system?

 

[McHrozni]

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

How about the center of mass? Surely one exists somewhere, right? Isn't that what we might call the center point of the universe?

More to the point, couldn't the center of mass of the universe be used as the universal frame of refference?

I'm sure I'm missing something here, can someone tell me what it is?

McHrozni