Universe Expanding: Space or Matter?

[Gear300]

When it is referred that the Universe is expanding/accelerating outward, do they mean to say that space is expanding or that matter is accelerating outward through space?

 

[Danger]

The space itself is expanding, carrying the matter with it.

 

[Civilized]

Since you can't observe space directly (you can only observe measuring sticks) , all the direct astronomical observations that we use to justify the statement "the universe is expanding" are based on light radiated by moving objects.

The primary theory behind the expansion of the universe is Einstein's General Relativity. In terms of the theory, it is space itself that is expanding, analogous to what would happen if our universe was a 2-d surface of a balloon and the balloon was expanding.

Therefore, if you believe General Relativity, then it is space itself that is expanding. If, however, you want to strictly limit ourselves to what we observe, we can only observe measuring sticks.

At a slightly more detailed level, here are two explanations for the cosmological redshift:

(1) Objects are moving away from us and so the frequency of emitted light is lower (redshifted). This is just like an ambulance moving away from you producing a lower tone from it's siren.

(2) In the time that the light has been traveling from the object, space has expanded , increasing the wavelength of the light, which decreases the frequency, leaving it redshifted.

In general relativity the answer is unambiguously (2). It turns out (1) and (2) predict different results, and (2) is the correct explanation of the cosmological redshift using general relativity.

 

[Gear300]

I see (to the extent I can)...thanks for the replies. But since it is space that is expanding, does that still imply that we might experience relativistic effects, or do we need to be moving through space for that to happen?

 

[russ_watters]

There is no such thing as "moving through space" and no, there are no relativistic effects associated with expanding space. There are actually galaxies that appear to be moving faster than C.

 

[Gear300]

Thanks for the answer.

There is no such thing as "moving through space"...

Then what is there?

 

[Civilized]

I'm not totally sure what Russ is getting at, but from a spacetime perspective, moving through space without moving at all through time would correspond to infinite velocity, and that is impossible in relativity.

 

[Gear300]

I'm not totally sure what Russ is getting at, but from a spacetime perspective, moving through space without moving at all through time would correspond to infinite velocity, and that is impossible in relativity.

I see. Thanks again for the reply.
An additional question: if space is expanding, doesn't that necessarily imply that there should be a higher-dimensional medium for it to expand (thus, we have indirect evidence for higher dimensions)?

 

[russ_watters]

Then what is there?

Space is not an object nor an absolute reference frame, so there is nothing for one to measure a velocity relative to. Velocity is measured between objects.

 

[Gear300]

Space is not an object nor an absolute reference frame, so there is nothing for one to measure a velocity relative to. Velocity is measured between objects.

Then what is space and what is meant by it expanding?

 

[sylas]

Then what is space and what is meant by it expanding?

I think the notion that matter is "carried by" the expansion of space is a bit misleading. (Reply #2 above.)

I like this definition: space is that which is measured by rulers and clocks. This is the converse of Einstein's operational definition of time: that which is measured by clocks. Spacetime is that which is measured by rulers and clocks.

Expanding space means that distances between things are increasing. We speak of expansion of the universe because on sufficiently large scales, the distance between any two objects is increasing. Since everything is moving further from everything else (on sufficiently large scales) you can equally say that the space between everything (on sufficiently large scales) is increasing. But I think it may be slightly better to say that space is expanded by the motions of objects, rather than objects are carried by the expansion of space. It's a bit like noting that spacetime is curved by massive objects.

You can also say that volumes are increasing. If you try to define a volume for a region of space, on sufficiently large scales, that volume is increasing.

Cheers -- sylas

 

[Ich]

Since everything is moving further from everything else (on sufficiently large scales) you can equally say that the space between everything (on sufficiently large scales) is increasing.

Yes, that's what one can say and usually does say.
The unambiguous meaning of it is that distances increase with time (with some subtleties concerning definitions and exact values). Every statement that goes beyond this fact, like space dragging objects with it or otherwise, is necessarily interpretation or mental picture, not fact. Interpreations may be useful or not, and one may pick in each event the one that is least misleading.

 

[Chronos]

General relativity is not so easily understood as special relativity. GR forces you to divorce many principles of SR when you look at the big picture. SR is only a special case of GR. Does that help? Nothing in SR forbids an expanding universe. Pretty much everything in GR demands it. The speed of expansion is irrelevant.

 

[Gear300]

I think the notion that matter is "carried by" the expansion of space is a bit misleading. (Reply #2 above.)

I like this definition: space is that which is measured by rulers and clocks. This is the converse of Einstein's operational definition of time: that which is measured by clocks. Spacetime is that which is measured by rulers and clocks.

Expanding space means that distances between things are increasing. We speak of expansion of the universe because on sufficiently large scales, the distance between any two objects is increasing. Since everything is moving further from everything else (on sufficiently large scales) you can equally say that the space between everything (on sufficiently large scales) is increasing. But I think it may be slightly better to say that space is expanded by the motions of objects, rather than objects are carried by the expansion of space. It's a bit like noting that spacetime is curved by massive objects.

You can also say that volumes are increasing. If you try to define a volume for a region of space, on sufficiently large scales, that volume is increasing.

Cheers -- sylas

Yes, that's what one can say and usually does say.
The unambiguous meaning of it is that distances increase with time (with some subtleties concerning definitions and exact values). Every statement that goes beyond this fact, like space dragging objects with it or otherwise, is necessarily interpretation or mental picture, not fact. Interpreations may be useful or not, and one may pick in each event the one that is least misleading.

General relativity is not so easily understood as special relativity. GR forces you to divorce many principles of SR when you look at the big picture. SR is only a special case of GR. Does that help? Nothing in SR forbids an expanding universe. Pretty much everything in GR demands it. The speed of expansion is irrelevant.

From all this, it seems that there is no stable definition for motion...

If I'm right, SR implies that all motion is evaluated relative to some reference that is also evaluated relative to another reference and so on. So then, as Ich said, there isn't too much against this interpretation:

But I think it may be slightly better to say that space is expanded by the motions of objects, rather than objects are carried by the expansion of space. It's a bit like noting that spacetime is curved by massive objects.

Still...if space is the one expanding...shouldn't it need to expand through a higher-dimensional medium and is that implied anywhere in GR or is it just interpretation?

 

[sylas]

From all this, it seems that there is no stable definition for motion...

If I'm right, SR implies that all motion is evaluated relative to some reference that is also evaluated relative to another reference and so on. So then, as Ich said, there isn't too much against this interpretation:


Still...if space is the one expanding...shouldn't it need to expand through a higher-dimensional medium and is that implied anywhere in GR or is it just interpretation?

No, there's no need and no implication for higher dimensions.

As for motion having no stable definition; I think it is more correct to say that when we describe things with conventional language or analogies, descriptions are incomplete. The maths of it is unambiguous. There are various ways you can measure a recession velocity, with different co-ordinates or distance definitions. If this is done with full rigour -- and this certainly can be done -- then there is no ambiguity about meanings or transformations between co-ordinates or alternative definitions.

Cheers -- sylas

 

[Ich]

As for motion having no stable definition; I think it is more correct to say that when we describe things with conventional language or analogies, descriptions are incomplete.

I'd say Gear300 is correct: there is no undisputable definition of "motion" in GR, if the concerned objects are at some distance.
You know thar SR has no concept of absolute speed, there is only relative motion. That's true also in GR, so I don't like the idea of "motion through space" - even if it is useful for specific calculations, and even though it works generally and can be defined appropriately.
In GR, even relative velocity between distant objects depends on how you define it. Even worse, relative velocities are not easy to calculate, so it's common to take some coordinate values and call them velocity.
But, of course, if you pick one definition and stick to it, everything will work out mathematically.

 

[Rymer]

'Space' only exists as a measure of the separation of 'objects'.

The measurement is determined relative to a 'standard' -- how this standard is chosen can vary.

The definition of an 'object' can vary.

My summation.

 

[sylas]

...
In GR, even relative velocity between distant objects depends on how you define it. Even worse, relative velocities are not easy to calculate, so it's common to take some coordinate values and call them velocity.
But, of course, if you pick one definition and stick to it, everything will work out mathematically.

Exactly. This is what I meant when I spoke of using different co-ordinates and transformations.

 

[Chronos]

The universe does not easily submit to human descriptions. All we know is it is really big.

 

[yogi]

In looking over the posts, in general, I think it presumptious to exclude some of the interpretations of space based upon what we think we know and what we know we don't know. Historically, Einstein premised his GR theory on the fact that the universe was static - and this was one of his primary arguments he used to criticize de Sitter's model - it wasn't static. GR is based upon the ad hoc postualate that inert matter affects static space - it really doesn't imply expansion per se. Then there are the statements by Einstein in his later years that embrace the notion of space as being more than just the distance between things - that it was a part of the universe that functioned in such a way as to make accelerated motion relative just as is inertial motion - that is, a reactionary force would be exerted upon a static mass if the entire universe (including its space) is accelerated, just as would be the case if the mass were accelerated relative to to the content of the universe.

Presently, the search is to find the energy needed to bring omega equal to one - and if this turns out to be hidden in the spatial stress or in the gravitational fields, then space cannot be relegated to nothing And as one one more point, the notion matter creation during inflation depends upon the energy released by an expanding negative pressure space.

In summary, I think it is good to keep some options open about what space could be - there seems to be a mainstream urgency to surpress interrogation in this regard

Who was it that said: "Cosmologists are frequently wrong, but never in doubt"

 

[Chronos]

Arp is the short answer. Cosmologists are no less surprised than anyone else about the existence of dark energy. At present, there is no other viable explanation.

 

[Rymer]

Arp is the short answer. Cosmologists are no less surprised than anyone else about the existence of dark energy. At present, there is no other viable explanation.

And I must comment, that I have NOT as yet seen ANY evidence for 'dark energy'. (and yes I have read the many papers on the subject -- not convinced -- data is far too poor)

 

[George Jones]

Who was it that said: "Cosmologists are frequently wrong, but never in doubt"

Lev Landau. When he said this, there was a paucity of good cosmological data. The situation today is much different, as Weinberg indicates in the preface of his new cosmology book,

"The new excitement in cosmology came as if on cue for elementary particle physicists. By the 1980s the Standard Model of elementary particles and fields had become well established. Although significant theoretical and experimental work continued, there was now little contact between experiment and new theoretical ideas, and without this contact, particle physics lost much of its liveliness. Cosmology now offered the excitement that particle physicists had experienced in the 1960s and 1970s."

 

[yogi]

Lev Landau. When he said this, there was a paucity of good cosmological data. The situation today is much different, as Weinberg indicates in the preface of his new cosmology book,

"The new excitement in cosmology came as if on cue for elementary particle physicists. By the 1980s the Standard Model of elementary particles and fields had become well established. Although significant theoretical and experimental work continued, there was now little contact between experiment and new theoretical ideas, and without this contact, particle physics lost much of its liveliness. Cosmology now offered the excitement that particle physicists had experienced in the 1960s and 1970s."

Correct George - Lev Landau - but I wonder if the statement should be thought of as appropriate only in the context of its historical utterance - perhaps it may be better classified as timeless.

There is definitely much new data - if we look back 20 years hence (if humanity lasts that long) I wonder if the recent discoveries will be viewed as pointing in the direction we presently interpret.

 

[Chronos]

The Legacy Supernova Survey is the best and only significant observational evidence for dark energy. It is difficult to dispute. See

http://www.kqed.org/quest/blog/2008/04/08/supernova-legacy/
http://mr.caltech.edu/press_releases/12767
http://www.gemini.edu/create-content/pio-websplash/the-last-supernova-legacy-survey-spectrum

Science plods along like a turtle, Turtles live for a very long time, not unlike science done properly.

 

[Rymer]

The Legacy Supernova Survey is the best and only significant observational evidence for dark energy. It is difficult to dispute. See

http://www.kqed.org/quest/blog/2008/04/08/supernova-legacy/
http://mr.caltech.edu/press_releases/12767
http://www.gemini.edu/create-content/pio-websplash/the-last-supernova-legacy-survey-spectrum

Science plods along like a turtle, Turtles live for a very long time, not unlike science done properly.

I've seen it. Worked with it for the last few years. I do DISPUTE that it shows anything like 'proof' for 'dark energy'. The so called 'acceleration' is ONLY seen when comparing to a known flawed model -- Milne or an empty model equivalent.

The departure 'curving' seen is due to the model, poor data quality, and poor data fitting techniques. Makes no difference whose name is attached to the paper.

Close analysis of the limited data does indicate a systematic divergence consistent with a slower CONSTANT expansion velocity than an empty model. This is considered to be a normal departure of comparing an 'empty' universe and one containing matter. THERE IS NO CHANGE. Its the same departure as seen in more redshifted data.

 

[sylas]

I've seen it. Worked with it for the last few years. I do DISPUTE that it shows anything like 'proof' for 'dark energy'. The so called 'acceleration' is ONLY seen when comparing to a known flawed model -- Milne or an empty model equivalent.

Flatly false. It is seeing in the fact that the rate of expansion is increasing rather than decreasing, as determined by the evidence. I have no idea how you can possibly claim to be "working" with this. Your answer doesn't make any sense at all. Prior to this evidence, the conventional model was a critical mass model; not the empty model.

Close analysis of the limited data does indicate a systematic divergence consistent with a slower CONSTANT expansion velocity than an empty model. This is considered to be a normal departure of comparing an 'empty' universe and one containing matter. THERE IS NO CHANGE. Its the same departure as seen in more redshifted data.

No, it doesn't. The empty model would give a constant expansion rate. The evidence indicates that the rate is increasing.

Cheers -- sylas

 

[Rymer]

Flatly false. It is seeing in the fact that the rate of expansion is increasing rather than decreasing, as determined by the evidence. I have no idea how you can possibly claim to be "working" with this. Your answer doesn't make any sense at all. Prior to this evidence, the conventional model was a critical mass model; not the empty model.



No, it doesn't. The empty model would give a constant expansion rate. The evidence indicates that the rate is increasing.

Cheers -- sylas

Have you LOOKED at the simple image attachment to see what can happen with two slightly different data fits to data? And this assumes the data doesn't have a systemic error. It appears to have one in very low z -- (suspected that a small gravitational correction is needed). This only effects data below z<<0.2

The evidence is the data. Do your own analysis.

 

[sylas]

Have you LOOKED at the simple image attachment to see what can happen with two slightly different data fits to data? And this assumes the data doesn't have a systemic error. It appears to have one in very low z -- (suspected that a small gravitational correction is needed). This only effects data below z<<0.2

The evidence is the data. Do your own analysis.

I looked at it, and it didn't seem to use the data at all.

People have said above that the "only" evidence for dark energy is the supernova light curves. That's not strictly true. As with all science, cosmology uses all the available evidence together. If you use supernova light curves and nothing else, you might consider the empty model as credible. The data does actually indicate acceleration, as I indicated, rather than merely constant expansion as in the empty model, but the difference is small.

But the empty model has worse problems than a slightly worse fit with the supernova data... and that is that the universe ISN'T empty. There are multiple lines of evidence for dark matter, on top of all the baryonic matter around... and worse, the universe is FLAT. Empty model can't give you that at all.

So, as I said before, it's just nonsense to say that acceleration only shows up by comparison with the empty model. It shows up much more strongly, in fact, when compared with other flat models, with critical mass density. Which, of course, was the preferred model before the evidence for dark energy came in, because the empty model is ruled out by other data.

The whole question remains open, particularly in the light of the fact we don't really know what dark energy or dark matter really is. But it's just silly to say that acceleration is only apparent in comparison with the empty model.

If you REALLY want to look at data, then you need to look at the observation; not just the curves you provided for some of the models from some spreadsheet (I presume). You don't seem to be doing "data analysis" at all.

Here's a better way to look at data. I am simply giving some figures from New Constraints on Ω_M, Ω_Λ, and w from an Independent Set of Eleven High-Redshift Supernovae Observed with HST, by Kopp et al (2003) at astro-ph/0309368.

This is a comparison of magnitudes with redshift.

The thing to note here is the cosmologies. The lowest line is the critical mass case. The middle line is the sub-critical mass. The upper line is a dark energy. The empty model would be between the middle and upper lines, but it is not displayed here. As you've noted yourself, this is a falsified model in any case. Why would you bother to show it?

The fact is, the empty model... on THIS data... would be hard to separate from the dark energy model. But that ISN'T the right comparison. And it most definitely is not the case that the dark energy model only shows up by comparison with the flawed empty model! Dark energy actually shows up BETTER by comparison with the critical matter model... which is not flawed... at least, until the data for dark energy came along.

I don't mind if you are personally skeptical on your own behalf. The major objection I had was your claim that dark energy only shows up when comparing with the empty model. That's just silly.

My own position is that I don't think the case is closed. Dark energy is able to explain the data best, at present; and more recent data has only strengthened that case. But until we know more about what dark energy actually is, I'm keeping an open mind.

Cheers -- sylas

 

[Rymer]

I looked at it, and it didn't seem to use the data at all.

People have said above that the "only" evidence for dark energy is the supernova light curves. That's not strictly true. As with all science, cosmology uses all the available evidence together. If you use supernova light curves and nothing else, you might consider the empty model as credible. The data does actually indicate acceleration, as I indicated, rather than merely constant expansion as in the empty model, but the difference is small.

But the empty model has worse problems than a slightly worse fit with the supernova data... and that is that the universe ISN'T empty. There are multiple lines of evidence for dark matter, on top of all the baryonic matter around... and worse, the universe is FLAT. Empty model can't give you that at all.

So, as I said before, it's just nonsense to say that acceleration only shows up by comparison with the empty model. It shows up much more strongly, in fact, when compared with other flat models, with critical mass density. Which, of course, was the preferred model before the evidence for dark energy came in, because the empty model is ruled out by other data.

The whole question remains open, particularly in the light of the fact we don't really know what dark energy or dark matter really is. But it's just silly to say that acceleration is only apparent in comparison with the empty model.

If you REALLY want to look at data, then you need to look at the observation; not just the curves you provided for some of the models from some spreadsheet (I presume). You don't seem to be doing "data analysis" at all.

Here's a better way to look at data. I am simply giving some figures from New Constraints on Ω_M, Ω_Λ, and w from an Independent Set of Eleven High-Redshift Supernovae Observed with HST, by Kopp et al (2003) at astro-ph/0309368.

This is a comparison of magnitudes with redshift.

The thing to note here is the cosmologies. The lowest line is the critical mass case. The middle line is the sub-critical mass. The upper line is a dark energy. The empty model would be between the middle and upper lines, but it is not displayed here. As you've noted yourself, this is a falsified model in any case. Why would you bother to show it?

The fact is, the empty model... on THIS data... would be hard to separate from the dark energy model. But that ISN'T the right comparison. And it most definitely is not the case that the dark energy model only shows up by comparison with the flawed empty model! Dark energy actually shows up BETTER by comparison with the critical matter model... which is not flawed... at least, until the data for dark energy came along.

I don't mind if you are personally skeptical on your own behalf. The major objection I had was your claim that dark energy only shows up when comparing with the empty model. That's just silly.

My own position is that I don't think the case is closed. Dark energy is able to explain the data best, at present; and more recent data has only strengthened that case. But until we know more about what dark energy actually is, I'm keeping an open mind.

Cheers -- sylas

I see a bunch of data -- but NOTHING that supports 'acceleration' without using a model for reference. THAT is the point. Further, there are other models that do NOT show acceleration using the same data. So that it seems to me that the model needs to be proved -- somehow -- independent of the data showing these departures -- BEFORE we invent entirely new kinds of energy just to protect a cherished model.

Frankly, the concept that matter density -- critical or otherwise -- having much to do with the expansion of the universe is also an question.

Sorry, a thunderstorm here and uploads were interrupted.

Attached in the thumbnails are three comparisons plots of the common format used to show acceleration (data from Ned Wrights Cosmology page).

The first is basically a reproduction of the typically shown support for dark energy.

The second is a comparison of empty models using Milne as reference. This indicates a possible problem with Milne -- in that the other empty model 'curves' to fit the data.

The third is a similar to the first plot BUT the reference is the SGM Empty model -- for the data and the curve ('full matter' model) fit. Note, NO HUMP .. and no apparent acceleration.

I'm NOT presenting this the tout a model. That is not the point. The point is the apparent acceleration is due to the functional structure of the Standard Model (mirrored by Milne at Omega=0). By itself it doesn't prove its wrong -- but it also doesn't prove it right.

The ONLY support I have seen for dark energy -- related to acceleration -- is when using Standard Model. There is NO model independent support for this. And that is my problem with it.

Note, the SGM model while relatively simple -- as it was designed to be -- still is far to numerically challenging for a spreadsheet (NEVER use them).

I have a MSC in Engineering Physics and have been doing data analysis for over forty years professionally. So far the claims I've seen (for dark energy) are not supported by the data I've seen.