Does Space Expand? What Do You Think?

[MeJennifer]

Commonly we are told that the local mass concentration 'overcomes' the expansion preventing this from occurring. This is one of the worst and most fallacious explanations you could possibly give someone!

I could not agree more on this.

Sorry for all the !'s I get rather excited when I talk about this stuff

Well, keep talking about it!

By the way an excellent explanation Wallace!

 

[oldman]

The FRW metric, and the 'expansion' does not have an effect on small scales in our universe. Local mass concentrations are far more than merely a perturbation to FRW on familiar scales. If anything there is a vanishingly small FRW element to the metric of bound structures.

I guess our disagreements are more a case of crossed wires (due to my careless writing about gravitationally bound structures) than a disagreement. At least I hope so!

I see it like this:

First, consider a classic dust-filled fluid-like universe described by the FRW metric, but filled with CMB radiation (no structures). Let all dust particles be at rest in their local CMB frames (no peculiar motions). The Cosmological Principle rules strictly here. Yet the proper separations of dust particles vary as a(t) changes with time! Strange but true! This is GR, remember ... not simple expansion.


In bound structures with finite volumes cohesion (due to any kind of nature's interactions ... gravitational, electroweak or strong) endows individual components of the structure with kinematic or coasting velocities relative to local CMB frames. These velocities can ensure that proper distances between components remain constant while H remains constant. (Please don't ask me what happens in an inflating, accelerating or decelerating universe!). Ask Pervect!

Thus, in a gravitationally bound galaxy, stars on opposite sides have coasting velocities relative to their local CMB frames. And, atoms in a molecule in deep space have (very, very, very tiny) velocities relative to their local CMB frames.

This is how, as you say:

galaxies maintain a constant size as the distance between them expands?

while the FRW metric nevertheless "prevails" everywhere (a poor word choice of mine, I'm afraid). I should have said "applies but imperceptibly in cases of practical interest". Perturbation was also the wrong word to use.

But then I could accuse you of my own sin, when you say:

The FRW metric simply is not valid on these scales.

valid?

 

[Wallace]

I think we pretty much agree on clarification and reflection. I just think it's important to be clear that the 'expansion' (which we both definitely agree is a bad term for it!) is a result of the FRW metric, in particular a(t). The metric in the region of bound structure looks nothing like the FRW metric, in particular it has no global time dependence (though will of course evolve). For this reason I stand by the statement that the FRW metric is not valid on scales which are significantly inhomogeneous, since the metric has no component that reflects the global a(t), and hence the FRW picture does not relate to the dynamics of the system.

 

[oldman]

I can't refrain from pointing out that during inflation, where H increases exponentially (if I remember correctly), even the tiniest structures experience a huge disruptive force as a result of the rule of the FRW metric. Pervect convinced me that these forces are vast enough to tear apart just about any structure we can imagine. Yet nobody seems to take notice of this extreme effect! Strange.

 

[Castlegate]

Isn't it possible that close galactic clumps currently receding could in a matter of say 10 billion years, be seen as a part of a larger system, by which anyone of them could have a blue shift from our vantage point? This is something tantamount to the universe evolving, wherein our current universal state of evolution is that of clumps of galaxies held together by gravity, and a later state of evolution we will see clusters of clumps held together by gravity, and further down the line ... clumps of clusters held together by gravity?

It would seem realistic to assume that earlier on in the history of the universe, that all galaxies were seen to be receding from our vantage point, and much earlier than that, matter the size of a standard average every day sun would see all suns receding from the vantage point of every sun. Taken much earlier - all atoms are seen from the vantage point of any atom to be receding.

If this is so, then the idea of space expanding seems frivolous. An accelerated expansion even more so.

 

[pervect]

Isn't it possible that close galactic clumps currently receding could in a matter of say 10 billion years, be seen as a part of a larger system, by which anyone of them could have a blue shift from our vantage point? This is something tantamount to the universe evolving, wherein our current universal state of evolution is that of clumps of galaxies held together by gravity, and a later state of evolution we will see clusters of clumps held together by gravity, and further down the line ... clumps of clusters held together by gravity?

It would seem realistic to assume that earlier on in the history of the universe, that all galaxies were seen to be receding from our vantage point, and much earlier than that, matter the size of a standard average every day sun would see all suns receding from the vantage point of every sun. Taken much earlier - all atoms are seen from the vantage point of any atom to be receding.

If this is so, then the idea of space expanding seems frivolous. An accelerated expansion even more so.

I'm not quite sure what your point is. A hypothetical universe where everything was inititally redshifting, continuously evolving towards less and less redshift, then started to blueshift is imaginable. Furthermore, it is imaginablie in the context of GR - such a universe would have a matter density over the critical, and would be a universe ending in a "big crunch".

However, experimental data does not support this. As is widely known, the universal expansion appears to be accelerating.

I don't understand why you think the experimental data that says that the universe is expanding and that the expansion is accelerating is "frivolous".

The redshift data is pretty clear evidence that the universe is currently expanding. Even more convincing is that fact that distant supernovae explosions appear to happen "in slow motion" due to time dilation - this basically rules out "tired light" type theories. So for starters, I hope we can agree that the universe is currently expanding.

It does require GR to go beyond saying that the universe is currently expanding to interpret the data as saying that the expansion is accelerating and will continue to accelerate. It is possible using some non-GR theories ,to have a non-accelerating expansion that fits the data. AFAIK there isn't any alternative theory "in play" that suggests that the expansion could reverse, however.

Basically, GR isn't "frivolous" - it's being tested even as we speak. One of the simplest forms of the alternative gravitational theories that would allow non-accelerating expansion has recently been ruled out by the Gravity probe B experiments, for instance. Other more complex forms of this theory may still be viable, but GR has a long history of making good, correct predictions.

 

[pervect]

I can't refrain from pointing out that during inflation, where H increases exponentially (if I remember correctly), even the tiniest structures experience a huge disruptive force as a result of the rule of the FRW metric. Pervect convinced me that these forces are vast enough to tear apart just about any structure we can imagine. Yet nobody seems to take notice of this extreme effect! Strange.

I think that the people concerned with large scale structure have to take this effect into account - but they don't tend to popularize their work in such simple terms.

There is a rather subtle point here - when one looks at the actual tidal forces generated by the Riemann, it is not the expansion of the universe that causes these forces, but the acceleration of the expansion of the universe. This is a point that's been discussed before, but I'm too lazy to look up the threads unless there is some renewed interest. Basically, if one considers a universe with no cosmological constant and no "dark energy", there could never be "tearing apart" tidal forces, there could only be compressive tidal forces.

Of course, inflation has effectively an extremely large cosmological constant - and this is what causes the "tearing apart" forces in the usual inflationary schenario.

 

[pervect]

I think we pretty much agree on clarification and reflection. I just think it's important to be clear that the 'expansion' (which we both definitely agree is a bad term for it!) is a result of the FRW metric, in particular a(t). The metric in the region of bound structure looks nothing like the FRW metric, in particular it has no global time dependence (though will of course evolve). For this reason I stand by the statement that the FRW metric is not valid on scales which are significantly inhomogeneous, since the metric has no component that reflects the global a(t), and hence the FRW picture does not relate to the dynamics of the system.

While it may not make sense to use a pure FRW metric to describe a bound system, there are some papers that take the approach of using for instance a Schwarzschild De-sitter metric. These approaches can be justified more rigorously if one imagines that the dark matter and dark energy are isotropically distributed, and that the lumpiness is only in the normal matter.

This approach won't model effects due to "lumpiness" in the dark matter, for instance, but one can probably gain some insight into the effects of lumpiness in the dark matter by the effects of lumpiness in the non-dark matter.

The whole dark matter issue is rather annoying if one isinterested in gravity - basically, the source of most of the gravity in the universe appears to be invisible :-(.

 

[Castlegate]

I'm not quite sure what your point is. A hypothetical universe where everything was inititally redshifting, continuously evolving towards less and less redshift, then started to blueshift is imaginable. Furthermore, it is imaginablie in the context of GR - such a universe would have a matter density over the critical, and would be a universe ending in a "big crunch".

Your assumption of a 'big crunch' would be true if you assumed a 'big bang'

I don't understand why you think the experimental data that says that the universe is expanding and that the expansion is accelerating is "frivolous".

I'm not saying that an accelerating expansion is frivolous. Only that if the assumption posted earlier by me were true, that the expansion of space is a frivolous thought. Do you consider the idea of greater masses evolving to even greater masses as frivolous? It is at least a logical progression one might expect irrespective of other known measurements.
My way of creating new angles by which to look at the same thing. In other words - If galaxy clusters are to eventually cluster themselves, what model of a universe would bring this about? Big bang does not fit this bill. This is just thinking out of the box that most of us put ourselves in, and I leave the box because all six sides represent dead ends.

A model that produces an evolving scenerio as described is possible by the way. Doesn't make it right ... but possible?

 

[Wallace]

Do you consider the idea of greater masses evolving to even greater masses as frivolous? It is at least a logical progression one might expect irrespective of other known measurements.
My way of creating new angles by which to look at the same thing. In other words - If galaxy clusters are to eventually cluster themselves, what model of a universe would bring this about? Big bang does not fit this bill. This is just thinking out of the box that most of us put ourselves in, and I leave the box because all six sides represent dead ends.

A model that produces an evolving scenerio as described is possible by the way. Doesn't make it right ... but possible?

Hmm the current model of the universe predicts that mass gets increasingly lumpy as time goes on. Lumps of mass do indeed get more massive as time goes on, clusters form etc etc. I don't follow your previous logic as to how this is a problem but rest assure that far from being incompatible with the Big Bang it is a prediction (and observation!) of current cosmology.

 

[cristo]

Your assumption of a 'big crunch' would be true if you assumed a 'big bang'

What do you mean by this? Are you meaning to say that if we assume a big bang, then this implies a big crunch in the future? If you are, then this is not true!

 

[Old Smuggler]

The point is though that there is not expansion to 'overcome' since the 'expansion' is merely the result of the metric formed by a homogeneous and isotropic mass distribution. If the mass dosn't obey these principles we shouldn't be surprised that we don't see any 'expansion'.

But the expansion is seen on scales where the mass distribution is far from homogeneous and isotropic - namely in the vicinity of or even within the local group of galaxies. Moreover, the Hubble law is obeyed tightly even at this (cosmologically speaking) local scale, and the local Hubble parameter is of the same order as the global one. This problem of "the quiet local Hubble flow" was first identified by Sandage, and it is still an unsolved problem.

For observational results and references to the original papers, see

T. Ekholm et al., Astron. & Astrophys. 368, L17 (2001) (astro-ph/0103090).

 

[Wallace]

Sure, so there is no harsh cut-off point where the metric goes from FRW suddenly to some other non-expanding form. I would have expected that the expansion would not be dominant on the scale of the local group, so that results is unexpected for me. Thanks for pointing it out, I will have to follow this up.

It remains that case though, that as the local region becomes more and more inhomogenous, the metric must have less and less 'FRWness' in it. The expansion most certainly does not occur on galactic scales or smaller, such as the scale of a solar system for instance.

 

[wolram]

Sure, so there is no harsh cut-off point where the metric goes from FRW suddenly to some other non-expanding form. I would have expected that the expansion would not be dominant on the scale of the local group, so that results is unexpected for me. Thanks for pointing it out, I will have to follow this up.

It remains that case though, that as the local region becomes more and more inhomogenous, the metric must have less and less 'FRWness' in it. The expansion most certainly does not occur on galactic scales or smaller, such as the scale of a solar system for instance.

For sure, it seems that some realistic idea are coming to the fore
.

 

[Wallace]

But the expansion is seen on scales where the mass distribution is far from homogeneous and isotropic - namely in the vicinity of or even within the local group of galaxies. Moreover, the Hubble law is obeyed tightly even at this (cosmologically speaking) local scale, and the local Hubble parameter is of the same order as the global one. This problem of "the quiet local Hubble flow" was first identified by Sandage, and it is still an unsolved problem.

For observational results and references to the original papers, see

T. Ekholm et al., Astron. & Astrophys. 368, L17 (2001) (astro-ph/0103090).

I've followed this up, it looks like this was a unfortunately timed paper, coming out just a month or two before the HST key project to measure H₀. This paper uses a best guess for H₀ of just over 50 km s^{-1} Mpc^{-1} substantially lower than the 72 km s^{-1} Mpc^{-1} found from the Hubble key project. This significantly changes the articles conclusions since it really shows that the local group Hubble diagram will not match the slope of the global expansion rate very well at all. I think this is closer to what I would have expected.

 

[hellfire]

You may be interested in the references mentioned in this post.

 

[Old Smuggler]

I've followed this up, it looks like this was a unfortunately timed paper, coming out just a month or two before the HST key project to measure H₀. This paper uses a best guess for H₀ of just over 50 km s^{-1} Mpc^{-1} substantially lower than the 72 km s^{-1} Mpc^{-1} found from the Hubble key project. This significantly changes the articles conclusions since it really shows that the local group Hubble diagram will not match the slope of the global expansion rate very well at all. I think this is closer to what I would have expected.

But they didn't measure the Hubble parameter; rather they took a value of 57 km/sMpc as
an input for their model. While it is reasonable to assume that the numbers would change
somewhat if a different input value were chosen, the main results should not be critically
dependent on this. That is, the linearity of the local Hubble law and the small local velocity
dispersion around it should still hold.

As Hellfire pointed out, the mystery of the quiet local Hubble flow could have an explanation in dark energy, suppressing the growth of velocity fluctuations. However, this explanation is not sufficient, since the parameter values necessary to achieve this would
be incompatible with other observational tests. For details, see

M. Axenides and L. Perivolaropoulos, Phys. Rev. D 65 127301 (2002) (astro-ph/0201524).

So what the observations of the velocity field of galaxies in the vicinity of the local group
is telling us, is that there is no obvious link between the expansion and the clumpiness of the Universe. Thus the fact that the Universe is even more inhomogenous on smaller
scales such as galaxies and planetary systems, is in itself not a sufficient argument
to conclude that the expansion cannot exist and be detectable on such scales.

 

[Hell_SD]

going back to the analogies...

...has anyone equated space expanding to a cell dividing ?

imagine a single red cell dividing in two. Those 2 red cells then split, but instead split into white cells and every division there after is also white, such that there are only ever 2 red cells. The faster the white cells divide exponentially, pushes the 2 red cells ever further apart and faster...

...it'd make a great cartoon i reckon, cos in the bigger picture you'd see that the 2 red cells are moving apart faster than the rate at which any single white cell is dividing at

now when there weren't many white cells separating the 2 red cells someone living on one red cell shined a torch in the direction of someone on the other red cell, but as the distance between the two cells increased, the light from the torch had to travel further and by a strange coincidence turned red itself...

 

[Wallace]

This doesn't seem like a very helpful analogy.

 

[Hell_SD]

...so its no more or less helpful than the raisin and balloon one. Care to explain why ?

watching cells divide would make for a better cartoon though. Theres just something about how they do that and how space is created which i find intriguing...

...like reconstituting matter from one thing to another in real time

 

[marcus]

...less helpful than the raisin and balloon one. Care to explain why ?

watching cells divide would make for a better cartoon though.

I agree with Wallace about cells analogy not helpful---think it is very likely to be harmful to understanding.
with pennies on the balloon, you go thru the exercise of imagining the pennies all drifting apart and then the all-important final step is IMAGINE THE BALLOON ISN'T THERE.

Picture the pennies all receding from each other, their separation distances increasing by some percentage with each time tick

and then imagine the rubber skin isn't there!

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

likewise if you use the raising dough analogy you should, at the end, imagine that the dough is not there

all there is is increasing distances between raisins
=============
the pennies, or the raisins, are the analogs of GALAXIES, where are the galaxies in your analogy?
=============

I would say your "cells" analogy is mind-damaging and pernicious because it tends to mislead people into thinking that space is a "something" like cells. and it gets mentally messy when you imagine finally that all the cells vanish from existence, then what is left?

 

[Hell_SD]

I would say your "cells" analogy is mind-damaging and pernicious because it tends to mislead people into thinking that space is a "something" like cells. and it gets mentally messy when you imagine finally that all the cells vanish from existence, then what is left?

fair enough but surely it's no more messy than the actuality of distances between objects increasing faster than the speed of light without the objects themselves moving...

...and considering there is supposed to be a universal speed limit based on how fast light travels through space which supposedly can't be exceeded or asking the messy question of where does this extra space come from ?

i just like the organicness of cells dividing so could you please tell me why space isn't like cells ? Whats to stop there being a unit of something which appears and vanishes at superluminal speed leaving space as it's residue ?

 

[marcus]

fair enough but surely it's no more messy than the actuality of distances between objects increasing faster than the speed of light without the objects themselves moving...

...and considering there is supposed to be a universal speed limit based on how fast light travels through space which supposedly can't be exceeded or asking the messy question of where does this extra space come from ?
...

you sound pretty confused HSD. I don't know what to advise you or where you could begin trying to get straightened out. I can't take time to try but maybe someone else can help you.

you are mixing up SR and GR. the speed limit in SR does not apply globally to rates of distance change in GR. you are mixing up recession speed with relative motion within one local framework

there is NOT supposed to be a universal speed limit that applies to recession speeds. if someone told you that they lied, or were mistaken, or maybe you just misinterpreted. (all that kind of talk is with SR only and SR has a very restricted applicability, if you apply it outside its area of validity it gives nonsense)

there is NO problem of "where extra space comes from" because space is not a material substance-----it is just the distances between things a web of geometric relations----you DONT HAVE TO MAKE MORE.

you mistakenly assert that this is messy, I do not see that anything i am telling you is messy.

between things that are not bound together there is no reason to expect distances to stay the same, in fact large scale distances tend to increase at a certain percentage rate----there is nothing messy or funny about this and no reason to expect it to not be the case---and in fact it is how the world is.

largescale distances typically increase by 1 percent about every 140 million years-----so naturally the longer they are to begin with the faster they increase in absolute terms---that's how percents work.
so naturally some long distances are increasing faster than any given speed, you can always pick a distance long enough that it is increasing faster than c.

Unless you immediately come around and stop talking nonsense i will have to give up on you, and I do not expect you to. So maybe someone else will be willing to try to explain. I hope so.

 

[Hell_SD]

distances between objects increasing faster than the speed of light without the objects themselves moving

c'mon marcus that is a hell of a concept to try and wrap your head around...

the fact that the objects aren't moving apart at superluminal speed but that space is growing between them at faster than lightspeed while maintaining that nothing can go faster than c

...sounds messy so surely I have a right to be confused ?

 

[Nick666]

Well there are theories that say space is nothing. So, it might be that space can move with speeds > c.

 

[BoomBoom]

Dividing cells is an absolutely horrible analogy...no offense. It seems to me that the main point of the discussion was to try and describe increasing distances without implying that space itself was driving it apart. Describing space as white cells in this analogy defeats that purpose and implies that space is like matter.

I've been struggling to remember what this gadget is called (perhaps someone can help me out) but if anyone has ever seen these toys that are a lattice type structure with moveable joints that when fully contracted is like a ball. When you pull it apart, it expands in size uniformly to many times it's original size. If you can pictue this and also picture each joint as a galaxy, I've always thought that would be a decent visualization of an expanding universe...

 

[Wallace]

distances between objects increasing faster than the speed of light without the objects themselves moving

c'mon marcus that is a hell of a concept to try and wrap your head around...

the fact that the objects aren't moving apart at superluminal speed but that space is growing between them at faster than lightspeed while maintaining that nothing can go faster than c

...sounds messy so surely I have a right to be confused ?

Superluminal recession is something that easily causes confusion. The issue is that velocity is the rate of change of distance and with but in GR neither distance or time are trivial concepts. The distance that we use in FRW cosmology is most commonly the proper distance, and if we look at the rate of change of this with 'cosmic time' (the time as measured by co-moving observers) we note that this velocity exceeds c for sufficient distance from the origin.

All this tells us is how one of many different possible definitions of distance change. If you accept the FRW metric then you have to live with that fact. Other metrics that use difference co-ordinates but make the same physical predictions do not contain any apparent superluminal recession.

You need to realize that your everyday notion of distance doesn't work on cosmological scales, and hence your everyday intuition is useless.

 

[wolram]

If we want to measure distances in our own galaxy, we can use a rigid ruler,
and we can get quite an accurate measurement from some point (earth) +,- bodies motion (orbits) within the galaxy.
If we want to measure between galaxies, we have to throw away the rigid ruler and use a flexible one, the rate of flex (stretch) depends on distance from our vantage point, the rate of stretch increases with distance.
The stretch factor is not considered as a (speed) just an increase in the total volume we, the universe exists in.
This sounds total nonsense, but may be i have the above wrong?

 

[wolram]

Another paper.

arXiv:0707.1350 (cross-list from gr-qc) [ps, pdf, other]
Title: Cosmological expansion and local physics
Authors: Valerio Faraoni, Audrey Jacques (Bishop's University)
Comments: 17 pages, LaTeX, to appear in Phys. Rev. D
Subjects: General Relativity and Quantum Cosmology (gr-qc); Astrophysics (astro-ph)
The interplay between cosmological expansion and local attraction in a gravitationally bound system is revisited in various regimes. First, weakly gravitating Newtonian systems are considered, followed by various exact solutions describing a relativistic central object embedded in a Friedmann universe. It is shown that the ``all or nothing'' behaviour recently discovered (i.e., weakly coupled systems are comoving while strongly coupled ones resist the cosmic expansion) is limited to the de Sitter background. New exact solutions are presented which describe black holes perfectly comoving with a generic Friedmann universe. The possibility of violating cosmic censorship for a black hole approaching the Big Rip is also discussed.

 

[pervect]

The issues arising with measuring the distance to distant galaxies are not really related to the problem of constructing a rigid ruler (though of course a rigid ruler is an idealization - the most rigid ruler that's actually possible would be counting the wavelengths on a lightbeam, which is only approximately rigid. This is essentially a limiting case when the velocity of sound in the ruler is equal to 'c'.)

Rather, the issue with measuring distance it is the problem of "what curve" to measure the length of. The usual notion of distance proceeds as follows - one takes space-time, and separates it out into space and time. One then measures the distance in some hypersurface of constant time.

Unfortunately, the split of space-time into space and time is in general arbitrary and depends on the choice of coordinates.

Note that the usual notion of distance ("proper distance") defined in this manner (measuring the distance along a curve of constant cosmological time) does not actually measure the distance along a straight line (or the equivalent of a straight line in a curved space-time, a space-like geodesic).

This is because a curve of constant cosmological time connecting two points in a FRW universe is not a "straight line", i.e. it is not a geodesic.