Does the Universe Rotate and Does it Have an Axis?

[utek1]

Since everything in the Universe seems to be rotating, be it atoms or planets or stars or galaxies, and since the Universe consists of everything, shouldn't the Universe also be rotating? And if it is rotating, what is its axis?

 

[wolram]

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

 

[Garth]

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

Well, that is the question, rather than an answer!

As a Machian asking this question I would say that the universe as a whole cannot be rotating, but I am willing to be proved wrong.

Garth

 

[wolram]

Well, that is the question, rather than an answer!

As a Machian asking this question I would say that the universe as a whole cannot be rotating, but I am willing to be proved wrong.

Garth

I have noticed that no machine with many moving parts is entirely static, may be the universe is in some perfect balance, but would that not be an exception?

 

[utek1]

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

I'm sorry if I'm being unclear, but I don't know how else to phrase it. My thought is this. If the Universe is expanding from the central point of the Big Bang, would not the Universe also be rotating around that point, so that a galaxy at the edge of the Universe would complete an orbit every gazillion years or so?

 

[cristo]

I'm sorry if I'm being unclear, but I don't know how else to phrase it. My thought is this. If the Universe is expanding from the central point of the Big Bang, would not the Universe also be rotating around that point, so that a galaxy at the edge of the Universe would complete an orbit every gazillion years or so?

According to the current cosmological model, there is no centre of the universe. If there were, then this would mean that there is a special reference frame in the universe, which contradicts the cosmological principle. I think your misconceptions come about from thinking of the big bang as an explosion in the usual sense of the word, which one should avoid. See here for more of a discussion.

 

[wolram]

You are quite clear utek1, i asked this question myself some time ago, the point is if the universe is rotating or not there is no way for anyone to observe it, as the universe is every thing there is.
And people on this board will also tell you the universe is not expanding from a central point
rather space is expanding at every point, so there is no center.

Cheers.

 

[Oliver981]

Hello! This is a very interesting question! I remember that during my cosmology course I wondered if the Hubble flow could be alternatively explained by a rotating Universe theory based on some sort of centrifugal force! That would be amazing! However, a centrifugal force implies a centre, and, actually, there is no one as wolram has just written!
The Big Bang is not a point around which the Universe expands, but it is the Universe "concentrated" in one point (the so-called singularity). Therefore, the Big bang was everywhere (in fact the expansion due to the Hubble flow is identical in every directions and every points of the space)! For the same reason, I think that a rotation around the Big Bang is not possible!

 

[wolram]

I an not sure, it seems cosmology is telling us that that on large scales things can only move in a prefer ed direction, and that there can be no large scale coupling, i may have this wrong but if frame dragging is a reality why is it not possible for some sort of coupling on the scale of the universe

 

[wolram]

As an aside to this question is there a prefer ed direction of rotation for galaxies referenced from our only observation point.

 

[Oliver981]

Standard cosmology is based on the Robertson-Walker metric, which stems from the cosmological principle and is not rotating because of the homogeneity and isotropy. The cosmological principle seems to be true only on very large scales (over than 200 Mpc). So in this framework rotations are not allowed because they would identify a preferred dierction (that of the rotation axis). But this just a model. Let's think, for example, to the Kerr metric, which indeed is a rotating one! It is a vacuum solution, but perhaps a similar solution of Einstein equations, in presence of a non-zero stress-energy tensor, could exist which could properly describe our Universe.

 

[Ulysees]

It may be very simple. Any collection of objects has a centre of mass, by definition. Gravity everywhere in the universe should average to point roughly to the centre of mass.

Therefore everything should tend to follow orbits around this centre. Just like if you throw lots of particles in a simulator, gravitationally attracted they tend to form rotating patterns.

(I have to admit though I have not understood some of what has been written, apologies if what I am saying has been said already or proven wrong).

 

[Ulysees]

Here's a "universe" of particles in a simulator. No surprise the end up as a rotating galaxy.

http://youtube.com/watch?v=Y_SkuI8X_Q8

For rotation to be prevented, everything should move in a straight line from the centre, which of course is not the case in the observed universe.

 

[Wallace]

It may be very simple. Any collection of objects has a centre of mass, by definition. Gravity everywhere in the universe should average to point roughly to the centre of mass.

As has been said, there is no centre of the Universe and hence no centre of mass. This is an important but hard to grasp concept. The simulation you link to has a bunch of particles in a spherical cloud surrounded by empty space hence there is a net attraction due to gravity in towards the centre of the cloud. Think of a particle on the edge of the cloud, it has a pull on one side due to the rest of the cloud but no pull on the other side, since there is empty space.

The Universe on the other hand consists of space that is full of roughly the same density of material everywhere, there is no edge beyond which there is empty space, so there is no centre to which everything is attracted.

 

[Ulysees]

The simulation you link to has a bunch of particles in a spherical cloud surrounded by empty space hence there is a net attraction due to gravity in towards the centre of the cloud.

If there was no empty space around the cloud, but there was nothingness, no space, then would the law of gravity not apply between the particles? The simulation shows the law of gravity internally only, the interaction does not change whether there is empty space or "nothingness" around the cloud.

The Universe on the other hand consists of space that is full of roughly the same density of material everywhere, there is no edge beyond which there is empty space, so there is no centre to which everything is attracted.

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.

Of course the big-bang theory is only a theory, theories come and go, so maybe in the future we will be told that the universe is infinite and swelling/contracting only locally in places.

 

[Ulysees]

In fact there are indications that the shape of the universe is a dodecahedron, based on the harmonics of background radiation according to a recent paper in Nature:

http://www.nature.com/nature/journal/v425/n6958/full/425566a.html;jsessionid=5E2030BCEF7F560045D5C638B91336A3

 

[Ulysees]

Here's the paper:

 

[Ulysees]

It should be bigger here:

http://img222.imageshack.us/img222/741/image5ln5.gif

 

[cristo]

If there was no empty space around the cloud, but there was nothingness, no space, then would the law of gravity not apply between the particles? The simulation shows the law of gravity internally only, the interaction does not change whether there is empty space or "nothingness" around the cloud.

But "nothingness" does not exist. It seems that you are confusing your term "nothingness" with empty space. Whether the universe is finite or not, there is nothing beyond it; that is, by definition, the universe is everything, be it stars, planets, or even spacetime itself. It makes no sense to talk about what is "beyond" the universe

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.

This does not follow. Why do you think the big-bang theory implies this? It seems like you are still thinking of the big-bang as a conventional explosion: this is not the case!

Of course the big-bang theory is only a theory, theories come and go, so maybe in the future we will be told that the universe is infinite and swelling/contracting only locally in places.

Indeed the big-bang theory is only a theory, but it's a pretty good one, that agrees with many observations. However, we are not in a position to speculate about what may or may not be discovered in the future here.

 

[Ulysees]

But "nothingness" does not exist. It seems that you are confusing your term "nothingness" with empty space.

That does not seem at all from what I wrote: "if there was no empty space around the cloud, but there was nothingness, no space", which implies one of two cases:

1. Either there is empty space around the cloud

2. Or there is "true nothingness" around the cloud, ie the extent of the spacetime continuum is entirely occupied by the cloud.

It makes no sense to talk about what is "beyond" the universe

You've just done it though:

Whether the universe is finite or not, there is nothing beyond it

Hey, no offense, I know what you mean.

This does not follow. Why do you think the big-bang theory implies this?

Alright, it does not follow that the extent of spacetime has an edge. But it does follow that the cloud of mass has an "edge". As the cloud is expanding under the limitation of the speed of light, at the present time in its history.

It seems like you are still thinking of the big-bang as a conventional explosion: this is not the case!

Surely the beginning was not like a conventional explosion, the laws of physics are believed by many people to not apply in the early universe, that was expanding faster than light, and both matter and spacetime were probably expanding, etc etc.

But in the present time, expansion follows the familiar laws. Anything that does not move right, they associate it with the presence of "dark mass". Or they change the law of gravity from the familiar inverse square to slight modifications to match the observations.

we are not in a position to speculate about what may or may not be discovered in the future here.

I agree, I take it back. But this is not because we're stupid or ignorant, it's because this place is for teaching wide-spread and practically useful models (like Newtonian physics or other well-established ideas), not for research.

Before you tell me research should only be done by "professionals", may I remind you that Einstein's revolutionary paper on relativity:

1. did not give a single reference,
2. was not reviewed at all,
3. was not by an academic, and
4. was 100% right (in its range of applicability of course).

 

[cristo]

Alright, it does not follow that the extent of spacetime has an edge. But it does follow that the cloud of mass has an "edge", as the cloud is expanding under the limitation of the speed of light, at this time in its history.

The limitation of the speed of light means that nothing with mass can move faster than the speed of light, it doesn't mean that distances between objects cannot increase faster than the speed of light.

And before you tell me research should only be done by "professionals" ...

I wasn't going to say anything of the sort, merely that PF does not allow discussions which are speculative in nature.

... may I remind you that Einstein's revolutionary paper on relativity did not give a single reference, was not reviewed at all, was not by an academic, and was 100% right. To its range of applicability of course.

I really hope that you're not implying that Einstein was a layman: as per the Nobel Prize webpage, Einstein obtained a diploma in physics and mathematics in 1901, and his doctorate in physics in 1905-- he was by no means a layman!

 

[Ulysees]

Everybody has a Doctorate nowadays.

Kidding, but higher education is certainly much more available than before. Many people are very highly qualified, it is not sound to assume that everyone who appears in a forum asking physics questions is an ignorant schoolkid or student until proven otherwise, and that therefore any new theory shown is probably crap. By I digress, the bottomline is that amateur research can be done in specialised forums and we can keep this one for well-established ideas.

So did you say there are galaxies that appear from Earth to be moving apart faster than light?

 

[Wallace]

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.

Quite to the contrary, the Big Bang theory is what says that there is not edge. On the pop sci bastardized version of the Big Bang theory would be interpreted in this way. I didn't follow the exchanges between you and cristo too closely, but I wanted to make sure this misconception had been resolved.

 

[Ulysees]

I wanted to make sure this misconception had been resolved.

Thank you Wallace. This has been resolved only partly. Spacetime has no edge we said, according to the big bang theory, but matter has a finite extent because to fill infinite space it would need infinite speed. Does the big bang theory say matter has an infinite extent?

 

[Wallace]

Does the big bang theory say matter has an infinite extent?

The Big Bang theory is consistent with matter having an infinite extent. It is also consistent with a finite Universe with no edge, in a similar way to how a the surface of a ball is finite without edge.

but matter has a finite extent because to fill infinite space it would need infinite speed.

There is nothing wrong with this, the further a galaxy is from us the greater its recession speed without bound. This doesn't violate relativity, in fact it is predicted by it. The limit of traveling at the speed of light applies only locally and in the absence of gravitational fields. I.e. if something moves through the location we are at it cannot do so at faster than the speed of light however something a great distance from us can increase its distance from us at any rate without bound, provided that it is not moving past the location it is at any faster than the speed of light and also, importantly, that there is matter and hence gravitational fields between us. I know we have had it drummed into us that nothing can move faster than the speed of light but like many popularisations of science this mantra doesn't give the full details of the context of this limit, so it makes it hard to grasp a lot of concepts in cosmology where this does not apply in the most naive sense.

The above explanation is rather short and incomplete, here is a link that might help give a more complete explanation:

http://www.astro.princeton.edu/~aes/AST105/Readings/misconceptionsBigBang.pdf" See the 5th page (page 40 on the numbers) for this specific issue, though the whole document is a very good read for getting your head around Big Bang theory.

 

[_Mayday_]

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

If I was spinning on a point it would be rotating relative to me.

 

[Ulysees]

You're mentioning the speed of light. I was careful not to mention it in the last post, because the problem is not the speed of light, the problem is the finite-ness of any speed:

Finite speed v (as large as you like) after finite time t (as large as you like) will give a finite extent to the distribution of matter. Even with relativistic considerations. Unless some laws are broken, as is said for the early universe. Is that how they account for an infinite extent of mass, by laws being broken?

And don't forget those guys with the theory of the dodecahedral universe. For them the distribution of mass is definitely of finite extent, as is spacetime itself (except that when you go out through one edge of the dodecahedron, you come in from the opposite side in Poincare dodecahedrons, I think).

 

[Wallace]

Finite speed v (as large as you like) after finite time t (as large as you like) will give a finite extent to the distribution of matter. Even with relativistic considerations. Unless some laws are broken, as is said for the early universe. Is that how they account for an infinite extent of mass?

You are still thinking that all the matter in the Universe must have been at one point at time t=0. This is the fundamental misconception about the Big Bang that causes so many problems. There is nothing in the theory that suggests that this is the case. To be clear if the Universe is infinite in extent then it has always been infinite in extent. We don't know much about the Universe at t=0 and indeed the Big Bang theory, despite its name, doesn't actually say anything about the Universe at t=0.

What the Big Bang theory says in that the Universe was hotter and denser in the past. We have a reasonable idea of what happened going back in time to just a small fraction of a second into the history of the Universe (so t=0 + a very small number). At this time the Big Bang theory is consistent with the Universe being infinite in extent. There is no requirement anywhere in the theory that says that at some point in time everything in the Universe was at one point.

As I say, the document I linked to explains a lot of these common misconceptions very well, so do have a read. What is in error is not your reasoning, that is quite sound judging by your posts, but unfortunately you are finding the faults in a 'strawman' Big Bang theory that is not the same as the scientific theory.

 

[Ulysees]

People even give dimensions for the early universe, like 10^-15 metres after 10^-12 seconds (something like this). They name like 4 or so phases of the universe in the first fractions of a second, each phase with its starting size, ending size, starting time, ending time. These are mathematically determined, well known physicists have listed the phases with their names on tv. Maybe you even remember the names of the phases.

 

[Wallace]

People even give dimensions for the early universe, like 10^-15 metres after 10^-12 seconds (something like this).

Again, things often get lost when crossing over from science to pop science. These dimensions refer to the size of the observable Universe, which describes the maximum extent of what part of the Universe we can see at any given time. When researchers refer to this as the 'size of the Universe' it is taken as obvious that they really mean the size of the observable Universe. Of course this isn't obvious and yet another piece of miscommunication leads to misconception in the general community.

Since light travels at a finite rate then the observable Universe clearly is always finite. However there is nothing to suggest that anything different occurs just beyond the furthest reach of what we can observe due to the finite age of the Universe and finite speed of light. The theories that correctly predict the evolution of the Universe require that the possibly infinite amount of space beyond our observable bubble of the Universe has essentially the same properties as the observable Universe.

 

[cristo]

Many people are very highly qualified, it is not sound to assume that everyone who appears in a forum asking physics questions is an ignorant schoolkid or student until proven otherwise, ...

Just as an aside (since Wallace has stepped in and helped out with the technical questions) I'd like to point out that neither did I assume you were an ignorant schoolkid, nor did I mean to imply it. Further, given that I am a student myself, I'm not about to pass anyone off as having no knowledge. It's just from past experience that when one starts to quote that Einstein was a layperson when he created his theory of relativity the discussions tend to drift far from mainstream!

... and that therefore any new theory shown is probably crap. By I digress, the bottomline is that amateur research can be done in specialised forums and we can keep this one for well-established ideas.

Indeed, I agree: that is why we have a special Independent Research forum here at PF for new theories to be discussed.

 

[Ulysees]

When researchers refer to this as the 'size of the Universe' it is taken as obvious that they really mean the size of the observable Universe. ... Since light travels at a finite rate then the observable Universe clearly is always finite.

I get the impression they were not talking about the observable universe in this context:

1. Planck era
2. hadron era
3. inflation era
4. lepton era
5. nucleosynthetic era
6. radiation era
7. matter era

I mean, what, they made all this theory just to describe the observable universe, and left the whole world thinking they were talking about THE universe, the cosmos? It doesn't sound right.

So are you saying that THE universe is probably of infinite extent and finite density? Putting the two together, infinite extent, times finite density, means infinite mass. But the mass of THE universe is finite. Or are they talking about their observable universe again?

This is a crime, everybody in the world was left thinking that the entire universe was like this, or like that, or it will do that, and so on. We want to know about the real cosmos.

 

[Ulysees]

We want to know about the real cosmos. Not what the speed of light limits our eyes to. Because the speed of light has been exceeded already, information has been transferred faster than the speed of light. We therefore care to learn about THE universe.

 

[Wallace]

I get the impression they were not talking about the observable universe in this context:

1. Planck era
2. hadron era
3. inflation era
4. lepton era
5. nucleosynthetic era
6. radiation era
7. matter era

Can you show me some examples of this? That might help make things clearer, i.e. picking out exactly what was meant by a particular statement.

I mean, what, they made all this theory just to describe the observable universe, and left the whole world thinking they were talking about THE universe, the cosmos? It doesn't sound right.

The theories that we have developed in cosmology certainly are thought to apply to the entire Universe, not just the observable part. The fact that we can only observe the observable Universe is true by definition, but the theories we have are certainly thought to apply everywhere.

So are you saying that THE universe is probably of infinite extent and finite density? Putting the two together, infinite extent, times finite density, means infinite mass. But the mass of THE universe is finite.

Who said the mass of the Universe is finite? Of course if the Universe is of infinite spatial extent then it clearly cannot have a finite total mass.

This is a crime, everybody in the world was left thinking that the entire universe was like this, or like that, or it will do that, and so on. We want to know about the real cosmos.

Of course we do. I can't answer for whatever crimes may have been committed in the communication of science to the public. All I can do is try and explain what the scientific theory of the Big Bang is and what it is not. But please, don't take my word for it. There are plenty of good books that explain things far better than I can here. A good place to start might be 'The Big Bang' by Simon Singh (last name might be spelt differently). It is a good, accurate and clear pop sci book on the Big Bang.

Plus you should read the Davis and Lineweaver paper I linked to.

 

[Ulysees]

Thanks for the book recommendations. Dunno the details of those eras, just saw that the Planck one has a size equal to the time light takes to travel a Planck length... probably the observable universe alright.

when one starts to quote that Einstein was a layperson when he created his theory of relativity

For the record, no one suggested Einstein was a layperson. Just that he did not do things the way so many people here value so much and quote all the time when they want to seem important ("peer reviewed" and so on). Einstein would likely have been asked to stop the over-speculative posts and personal theories or something. Only exaggerating here, I hope the point is understood: this is for teaching well-established ideas, not research. In fact I don't dare to go to the Research sub-forums here, people don't switch from infallible-professor mode to genuine free-minded pioneer mode in a flash.

 

[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.

⚛​

 

[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.

⚛​

 

[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.

⚛​

 

[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.

⚛​

 

[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?

⚛​

 

[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.