[neilep (OP)]

Dear Universologists,



The Universe Just Moments Ago !! Uh OH !!!....all the Flat Earthers out there will now pronounce the Universe is flat !!




CD's spin, the Earth Spins, The Solar System Spins, The Galaxy Spins, Globular Clusters Spin....everything spins.....even candy floss spins !!




so, d'ya fink the Universe Spins ?




I dunno !!


I'm asking ewe cos ewe know stuff !!






Sheepy
xxx








Ewe spin me right round Uni
Right round Like Candy-floss Uni
Right round !!round round !!

[paul cotter]

Spin in relation to what? A spin is usually observed from some static frame of reference. Maybe it is spinning and this causes the observed expansion but there is no way to determine this without said frame of reference which by definition does not exist.

[Halc]

Everything spins, but the universe isn't a thing. An object without a bounded size cannot meaningfully spin.

Ewe spin me right round quickly,
tight round, getting dizzy,
ralph a one pound ground round mound

Spin in relation to what?

Spin is absolute, and need not be in relation to any particular frame, although something's angular momentum is at least relative to an axis, but angular moment and spin (RPM say) are different things.

[paul cotter]

Yes of course halc, you are correct, In total isolation spin would be detectable. I didn't think clearly, mea culpa. However I think I would disagree with the statement that the universe is without a boundary. How can we know when we can only see so far? I realise a boundary is problematic as it would be the transition zone between space and nothingness. Its confusing(I think i'm confusing myself!) and I would like to hear some elaboration on this off-topic.

[neilep (OP)]

Spin in relation to what? A spin is usually observed from some static frame of reference. Maybe it is spinning and this causes the observed expansion but there is no way to determine this without said frame of reference which by definition does not exist.

Good point Paul. hmmm..well it's expanding so it's moving yes ? Does the Universe specifically have to spin in relation to something else ?

[neilep (OP)]

Why is the Universe  not a thing ? isn't it every-thing ?  which is also a thing ?

Everything spins, but the universe isn't a thing. An object without a bounded size cannot meaningfully spin.

Ewe spin me right round quickly,
tight round, getting dizzy,
ralph a one pound ground round mound

Spin in relation to what?

Spin is absolute, and need not be in relation to any particular frame, although something's angular momentum is at least relative to an axis, but angular moment and spin (RPM say) are different things.

[Halc]

Why is the Universe  not a thing ? isn't it every-thing ?  which is also a thing ?

Things are typically objects, which exist in time and space and thus have bound which constitute their size and duration of existence. They are caused/created.

The universe is contained by neither time nor space, but rather contains them. It has none of the properties listed above.

So any finite subset of material can be said to spin if you want because the material might have a net rotation to it. The universe itself cannot spin in that way, but I suppose it could spin simply by considering it in a rotating reference frame. The universe spins round once a day relative to a sheep ranch in Ireland.

[geordief]

Why is the Universe  not a thing ? isn't it every-thing ?  which is also a thing ?

Everything spins, but the universe isn't a thing. An object without a bounded size cannot meaningfully spin.

Ewe spin me right round quickly,
tight round, getting dizzy,
ralph a one pound ground round mound

Spin in relation to what?

Spin is absolute, and need not be in relation to any particular frame, although something's angular momentum is at least relative to an axis, but angular moment and spin (RPM say) are different things.

I think the universe  is only a "thing" if you can point to it  some way.
I can't see how we could do that even in principle.

Of course we have the word in the dictionary and  it can be used  in the system of language that we use

So ,in that sense it exists(semanticaly) but not in the sense that you seem to be asking

If  you were to ask about the observable universe I would be interested to know if there was any detectable spin ,but I am not sure what frame of reference could be chosen even then that might give an answer that would  apply to the observable universe as a whole.

[Halc]

I think the universe  is only a "thing" if you can point to it  some way.

One could say that a 'thing' is something to which you can point, and also be able to point to not-the-thing.
So I can point to both an apple and the table on which it rests, the latter qualifying as not-apple.

it can be used  in the system of language that we use

Language is a source of a lot of misguided intuition about such topics. It leads to assumptions for which there isn't any actual evidence.

If  you were to ask about the observable universe I would be interested to know if there was any detectable spin

OK, by my definition just above, an OU (defined as all the material that can ever have had a causal effect on an observation event) is a 'thing'. The odds of it having exactly zero angular momentum is nil. Over time, new material is always coming into any given OU, and that material is not going to have zero angular momentum relative to the center of the given OU, so it has spin of sorts. Detectable is a very different story.

Totally unrelated fun fact: Concerning new material coming into a given OU: There is a max total mass/energy that will in infinite time eventually be part of any given OU. When the universe gets to be about 14 BY old (just a short time from now), we will reach the halfway point.  Around that time, exactly half of the material that will eventually be part of our OU will be our OU at that time, if the current FLRW model doesn't get its mass/dark-energy ratios updated much.

[geordief]

One could say that a 'thing' is something to which you can point, and also be able to point to not-the-thing.
So I can point to both an apple and the table on which it rests, the latter qualifying as not-apple.

Yes, that's better.

Language is a source of a lot of misguided intuition about such topics. It leads to assumptions for which there isn't any actual evidence

Oh,yes.

[evan_au]

It is possibly to detect that you are spinning, without reference to the outside universe.
- The Foucault pendulum (1851) was able to show that the Earth was rotating once per day, while sitting on the Earth, inside a building (there were still some people in 1851 who didn't believe Copernicus, Galileo & Newton)
https://en.wikipedia.org/wiki/Foucault_pendulum
- Of course, if it were rotating once per billion years, that would be much harder to detect!

When we look into deep space, we see many individual spiral galaxies rotating, in many different planes and directions, so no overall rotation at that level
- We also see swirling clusters of galaxies, with the individual galaxies orbiting in different directions, so no overall rotation at that level

If there were to be some overall rotation to the visible universe (with us at the center), when you look at high redshift galaxies, then a portion of their redshift would be due to their tangential velocity
- But if science since Copernicus has taught us anything, it is that humanity is not the center of the universe.
- So if you were to develop a model where everyone sees the same effect from every position in the observable universe, I suspect that you would end up with a model dominated by the rotation of your galaxy, and it's random peculiar motion within its supercluster, rather than some large-scale rotation of the observable universe(?)
- While it is possible to have a Hubble-style expansion of the universe, with everyone seeing the same expansion, I think that this cannot happen with rotation, because rotation has a special "axis of rotation".

[Eternal Student]

Hi.

   I must have missed this post when it was new but I'd like to join it now, if that's OK.   It doesn't look like anyone has mentioned   Mach's Principle   or   Frame Dragging in General Relativity.

    Spin is often thought to be absolute.    Start with a bucket of water.   Here's one:


Notice that the water is fairly flat and level across the surface at the moment.
Now put it on a potters spinning wheel and start it spinning....    After a while the the water inside the bucket will start to acquire some spin and something changes in the water level at the top of the bucket.   This is what you get...


The outer edges of the water seem to climb up the sides of the bucket, while the centre of the water becomes a bit depressed.  Overall the water surface becomes a parabolic shape ( a curved shape like a breakfast bowl).

Now the thing is,  why did that happen?   Well... because of the physics and mathematics as shown in that website, you might say.     It would seem that if you kept the bucket still and instead spun everything else in the universe around the bucket then this would not happen.   The water should stay flat and level in the bucket,  right?   In this way rotation is absolute and not relative.   You know which thing is rotating because you can look at the water surface and there is a difference.

   However.... that isn't thought to be what happens.   General relativity suggests that a phenomena called "frame dragging" will occur  if the whole universe was made to spin around the bucket.   Moreover,  the frame of reference in which the bucket is still and not rotating will simply STOP being an inertial frame of reference.   It will behave like an accelerated (rotating) frame of reference and the water level in the bucket would become bowl shaped again.   Overall, there would be no way to know if the bucket was spinning or if the universe was spinning around the bucket.

    It seems that "rotation" is not as absolute as we might have thought,  provided you move to General relativity instead of just Special relativity then rotation is not absolute, it is relative just as translational motion (ordinary straight line motion) is relative.

   Mach's principle is also interesting and very much relates and connects to this discussion.   It can be heavily paraphrased and shortened to the following idea:      The mass and movement of mass out there in the universe.... influences the inertia (the movement of mass) here.     More generally, the movement and behaviour of objects locally is influenced by the movement and behaviour of every other object anywhere and everywhere in the universe.
    This is what Wikipedia says about Mach's principle:
You are standing in a field looking at the stars. Your arms are resting freely at your side, and you see that the distant stars are not moving. Now start spinning. The stars are whirling around you and your arms are pulled away from your body. Why should your arms be pulled away when the stars are whirling? Why should they be dangling freely when the stars don't move?
Mach's principle says that this is not a coincidence—that there is a physical law that relates the motion of the distant stars to the local inertial frame.

    Anyway... I find the whole idea of Mach's principle and of frame dragging quite interesting on their own but it strongly relates to what you  ( @neilep ) was asking:
    Does the Universe spin?
Well, quite possibly  -  but we wouldn't know and it would make no difference.   A frame of reference that didn't spin with the universe wouldn't be an inertial frame of reference   (for example, Newton's laws would not apply).     We can't say... well look, I'm certain that my frame of reference is the right one to use,  the only possibility then is that the universe is spinning.   We just don't know which frame of reference is the "right one".   More generally there doesn't need to be one absolute "right one" to use.    The laws of physics hold in any inertial frame of reference.    So we will naturally tend to use a frame of reference that spins with the universe.

Hope that helps.
Best Wishes.

[Colin2B]

It doesn't look like anyone has mentioned   Mach's Principle 

I don’t think anyone has ever pinned down Mach’s principle to a complete theory, although I do find Lense–Thirring interesting when applied to an outer shell of mass.
Interesting thought, if we can’t see beyond the visible limit how much would any mass out there affect our perception of rotation if it is spinning relative to the bit we can see. Mach thought it would.

I think the universe  is only a "thing" if you can point to it  some way.
I can't see how we could do that even in principle.

Scientific and mathematical terms are usually well defined for good reasons; general language is often more open to interpretation and misunderstanding as you say. ‘Thing’ is pretty vague and could cover an idea:
Merriam-Webster for eg:
1: an object or entity not precisely designated or capable of being designated
2a: an inanimate object distinguished from a living being
b: a separate and distinct individual quality, fact, idea, or usually entity
c: the concrete entity as distinguished from its appearances
d: a spatial entity

Edit: by the way, I agree with @Halc that it doesn’t make sense to say the universe is spinning. If, say, we define the universe ‘thing’ as everything that exists, then what could it spin relative to?
Getting too close to philosophy for my liking 

[Eternal Student]

Hi.

   I thought I had replied to this already but either I forgot to press "Post" or else I've accidentally posted it somewhere else.

I don't think anyone has ever pinned down Mach's principle to a complete theory,

    Yes, it is just a conjecture and Mach himself didn't suggest a convincing mechanism for it (to the best of my knowledge).    Frame dragging in General Relativity does provide one possible mechanism to explain it.

Interesting thought, if we can't see beyond the visible limit how much would any mass out there affect our perception of rotation if it is spinning relative to the bit we can see. Mach thought it would.

    This is what I would have thought:
    The EFE is    G_μν = k T_μν   (with a cosmological constant if you want it).    The mass and momentum of the very distant parts of the Universe are in the stress-energy tensor tensor whether you can see it or not   (i.e. even it it is outside the visible universe).   So if it is spinning then this does influence the Riemann curvature tensor and hence the metric and corresponding co-ordinate system that you can obtain as a solution to that EFE.    To say it more simply,   if the matter-energy outside the visible universe is spinning relative to the visible universe then it can and should have an effect right here and now.
    How much effect is more difficult to answer.    I haven't done the calculations, so everything is outright speculation from here on.   Usually, Newtonian gravity isn't a bad intuitive guide to what happens.   We would expect the increased distance from planet earth to reduce the effect of frame dragging, something like ~ 1/r².  However, we would also have the amount of material at a distance r from earth grow as  4πr².  So similar to the hollow shell theorem...  the universe spinning outside of our visible universe could have a substantial effect.
     Light from those distant parts of the universe may never reach us but propagation of effect from those distant parts of the universe is not an issue because we can assume conservation of angular momentum applies.   So the universe has always been spinning this way overall and the frame dragging effect has always been here.

Best Wishes.