Expansion of Space vs Movement in Space

[Martin]

Didn't you say that you could be anywhere in the universe and that the age of the universe would be the same when calculated as long as you had a constant CMB temperature in all directions at each location?

 

Yes. In cosmology there is an idea of global time. sometimes called universe time. also Friedman time, because it goes with the Friedman model.

 

that gives a standard way to slice spacetime into a succession of spatial slices, 3D hypersurfaces, each of which is a "now" containing all the simultaneous events, events which are simultaneous in the sense of occurring at one particular moment of the global time.

 

But these spatial hypersurfaces are not flat. There is no way to put rectilinear space coordinates on that cover the whole 3D space.

 

 

Wouldn't that make it a common reference frame?

 

No. It wouldn't in the way I understand "frame". I think of a reference frame as involving not only a time coordinate but also spatial x, y, z.

 

Maybe we should have a poll to find out what people here at SFN think a "frame" is. I think we are just having a minor verbal difficulty here.

 

I have never said that cosmo has a preferred reference frame.

 

I have always said that cosmologists have a preferred time. They do. It is involved with their main equations and built into the standard cosmo model and absolutely essential.

 

But personally I always avoid saying preferred reference frame because I have never seen one in cosmology. What I would call that. They use several different systems of space coordinates, whatever is suitable for the purpose at hand. I don't think there is ever assumed one preferred x,y,z that covers the whole. I can't imagine what that would look like or how you could make sure it was right, or how you would define it, or on what basis you would prefer it. I feel a bit inadequate here even talking about it because I just can't imagine what it would mean.

 

Maybe someone else has a different more clear idea, and can talk about it.

[J.C.MacSwell]

There is something I'm missing here. A "preferred reference time" is fine, the professionals already have that and it is standard.

But how do you get a global frame from that?

 

What do you do for spatial coordinates? We don't know the largescale topology of space. One set of spatial coordinates might not fit the whole thing!

Like one flat map does not cover the whole earth. You can only make flat maps of a curved surface if you work in small local patches.

 

I don't see how anybody can talk about a preferred reference frame (except limited and local, for a given observer). What am I missing?

 

This is a good analogy. A flat map may not fit the whole Earth but "sea level" can be used as a preferred reference frame. It is pretty much flat everywhere locally, is a continuum of sorts etc. etc so to me it seems like the CMB isotropy frame (is there a better term- I have also called it the Big Bang Track) can be used as well.

 

If the Universe , say, turns out to be a hypersphere, then it might be OK. If it was something very different it might lead to inconsistencies, but may still be more useful over greater distances than, say, an SR inertial frame.

[NowThatWeKnow]

...Maybe we should have a poll to find out what people here at SFN think a "frame" is. I think we are just having a minor verbal difficulty here...

A frame could be any number of coordinates where there was no motion relative to each other and, considering GR, they would also have to share the same gravitational potential. Is that not right?

 

It wasn't long ago I heard that our current technology indicated space was flat, or so nearly so we could not determine any curve. Is there new proof that space is curved?

[Martin]

A frame could be any number of coordinates where there was no motion relative to each other and, considering GR, they would also have to share the same gravitational potential. Is that not right?

 

It wasn't long ago I heard that our current technology indicated space was flat, or so nearly so we could not determine any curve. Is there new proof that space is curved?

 

Current measurements do not indicate that space is flat, even on average. They indicate that on average it is nearly flat. There is a difference if you are talking about a global frame.

 

Didn't we discuss the WMAP 2008 estimate of the radius of curvature? They said if space is a hypersphere then the radius of curvature (with 95% confidence) is at least 100 billion light years.

This was based on the combined data from WMAP, the galaxy counts, and the supernova data available at the time.

 

My understanding of a reference frame is that the spatial part is an infinite flat Euclidean space----orthogonal x,y,z. Obviously you can't fit such a thing onto a hypersphere.

 

You have really good intuition and a lot of inititiative, you think vigorously (is my impression). But you seem to think "reference frame" can mean what makes sense to you for it to mean.

 

My experience is different. I find that when people say "reference frame" they mean something technical with some kind of rectilinear coordinates. Either it is flat Minkowski space of special rel, or it is conventional Euclidean space. That might not be what I would like them to mean---it is simply what they do mean. It is not my choice, it's just how it is.

 

If we could force people to use words the way we want, that makes the best sense to us, then I would agree with you that a hypersphere together with time could be a "reference frame".

 

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where did you hear that space was flat? As far as I can remember I have never said such a thing. We don't know. Of course it is obviously not flat locally. The sun bends light rays. All sorts of things do grav. lensing. Space is very wavy gravy. But it MIGHT be zero curvature if you take a very largescale average. We just do not know, and as far as I'm aware there has never been a scientific journal report that says we do know the overall curvature. Since 2004 anyway, what I have seen always has a confidence interval on the overall average curvature. The confidence interval is always around zero, so it includes zero curvature as a possibility, but not a certainty.

 

I'm getting tired of talking about "reference frame". It is just quibbling about words. We agree that there is a global universe time that is used in cosmology. Only approximately measurable but still practical. And fairly natural. We don't agree that there are natural rectilinear space coordinates.

(and we don't even know the largescale overall spatial shape)

 

Instead of talking about "reference frame" I'd like to talk about largescale curvature.

The latest data I know about that is March 2008. That is over a year ago. You and I may have discussed it. Did you look at Table 2 in that paper Komatsu et al? Ned Wright was one of the co-authors.

 

Do you remember the confidence interval for the curvature? The data was WMAP+BAO+SN*. Have you seen anything more recent? This was a major NASA report. As far as I know it is still the most reliable and authoritative source.

 

*microwave background+galaxy counts+supernovae data

Edited May 25, 2009 by Martin

[J.C.MacSwell]

 

My understanding of a reference frame is that the spatial part is an infinite flat Euclidean space----orthogonal x,y,z. Obviously you can't fit such a thing onto a hypersphere.

 

You have really good intuition and a lot of inititiative, you think vigorously (is my impression). But you seem to think "reference frame" can mean what makes sense to you for it to mean.

 

My experience is different. I find that when people say "reference frame" they mean something technical with some kind of rectilinear coordinates. Either it is flat Minkowski space of special rel, or it is conventional Euclidean space. That might not be what I would like them to mean---it is simply what they do mean. It is not my choice, it's just how it is.

 

If we could force people to use words the way we want, that makes the best sense to us, then I would agree with you that a hypersphere together with time could be a "reference frame".

 

=======================

 

 

I hate to quibble Martin, but I think this is a very restricted use of the term "reference frame".

 

I think you can define a reference frame as you like and often there are advantages (or disadvantages) to doing so.

 

I don't think I am using the term incorrectly. I would happily refrain from doing so if it is incorrect or even misleading as I think it is important to define our terms and speak a common language.

 

If it is not obvious what frame of reference I'm referring to I try to qualify it. (rotating, inertial, or in this case expanding etc.)

Edited May 25, 2009 by J.C.MacSwell

[NowThatWeKnow]

...You have really good intuition and a lot of inititiative, you think vigorously (is my impression). But you seem to think "reference frame" can mean what makes sense to you for it to mean...

Thank you, I think. I have no training in cosmology and most of what I know is from reading and posting in the last few months. It does seem that most examples of frames have to do with motion of one object relative to another. And if you are in the same frame you will share the same time. I would like a definition of a "frame" that I could understand but would also make sense to you.

 

...where did you hear that space was flat? As far as I can remember I have never said such a thing. We don't know. Of course it is obviously not flat locally. The sun bends light rays. All sorts of things do grav. lensing. Space is very wavy gravy. But it MIGHT be zero curvature if you take a very largescale average. We just do not know,...

Maybe I should have said that I heard the universe was flat, not space. I know you do not think much of the History channel but they said that during measurements, the triangle totaled 180*. And on a scale our technology was capable of dealing with, the universe was flat or nearly flat from what we can tell. I guess that kind of goes along with what you are saying.

[Martin]

I hate to quibble Martin, but I think this is a very restricted use of the term "reference frame".

...

 

All right. Both you and NowThat think so. I'll try to adapt and use it more flexibly. But I'll sometimes call attention to it's meaning different things.

 

Maybe I should have said that I heard the universe was flat, not space. I know you do not think much of the History channel but they said that during measurements, the triangle totaled 180*. And on a scale our technology was capable of dealing with, the universe was flat or nearly flat from what we can tell. I guess that kind of goes along with what you are saying.

 

will get back to this, have to go.