Science Questions

How did the Universe come by its angular momentum?

Sun, 1st Apr 2012

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Alan Alderson asked:



Thanks for the show, I find it totally fascinating.


It seems the universe is full of angular momentum. Planets revolve and orbit stars, stars revolve and orbit galactic centres even sub atomic particles spin.


Yet the origin of the universe was the big bang where everything exploded outward froma point source. As I understand it there is a law of conservation of angular momentum saying it cannot be created or destroyed onlt transferred. So I am puzzled where did all the angular momentum come from?



Freddie Alderson




You don't actually have to have any overall rotation in the whole universe for objects within the universe to be rotating in various different directions.

Imagine that you have a large cloud of gas that has no rotation to begin with, but that you split it up into lots of little pieces which go on to form into lots of different galaxies.  Each piece by random chance will have some particles which are swirling around in some particular direction, and others which are swirling around in other directions, so it will have some very tiny rotation in some particular direction.  Now as that cloud collapses down to form a star or a galaxy, there's an effect called the conservation of angular momentum which means that spin is accelerated as the object gets smaller.

That's similar to the effect when a figure skater is skating on ice and they start off spinning quite slowly, when they pull their arms in you'll see they start spinning incredibly quickly.  So, that means that an object, even if it only has a tiny rotation to begin with, can actually end up rotating quite appreciably as we see planets and stars in the universe doing.  Certainly, if you look at the rotations of different stars, you will see that smaller stars tend to rotate much more quickly than bigger stars and that is because they're being spun up as they've contracted down.


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Angular momentum is a vector. If two bodies have equal amounts of angular momentum, but they are spinning in opposite directions, the total angular momentum of the two bodies is zero. Two bodies can begin motionless beside one another, give each other a push and move apart spinning in opposite directions. That does not change the total angular momentum of the pair. Phractality, Sun, 1st Apr 2012

Brilliant OP question.

The answer, is that there is a growing number of physicists who actually believe the universe once spun (rotated) very early on it's history, making very early galaxies spin. There is in fact a specific handedness of spin in the universe, though admittedly some other galaxies do rotate the other way. Ęthelwulf, Sun, 1st Apr 2012

Of course the universe may be spinning right now, a so-called Machian Universe. However, this presumes, just as it did very early on, that the universe has a boundary. What is it relatively a boundary between is a new question. Ęthelwulf, Sun, 1st Apr 2012

Although the initial expansion seems to have been very smooth gravitational collapse and supernova explosions rapidly create turbulence at all scales.  this should integrate to zero if the universe does not have any net rotation

There is a considerable amount of work trying to assess how well balanced the net angular momentum expressed in galaxy rotations to try to find out of there is a net rotation in any direction.

The cosmic microwave background does appear to have some sort of rotational axis (sometimes dubbed the axis of evil). Soul Surfer, Mon, 2nd Apr 2012

No, it doesn't 'rotate'.

It's a meaningless idea to me as any rotation needs a reference frame from where it can be said to 'rotate' according to Mach's principle. So if the universe would 'rotate' it would need something from that rotation needs to be defined. You could assume that we in a 'lab' can prove a rotation but if you're honest you know that any thought up 'lab' in SpaceTime actually has another reference frame outside it, even if not 'known' for you.  Also it would destroy the isotropic (being the same in all directions) universe we expect to exist, and so question relativity. And as far as I've seen there are no proofs in the CBR so far for it. yor_on, Tue, 3rd Apr 2012

From Lisalotte who I wish would register here some day :)

"Pull the plug on a bathtub full of water and observe the motion of the water as it drains.

"all right"-"imagime this. Right You get this bath. Right. A large round bath. And it's made of ebony"
"doesn't matter...!"
"So you keep saying."
"listen. You get this bath, see...? Imagine you've got this bath. And it's ebony. And it's conical."
"Shhh!. It's conical. So what you do is, you see, you fill it with fine white sand, all right? Or sugar. Fine white sand, and/or sugar. Anything. Doesn't matter. Sugar's fine. And when it's full you pull the plug out, are you listening?"
"I'm listening"
"You pull the plug out and it just twirls away, twirls away you see, out of the plughole."
" I see"
"You don't see, you don't see at all. I haven't got to te celver bit yet. You want to hear the clever bit?"
"Tell me the clever bit"
"I'll tell you the clever bit. The clever bit is this. YOu film it happening."
"you get a movie camera and you film it happening"
"thet's not the clever bit. This is the clever bit, I remember now that this is the clever bit. The clever bit is that you then thread the film in the projector... backwards!"
"Yes. Threading it backwards is definately the clever bit. So then, you sit and watch it, and everything just appears to spiral upwards out of the plughole and fill the bath. See?

"And thet's how the universe began, is it?"

"no. But it's a marvellous way to relax."

gosn. Prdzadza." yor_on, Tue, 3rd Apr 2012

There seems to be a construct of the "fabric of space" or "fabric of space-time", or in some theories, aether that creates a universal reference frame necessary for matter to exist in.  So, the all the matter in the universe can be rotating with respect to the fabric of space reference frame.

Rotation may be a natural consequence of gravity.  Two moving bodies will tend to be attracted towards each other.  If not perfectly matched, they will invariably miss, and thus orbital rotation is born. 

As far as a preferred direction?  Obviously if the motion is random, those rotating in opposite directions would increase the probability of collisions.  Once rotation is established, it may also induce more rotation in the same direction. 

Angular momentum from a condensing particle cloud would also be preserved. CliffordK, Tue, 3rd Apr 2012

If you think of a solar system evolving from a cloud of dust where all particles are moving independently, then there will be a random angular momentum in the cloud of dust about its particular centre of gravity. But as the gravitational forces start to pull the system together, the angular momentum will be conserved, but the mass will gradually accumulate close to the centre. To conserve the angular momentum, the cloud in general, and the central mass in particular will have to rotate faster and faster -- smaller effective radius means larger angular velocity, since j = m * omega * r, and j and m are both constant. (Actually some mass will be lost to the system in the energetic turbulence of the process, but not enough to change this conclusion). Once the rotation speed of the central body gets to a particular level, the remaining material outside it is moving too fast to accrete with it, and so it will orbit. But irregularities and gravitational forces in this material will cause a similar process to repeat in the accumulation of the proto-planets and their satellites. That accounts for the fact that in general nearly every massive body in the solar system rotates and orbits in the same direction, and in nearly the same plane as everything else. But all of that angular momentum arises from a simple random sum of the angular momenta of the material that has accreted in the first place.

There -- I have told my story -- is it roughly right? More importantly, could I have applied it to a galaxy rather than just a solar system? damocles, Tue, 3rd Apr 2012

Maybe now; according to recent work, it seems very possible it did very early on in it's history Ęthelwulf, Tue, 3rd Apr 2012

That is interesting Wulf :)
Although I'm unsure if it can be used to prove a rotation.

And I'm still finding it difficult to imagine a rotation of anything, without having a reference frame from/in which it 'rotates'.
If you check this one out from Galaxy Zoo.  they discuss it too.

"We do not expect the same number of ellipticals as spirals. But indeed as you point out, whatever the ratio is then we expect it to stay roughly the same with distance from us. What we actually find is that the proportion of ellipiticals increases with distance from us, and this is mainly because as things get fainter and fuzzier then they tend to look more like ellipticals! One of the Zoo papers out later this month is exactly on this point - Steven has shown that we can statistically correct for this kind of elliptical biasing that happens with distance: he finds the bias correction as a function of size on sky, and luminosity. And once you correct for this then you find the ratio is constant with distance (redshift). Hoorah!"

And they used/had access to a larger set of statistics as I understand it?

There could be something new to this study though, they should have checked up the study Galaxy Zoo did before as theirs are newer. But it would be nice to know how and what? And then there are other arguments one can use. But it becomes at best educated guesses from both sides as we do not 'know' where we are relative any 'center' of a universe, or even if you can think of it that way, which I sincerely doubt. I find the question of a reference frame good enough myself. yor_on, Tue, 3rd Apr 2012

That Lisalotte is a hoopy frood who sure knows where her towel is - and perhaps she should stop ripping off the HHG2TG and Prof Ford Prefect imatfaal, Tue, 3rd Apr 2012

In hitchhikers?

That Ford Prefect? :)

I'll be da**ed yor_on, Wed, 4th Apr 2012

  Ford explains it to Arthur whilst at the Restaurant at the End of the Universe

Its Chapter 17 of the R at the E of the U imatfaal, Wed, 4th Apr 2012

I need to reread it :)

It's an immortal classic. yor_on, Thu, 5th Apr 2012

The Doc has presented a simple question in a perfectly eloquent fashion. This is a question which has been bugging me for some time and respectfully I suggest none of you have answered it. Not that I expect to either but here goes.

The Doc pointed out that matter must have been travelling outwards in all directions, subject to the same force. I almost wrote velocity not force but that would be wrong. Matter of differing densities, if that's how it was, with differing masses, would presumably travel at different velocities. Even so the differences would intially have been very small.

At the initial singularity there was presumably just energy, which coverted to matter over time. How much time? Who knows? So since gravity has no effect on energy, gravity has been developing so to speak, over time since the initial moment. I.E. there was no gravity at the point of origin. Is that right, does pure energy have gravity or is gravity a characteristic or matter only? When earth is flung from an exploding bomb there are all sorts of bits and pieces and gravity is of course present having it's persistent way with the 'falling' debris. It seems that lumps of varying density will interact and begin to rotate around each other as they fly out of the crater being formed. But if in the begining there was no gravity; in the first few minutes or hours as we know time. Of course as space has expanded so as time so what we understand as a second was a 100th of a nanosecond , or something like that, I don't know. I just think that in the initial period long before any astronomical body, galaxy, star or planet, was formed there was no or very little gravitational effect to force matter to rotate whether about a partner or itself. I think matter must have been travelling outwards in all directions and spreading further away from all other matter with little or nothing to stop what must have been a fairly linear expansion. And with greater separation even less interactive gravitational effect.

Side question; has all the matter that could be created from the initial energy been created? Or is matter still being created at the same rate or lesser rate, albeit spread over an entire universe?

Contributor Damocles suggests the image of matter moving 'independently', but matter wasn't moving independently was it? In the same way a bullet travelling from a gun is not independent; it cannot randomly stop, it must keep going until another object or gravity brings it under control. Until then it is dependent on the energy imparted by the gunpowder exploding in the cap.  Likewise the matter at the outset of the big bang was travelling outwards, as, I guess, it still is, with nothing to either stop it or hold it back.  There is no way in my imagination that I can see how we get to the point where everything, it seems, is busy rotating around either it's neighbour or itself.

Could it be, that just as the tiny force of gravity builds into a massive force when matter is drawn together, very slowly over eons, that the angular momentum of electrons whizzing around inside atoms is the force that has developed into the almost universally apparent rotation we see now?  Hey it's just a thought! Tunsarod, Sat, 21st Apr 2012

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