Does the Universe Rotate and Does it Have an Axis?

[CaptainQuasar]

Responding to things way back on the first page of this thread…

I think the answer will be , rotating in reference to what.

That question actually doesn't need to be asked because rotational references frames aren't relative - inertial reference frames are relative. I asked about it in [THREAD=214004]this question[/THREAD] a while back but people either didn't understand or weren't interested.

If the universe were rotating you would be able to detect the same phantom forces you detect within the Earth's rotating reference frame, like the Coriolis force. I don't believe we detect anything like that, so the universe isn't rotating. As I said in my post, it seems to me that this means that although the universe doesn't have absolute space or time it does have absolute directionality.

There's got to be an edge of space, with space on one side, and nothingness on the other, if you follow the big-bang theory.

That isn't true. See this http://www.astro.princeton.edu/~aes/AST105/Readings/misconceptionsBigBang.pdf" article that marcus frequently posts:

This ubiquity of the big bang holds no matter how big the universe is or even whether it is finite or infinite in size.

The Big Bang does not imply that there is an edge to space or that the universe is finite in size.

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[Ulysees]

We went through the bit about edges exhaustively, it has been corrected.

If the universe were rotating you would be able to detect the same phantom forces you detect within the Earth's rotating reference frame, like the Coriolis force.

Those phantom forces due to self-rotation can be detected because the Earth is a connected body, not a set of independent particles. But you can't detect inertial forces due to rotation about the sun, or the galaxy, or about anything bigger.

Or you have any suggestion how to detect forces from the galaxy onto the earth?

 

[CaptainQuasar]

We went through the bit about edges exhaustively, it has been corrected.

Sorry about that, didn't pick up on it in the course of my skimming.

Those phantom forces due to self-rotation can be detected because the Earth is a connected body, not a set of independent particles. But you can't detect inertial forces due to rotation about the sun, or the galaxy, or about anything bigger.

Or you have any suggestion how to detect forces from the galaxy onto the earth?

If you set up the Earth in a rotational reference frame so that it and the axis of the Milky Way remain stationary - in which you wouldn't be calculating any angular momentum of the solar system circling around the galaxy due to gravity - there would be phantom forces that could not be accounted for by applying Newtonian physics as if you're in a non-rotating reference frame, yes. And as I pointed out in the post I linked to, within the rotational reference frame other galaxies would appear to be moving faster than light.

I'll elaborate on that if you want me to but I think you aren't thinking this through. It's a matter of picking an invalid reference frame. Remember how early astronomers thought that the orbits of Mars and Jupiter and the other planets had “epicycles” in them, because they thought the other planets orbited the Earth instead of the Sun? It's that kind of thing.

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[talmans]

Thanks for the link to Misperceptions article. It clears up a lot of things but not a question I always wanted to ask.

If I understand it right space is expanding all around us and gravity and the forces of nature keep verything intact. So the space between the Earth and moon is expanding and gravity keeps them together. But that expanding space has to go somewhere. Would be possible to measure that at short distances? Is it right to say that new space is created or can you only say that space itself is somehow expanding?

 

[CaptainQuasar]

Thanks for the link to Misperceptions article. It clears up a lot of things but not a question I always wanted to ask.

If I understand it right space is expanding all around us and gravity and the forces of nature keep verything intact. So the space between the Earth and moon is expanding and gravity keeps them together. But that expanding space has to go somewhere. Would be possible to measure that at short distances? Is it right to say that new space is created or can you only say that space itself is somehow expanding?

I'm not a scientist but I think the response might ask what the difference between those two things is. How could you tell the difference between new space that has been created and space created by the stretching of existing space? Your question is a good question but unless I'm misunderstanding you it may be a matter of semantics.

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[Wallace]

Thanks for the link to Misperceptions article. It clears up a lot of things but not a question I always wanted to ask.

If I understand it right space is expanding all around us and gravity and the forces of nature keep verything intact. So the space between the Earth and moon is expanding and gravity keeps them together. But that expanding space has to go somewhere. Would be possible to measure that at short distances? Is it right to say that new space is created or can you only say that space itself is somehow expanding?

The short answer is that space does not expand in the 'naive' sense of the word (not to call you naive!). The words 'expansion of space' are an intellectual shorthand that helps to guide intuition about an expanding Universe. Things like galaxies and solar systems however are not expanding and hence try to use the same concept simply doesn't work.

The expansion of space is the description of the effects of General Relativity on an expanding homogeneous universe. Using GR in certain limits we can also model what things like galaxies and solar systems embedded in expanding universes look like. The result is that we don't see any 'expansion of space' in the region of these bound objects.

One way to think about it is that 'the expansion of space' is the effect of the expansion of the matter in the Universe, rather than what people often assume which is that the matter moves away because of the expansion of space. If you get the causality direction right then it helps guide your intuition much better. The matter in a galaxy or solar system is not move away from itself and hence we don't see any of the hallmarks of the expansion of space that you see on larger scales in which the matter is moving away.

For more details have a read of http://arxiv.org/abs/0707.0380" paper. It's a scholarly article, rather than pop sci, so a little more heavy going than the misconceptions link however as scholarly articles I don't think it's too bad in terms of understandability to a lower level audience. If there is anything in that you don't understand I can explain it for you (I'm actually one of the authors of that paper).

 

[talmans]

Thanks for the paper. I like how you start at first principles. Unfortunately, I wasn't schooled in those so it's a little tough. I'll explain my take aways and you can fill in a few things from there. But first can you explain what the cosmological fluid is?

My interpretation is that space expands only when certain physical conditions are met and that doesn't happen when matter is clumped together in galaxies. This seems counterintuitive since I thought there was plenty of empty space there.

I didn't get the explanation of red shift but its interesting this is discreet and not continuous since the photon is also described as a stretched wave.

This all really goes against the gut feeling doesn't it. What happens in frames where space expands?

 

[Wallace]

Thanks for the paper. I like how you start at first principles. Unfortunately, I wasn't schooled in those so it's a little tough. I'll explain my take aways and you can fill in a few things from there. But first can you explain what the cosmological fluid is?

The equations that describe cosmology model the contents of the Universe as a fluid with a given relationship between the pressure and energy density of the material. Don't worry to much about it, just read 'fluid' as 'stuff'. Galaxies, dark matter etc are all part of the 'fluid'.

My interpretation is that space expands only when certain physical conditions are met and that doesn't happen when matter is clumped together in galaxies. This seems counterintuitive since I thought there was plenty of empty space there.

Why do you expect space to expand just because it is empty? Remember that space never really expands at all, in the sense of it causing the motion of any material. The phrase 'expansion of space' is just a concise description of the motion of material in an expanding Universe, not a physical theory describing how gravity works. It is an analogy, not a physical effect.

What happens in frames where space expands?

I don't understand the question? By definition a momentary co-moving reference frame (which is the full version of what people dub 'frames' in GR) is equivalent to special relativistic flat space. The phrase 'frame in which space expands' therefore doesn't make sense in the normal use of the terminology. If you explain what you meant by that in more detail I can answer the question.

 

[CaptainQuasar]

Wallace, I haven't gotten through the paper you linked to - thank you btw - but I wanted to ask, does the rate of expansion differ at different locations in space, to our knowledge? I'm not sure but it seems like talmans might be asking a question along those lines, whether cosmological expansion happens in quite the same manner within galaxies as it happens in the voids between galaxies and clusters and superclusters.

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[talmans]

I obviously misinterpreted things from the article. In section 2 it clearly says that the expanding of space is not a physical phenomenon, or at least, its better not to view it that way. However, the last sentence or two in section 2.2, local expansions, says

The
expansion of space fails to have a ‘meaningful local
counterpart’ not because there is some sleight of hand
involved in considering the two regimes but because
the physical conditions that manifest the effects described
as the expansion of space are not met in the
average suburban bedroom

The statement "...the physical conditions that manifest the effects described
as the expansion of space are not met in the
average suburban bedroom." catches my eye.

Earlier in section 2.1 it states " ...It may be
misleading to suggest that the space that was there
stretched itself as the universe expanded. Perhaps a
better description, in simple terms, is to suggest that
more space appeared, or ‘welled up’ between the two
observers, however this is a largely semantic distinction."

This last statement and the conclusion is the basis of my question. The welling up occurs only in non-local frames. If not local then where? I'm just trying to envision what this welling up looks like.

My view is that galaxies, and such, are carried along with the expanding space, stuck in the amber of the cosmological fluid in a sense. But the amber doesn't ooze out in non-local frames.

 

[wolram]

The be all and end all of this is that some guys have got observation to fit with theory, those guys have spent thousands of hours getting things to fit in a scientific way, so who is going to replicate those thousands of hours, they may be wrong but it will take life times to prove them so.

 

[cristo]

I'm not sure but it seems like talmans might be asking a question along those lines, whether cosmological expansion happens in quite the same manner within galaxies as it happens in the voids between galaxies and clusters and superclusters.

No, galaxies themselves are not seen to be "expanding." As stated in the article, local regions of spacetime are not homogeneous, since they contain matter, and so the local spacetime cannot be described by the FRW metric. In any gravitationally bound system one must write down a different metric for the system. The FRW metric is only a description of the universe on large scales; that is, treating any gravitationally bound system as a point.

 

[CaptainQuasar]

No, galaxies themselves are not seen to be "expanding." As stated in the article, local regions of spacetime are not homogeneous, since they contain matter, and so the local spacetime cannot be described by the FRW metric. In any gravitationally bound system one must write down a different metric for the system. The FRW metric is only a description of the universe on large scales; that is, treating any gravitationally bound system as a point.

Ah, I think I understand that, thank you. I know that galaxies themselves are not expanding, but does cosmological expansion manifest itself as something like a force opposing the attractive force of gravity that holds galaxies together? Or is it completely undetectable on a galactic scale?

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[George Jones]

There's got to be an edge of space, with space on one side, and nothingness on the other, if you follow the big-bang theory.

Any standard classical Big Bang spacetime has an edge.

I wouldn't say "space," and I wouldn't say "nothingness on the other" side.

 

[Ulysees]

It's only by definition that edge, they say the "universe" but they really mean the observable part of the universe.

 

[George Jones]

It's only by definition that edge, they say the "universe" but they really mean the observable part of the universe.

No, that's not the edge to which I referred.

 

[Ulysees]

Interesting. What edge were you referring to then?

 

[BoomBoom]

No, that's not the edge to which I referred.

He must mean the only edge that we know of and are intimately connected to:

The temporal edge of the universe or... time.

Our only source of information of what the universe is doing "right now" in our time frame is our local universe...which ironically is contracting. :O

As far as an expanding universe goes... if the universe is truly infinite, I find it somwhat suspect to assume that the "whole" universe behaves as our "observable" universe does. Infinite is quite large indeed! :P The continued assertion that it does not expand "into" anything and the description of empty space as a physical thing I think confuses many...as it should. IMHO empty space cannot be "added" to a universe unless all the matter is expaning "into" it.

(ok you can all jump all over me now for my "blasphemy") :P

 

[Ulysees]

Are the most distant objects ever observed, all over the sky? Or do they tend to be in one direction?

1. If they tend to be in one direction, then that indicates the universe is like an expanding cloud, mass has a finite extent and therefore there's a centre of mass and therefore the universe has a tendency to rotate due to gravity.

2. If distant objects are all over the sky, then that indicates there's no edge to the extent of matter that we can observe, we can draw no conclusion whether the universe is finite or infinite from this or whether it rotates or not.

 

[Wallace]

Are the most distant objects ever observed, all over the sky? Or do they tend to be in one direction?

Observations of every type of object observed, whether near or far, are isotropic (the same in all directions). This is a very fundamental observation that is at the heart of our current cosmological model.

 

[edjoyce]

I would propose that the universe is no rotating for the following reason.

If you have a laser fired at you it does not matter whether you are stationary or moving closer or further from the laser the light will strike you at the same time. The same effect means that everywhere appears to be the centre of the universe. If everywhere is the centre then everywhere is expanding away from us therefore it cannot be spinning. We appear to be at the centre of an expanding bubble but then so does everywhere else.

This is my working assumption and I would be happy if someone could put me right if this is not so.

Ed Joyce

 

[stiv]

If the universe is rotating, then we have no possible way in which to observe its rotation, since we are observers from within the metric which is dependent upon the observable structure. The observable universe could be doing star-jumps, or stretching like a tired bear, but from who's perspective? We are within a space-time phenomenon and, it must follow, that all methods of measurement remain inside the same metric. From an observer outside of our metric, a metre "here" may well be a billion kilometres long "elsewhere", or a second "here" may as well be Planck time "there". We ourselves have no possible choice, but to use our universe within its given metric.
However: Godel solved a non-metrical solution to space-time. We need to leap forward a few years and then come back again: Penrose et al developed the idea of a light cone within space-time. For simplicity, let's flatten it out. You have an event (planet, fart, whatever) as a single point. You know that light has a definite speed, so run light escaping up the y-axis. You also know that it can't escape sideways any faster than usual, so run distance up along the x-axis. What you end up with is a cone of events that can be influenced by the event at the origin. There remains outside of the cone events that cannot be observed, nor can they influence actions within the cone.
So: We have so far talked about a flat universe - or at least one that follows geodesic principles - under those circumstances, there is no physical possibility of a photon from outside the light cone impinging within our own reality. Godel's maths suggested that we could rotate the universe and skew the light cones, so that events which we should see in the future, we see now: so we observe future history. Or, if bent the other way, we can observe events who's light cone is separated, but by bending space-time we can observe and manipulate previous events.
My maths isn't good enough to run through the above. Perhaps someone can help.

 

[bcrowell]

If the universe is rotating, then we have no possible way in which to observe its rotation, since we are observers from within the metric which is dependent upon the observable structure.

This is incorrect.

FAQ: Can we tell whether the universe is rotating?

It is possible according to general relativity to have cosmologies in which the universe is rotating. This is a non-Machian feature of GR, since the rotation is not relative to anything else. There does not have to be a center of rotation, and such solutions can be homogeneous. One of the earliest cosmological solutions to the Einstein field equations to be discovered was the Gödel metric, which rotates and has closed timelike curves.

Solar-system observations[Clemence 1957] put a model-independent upper limit of 10^-7 radians/year on the rotation, which is an order of magnitude too lax to rule out the Gödel metric. Observations of the cosmic microwave background's anisotropy impose a limit that is tighter (perhaps 10^-9 rad/yr[Su 2009] or 10^-15 rad/yr[Barrow 1985]), but model-dependent.

Because all of the present observation are consistent with zero rotational velocity, it is not possible to attribute any prominent cosmological role to rotation. In particular, centrifugal forces cannot contribute significantly to cosmological expansion.

Clemence, C.M. (1957). 'Astronomical Time', Rev. Mod. Phys. Vol. 29, p. 2

Hawking, S.W. (1969). 'On the Rotation of the Universe', Mon. Not. R. astr. Soc. Vol. 142, p. 529.

Collins, C.B., and Hawking, S.W. (1973). 'The Rotation and Distortion of the Universe', Mon. Not. R. astr.Soc. Vol 162, p. 307.

Barrow, J. D., Juszkiewicz, R., & Sonoda, D. H., "Universal rotation: how large can it be?," 1985 -- http://adsabs.harvard.edu/full/1985MNRAS.213..917B

Su and Chu, "Is the universe rotating?," 2009, http://arxiv.org/abs/0902.4575

 

[Flatland]

It's not meaningful to ask if the Universe is rotating or not. I think what the OP really means is whether the Universe as a whole has any angular momentum.

 

[MACHO-WIMP]

"... Our universe has zero spin. Although for years Kurt Gödel tried to show that the universe was spinning by adding up the spins of the various galaxies, astronomers today believe that the total spin of the universe is zero." -Michio Kaku, Physics of the Impossible

 

[bcrowell]

It's not meaningful to ask if the Universe is rotating or not. I think what the OP really means is whether the Universe as a whole has any angular momentum.

No, it's perfectly OK to talk about rotation. Note that four of the papers listed in #58 explicitly refer to rotation in the title. Actually, I think it would be problematic to discuss it in terms of angular momentum, since GR doesn't have any standard way to define the total angular momentum (or momentum, or energy) of a cosmologial solution.

"... Our universe has zero spin. Although for years Kurt Gödel tried to show that the universe was spinning by adding up the spins of the various galaxies, astronomers today believe that the total spin of the universe is zero." -Michio Kaku, Physics of the Impossible

I think Kaku is oversimplifying here in a popularization. It's not possible to prove that it has exactly zero rotation. One can only set an empirical upper limit. See the references in #58.

 

[Khashishi]

When I pirouette, the universe is rapidly rotating around me.

 

[Lino]

When I pirouette, the universe is rapidly rotating around me.

... and you are the centre of rotation!

 

[Ken G]

FAQ: Can we tell whether the universe is rotating?

It is possible according to general relativity to have cosmologies in which the universe is rotating. This is a nonMachian feature of GR, since the rotation is not relative to anything else.

Your post is clearing up a lot of misunderstandings, but there is also a kind of fine line to walk here. You are right that GR does not embed Mach's principle, it is possible to violate Mach's principle and still be GR. But this does not imply that we can have cosmologies that are rotating "according to GR." The fact that a solution to GR is possible does not mean that GR takes a stance on the physical allowability of that solution. We face physics solutions all the time that are not physical, and we don't say that our theories say the solutions are possible, we just don't look to the theory to tell us they are not possible.

Let me give an example to clarify. When I elastically scatter two spheres of unequal mass, I can use conservation of momentum and energy to calculate the possible solutions. But those equations allow a solution where the two spheres just pass right through each other. Do we then say that "according to conservation of energy and momentum, it is possible for two solid spheres to pass through each other"? No, we say that the conservation laws are moot on the point, so we need some other physical requirement to tell us what solutions are possible and what ones aren't. So someone who claims "the universe can't rotate" is not in contradiction with GR any more than someone who says "two solid spheres cannot pass through each other" is contradicting the conservation laws.

But I agree with you that we must recognize that someone claiming the universe can't rotate is going beyond GR to assert a physical truth, based on Machian philosophy, that we don't actually know is true. Still, the fact that the unvierse is not observed to be rotating (which is indeed a meaningful statement as you point out) is taken by some to be a sign of support for the idea of adding Mach's principle to GR as a kind of additional postulate. It's a judgement call, and does not have practical ramifications but is interesting to ponder. It's ironic that Mach's principle, which was so instrumental in motivating Einstein's thinking, didn't end up in the formal machinery of GR.

 

[DavidMcC]

If a Kerr ring-"singularity" loses its angular momentum by gravitational radiation before fully collapsing to a non-rotating BH, that would explain the lack of rotation in a BH-based universe, wouldn't it?

 

[George Jones]

If a Kerr ring-"singularity" loses its angular momentum by gravitational radiation before fully collapsing to a non-rotating BH, that would explain the lack of rotation in a BH-based universe, wouldn't it?

But this doesn't happen. Observations indicate that many black at near the maximum rate.

http://news.bbc.co.uk/2/hi/science/nature/7184526.stm
http://arxiv.org/abs/astro-ph/0612354

There are more recent observations as well, but I don't have them at my fingertips. More direct methods for observing black hole spin should be available in few years,

https://www.cfa.harvard.edu/~loeb/sciam2.pdf.

 

[bcrowell]

If a Kerr ring-"singularity" loses its angular momentum by gravitational radiation before fully collapsing to a non-rotating BH, that would explain the lack of rotation in a BH-based universe, wouldn't it?

The universe isn't a black hole.

The Godel metric is the best known example of a rotating cosmology, and it doesn't contain gravitational radiation.

The Godel metric isn't realistic, but even in the case of more realistic rotating cosmological models, it seems implausible to me that the rotation would dissipate by radiation of gravitational waves. The reason is that there are only two time scales in such a model: the time scale corresponding to the Hubble constant, and the period of the rotation. Both of these are extremely long (billions of years), so the only gravitational waves you could get would be ones with periods of billions of years. But the efficiency of gravitational radiation typically goes like some high power of frequency, so it doesn't seem plausible to me that you could get strong gravitational radiation when the frequency is so incredibly low.

There is also the question of whether symmetry rules out such a process for a homogeneous cosmology. If I had to guess, I'd guess that it does.

There are also observational constraints. There are upper limits on the strength of the ambient gravitational radiation in our universe, and these upper limits are extremely low -- they say the radiation degrees of freedom of our universe are basically not activated at all (contrary to what would be expected on thermodynamic grounds).

 

[Shenstar]

Where does the solar systems angular momentum come from? And is it possible that everything in the universe is rotating because the universe itself is rotating. Similar to how eddies and weather on the Earth is affected by the Earth's rotation. Round swirls of cloud migrate across the Earth due to rotation.

 

[bcrowell]

Where does the solar systems angular momentum come from? And is it possible that everything in the universe is rotating because the universe itself is rotating. Similar to how eddies and weather on the Earth is affected by the Earth's rotation. Round swirls of cloud migrate across the Earth due to rotation.

If that were the case, we'd see a preferential alignment of angular momentum vectors. We don't.

 

[Shenstar]

Isn't there some data frm the CMB that shows there is some alignment to the solar plane or elliptic alignment?

See here: http://en.wikipedia.org/wiki/Copern...ent_of_cosmic_microwave_background_anisotropy

Or is this another issue?