Why Does Everything in the Universe Rotate?

[Jimster41]

This is what I'm confused about. If Newton is the mechanism of conservation. I don't see how asymmetry can "develop". It has to just suddenly exist? What am I missing?

${ L }[t]\quad ={ \quad I }_{ { R } }{ \omega }_{ { R } }+\sum _{ i=0 }^{ \infty }{ { I }_{ i } } { \omega }_{ i }\quad \\ { L }[t+\tau ]\quad =\quad { I }_{ { R } }{ \omega }_{ { R } }+\sum _{ i=0 }^{ \infty }{ { I }_{ i } } { \omega }_{ i }$

If there is some sense in which gravitational propagation is required between particles, then "spontaneous" asymmetry seems natural, because you can have that third (or eventually uneven) moment.

 

[wabbit]

But even if the total momentum is zero, any finite region will generically have some (perhaps small) angular momentum.

I suspect this is true except for subsystems relative to which the rest of the universe is exactly spherically symmetric.

 

[PeterDonis]

If Newton is the mechanism of conservation. I don't see how asymmetry can "develop".

Global asymmetry being zero is not the same as no asymmetry at all. The global asymmetry is the total of it over the entire universe. You can have individual systems within the universe that have nonzero angular momentum, as long as the total of all of them works out to zero.

 

[HallsofIvy]

If you rephrase the question- "why is there nothing in the universe that has rate of rotation exactly equal to 0?" you should be able to see that it would be extremely unlikely for any object to have exactly any given rate of rotation, including 0.

 

[rootone]

It makes sense that locally condensing gas clouds can have an angular momentum, but the net momentum of the Universe is zero.
Hard to imagine it being otherwise because then you have to figure out what the Universe is rotating in.

 

[Jimster41]

Bear with me, this is something that has been bugging me awhile...

I'm trying to get a clear picture of how rotation "starts" in isolation. I'm picturing a uniform volume, isotropic, homogenous, that super high z instant, whenever it was, where suddenly the volume was differentiated. If this was not "last scattering", there still had to be a that instant, right. I want to say, at that instant the number of degrees of freedom has just gone up.

But even if the total momentum is zero, any finite region will generically have some (perhaps small) angular momentum.

I suspect this is true except for subsystems relative to which the rest of the universe is exactly spherically symmetric.

What subsystem get's to start off being different, in the homogenous isotropic case?

I am totally fine with saying, "a random one". The part I'm just trying to clarify, is what mechanism of clumping, can account for it.

Newtons law of gravity, because it is not a function of time (is it?), or sequence, doesn't seem to me to be able to explain how angular assymetry developed in that initial situation, where we really do have to account for it spontaneously occurring. In that first differentiated instant, For Newton, aren't all angular gravitational moments, from all random motions, in all random regions exactly what they were one instant to the next. Unless angular asymmetry is introduced into that gravitationally "frozen" system, what can cause it to change?

A propagation limit suddenly applied to those Newtonian relations however, a flow, seems to me to provide the means by which a single random movement, or all single (pair wise) random movement, suddenly becomes candidate for the formation of the first ever "plane of gravitational asymmetry".

To me it seems to require picturing a "medium" through which gravity must travel at some limit, through which gravity propagates... over time.

And this then fits the bill of a mechanism that took off in lots of random places, more or less at once. That still abides.

In our highly asymmetrical angularly rotating neighborhood way down the cloud of spacetime helictites, (which should represent any neighborhood of course), it's easy to invoke the "initiating" assymetric moment. But that doesn't mean the same process of spontaneous angular symmetry breaking isn't still all over the place.

Where my head is truly stuck, is in associating the instant of "propagation limit begins" with the instant of "+1d for g" with the instant of "start expansion". Suddenly, at that moment, a counting process in 3d+1 gravity requires helical trajectories of all worldlines, henceforth.

@PeterDonis, you said awhile back that not everything rotates. I'm prepared to agree but I'm having trouble thinking of an example. Don't all quantum particles (except the Higgs Boson, which as I understand it is responsible for giving mass to other particles!) have spin, even if in combinations, some spins cancel.

 

[Bandersnatch]

If this was not "last scattering", there still had to be a that instant, right

I think you're getting too hung up on this perfect-homogeneity picture. While it's an alright analytic tool, there's no reason to think there actually ever was such a state in the history of the universe.

There were periods when the universe was in a state of high homogeneity, such as before recombination (last scattering) when all the plasma constituting the (our patch of) universe was thermalised, and any overdensities tended to rebound due to overpressure, or after inflation (which was invented to deal with the question how come all the universe was thermalised in the first place). But it never was a perfect homogeneity.

A good analogy, I think, would be any arbitrary close volume of gas at a set temperature. You can say it's globally homogeneous, you can say it's got no momentum, angular or otherwise, but look close enough at a small enough volume and you'll see how neither of those qualifiers apply.
If you then cool this volume of gas to a low enough temperature, the thermal motion keeping it globally homogeneous will drop and the particles will start to clump, and all that motion and overdensities that required special attention to notice earlier will become immediately apparent.

 

[wabbit]

I'm not sure I understand your question, but I see two points : first, starting from a perfectly symmetrical universe, it seems to me pretty much any quantum fluctuation results in some asymetry.

In a classical setup, symmetry is preserved if it is there to start with - those are the FRW solutions, where all matter is comoving and nothing happens at all in the universe except global uniform expansion or contraction. But these are extremely special spacetimes and it would be rather extraordinary if our universe matched that exactly - it doesn't of course, not any more than the surface of the Earth is a perfect sphere, these are just simplifying asumptions valid as a first approximation.

In general, asymmetry is generic, symmetry is very special and requires an explanation. This is the case even if the underlying equations are symmetric, only in rare cases are generic solutions symmetric too I think.

 

[Chronos]

The issue of the univetse rotating with respect to 'what' remains unanswered.

 

[wabbit]

The issue of the univetse rotating with respect to 'what' remains unanswered.

If about the whole universe, hypothetically, rotating in the sense that inertial observers see each other rotate I suppose, as in a Gödel spacetime. I don't know what the "standard rotating cosmological model" might be or if it exists.

 

[Jimster41]

I had been operating on from the point of view that primordial anisotropy was a puzzle? (for information conservation purposes I guess)?

http://arxiv.org/pdf/1410.1562v1.pdf
Quantum collapse as source of the seeds of cosmic structure during the radiation era
Authors: Gabriel León, Susana J. Landau, María Pía Piccirilli
(Submitted on 29 Sep 2014)
Abstract: The emergence of the seeds of cosmic structure, from a perfect isotropic and homogeneous Universe, has not been clearly explained by the standard version of inflationary models as the dynamics involved preserve the homogeneity and isotropy at all times. A proposal that attempts to deal with this problem, by introducing "the self-induced collapse hypothesis," has been introduced by D. Sudarsky and collaborators in previous papers. In all these works, the collapse of the wave function of the inflaton mode is restricted to occur during the inflationary period. In this paper, we analyse the possibility that the collapse happens during the radiation era. A viable model can be constructed under the condition that the inflaton field variable must be affected by the collapse while the momentum variable can or cannot be affected. Another condition to be fulfilled is that the time of collapse must be independent of k . However, when comparing with recent observational data, the predictions of the model cannot be distinguished from the ones provided by the standard inflationary scenario. The main reason for this arises from the requirement that primordial power spectrum obtained for the radiation era matches the amplitude of scalar fluctuations consistent with the latest CMB observations. This latter constraint results in a limit on the possible times of collapse and ensures that the contribution of the inflaton field to the energy-momentum tensor is negligible compared to the contribution of the radiation fields.Which is why I am sort of hung up on a mechanism by which something uniform, or "perfectly gaussian" goes to a "differentiated" cloud of randomly moving particles then to "swirling" around specific loci.

If when gravity "turned on" it was Newtonian then all Newtonian moments were accounted for everywhere weren't they? At time 0, it is instantly an n-body solution where n is all bodies there are. And at time t+tau it is a continuous extension of the same solution. So if it started off gaussian at time 0, how did it get non-gaussian later. Sine it was non-gaussian later, it couldn't have been gaussian at t=0. So if things were swirling around specific locations later, then the distribution of moments (and momenta) had to contain that outcome to begin with. And maybe that's what happened.

On the other hand if when gravity "turned on", it had to "flow" to connect masses over time (through some sequence) then the swirling foci seen later were just those locations where the differentiated and "suddenly massive" particles were closest to each other. It is a subtle distinction. But it just seems more direct, simpler. And it's what I thought the theory said. It still leaves the question of information content, why the initial quanta were distributed spatially the way they were. But it is more easily flipped in my mind as information contained in the expansion field. And that seems nice.

... because it means that the fact everything rotates now, does not necessarily separate from the reason it started rotating then. If expansion/inflation (I do conflate them) and "geometry flow" were the cause then, what has changed? There is a vast backdrop of resulting "helicity" but that doesn't mean it is not an ongoing process?

 

[Jimster41]

"closest to each" other, as in entangled?

 

[PeterDonis]

something uniform, or "perfectly gaussian"

When that paper says "a perfect isotropic and homogeneous Universe", it is talking about an idealized model, not the real universe. The real universe was never perfectly isotropic and homogeneous. So we don't need an explanation of how something perfectly uniform could produce non-uniformity; that never happened. That's why you should stop being "hung up" on finding such an explanation; it's a wild goose chase.

If when gravity "turned on" it was Newtonian

Gravity never "turned" on; it was always there. And gravity was never Newtonian; it was always described by GR.