# B Is 'space' expanding or not?

1. May 6, 2016

### rede96

I've spent quite a bit of time researching cosmology on here and various other sources (NOT pop science) and although I feel I have a much better understanding in general, there is one specific thing that I just can't get my head around. Is the space between galaxies expanding or is it just the distance between galaxies growing.

The reason this seems so confusing is that from posts I've made before I understood it as just the distance between galaxies growing. (at an accelerated rate due to dark energy) and not because there was some property of empty space the was 'expanding' causing the galaxies to move apart. But I read in many posts here that it's the space that is expanding.

One of the most popular references is related to photons from distant galaxies that are receding >c won't reach us because the 'space' in between is growing faster than c. Where I would have thought that if it is just the distances growing at a rate >c the photons would still reach us eventually.

So can someone help clear this up for me once and for all! I'm sure I'm missing something simple, but I just cant get it!

2. May 6, 2016

### Chalnoth

The average distance between galaxies certainly increases over time.

Whether you refer to this increasing distance as the expansion of space is more or less a matter of taste. The purpose of the "expanding space" description is to make it easier to grasp certain features of the expansion, namely that recession velocity can exceed the speed of light and that the expansion also affects the wavelengths of photons in the same way that it does the distances between galaxies.

It depends upon what the expansion does in the future. If the rate of expansion slows down sufficiently, yes, those photons will eventually reach us. But if the dark energy is indeed a cosmological constant (or acts like one in the future), then the rate of expansion won't slow down far enough for the light from many far-away galaxies to ever reach us.

Last edited: May 6, 2016
3. May 6, 2016

### ogg

Please explain the difference between the expansion of distance (in all directions) and the expansion of space. I am unable to understand why you think there IS a difference. Space is expanding. The distance "between galaxies" is not - unless the galaxies in question are NOT GRAVITATIONALLY BOUND. (Another way to put it is that the expansion rate is so small, that even the weakest force (gravity) is sufficiently strong to keep locally interacting objects bound.) So, it is on the scale of superclusters where you can observe the expansion of space VIA the increase in distance (as measured by red-shift). Our current model (theory) is that the expansion is the same for each point in space at a given time. (Please don't ask what I mean by "a given time"!). Our models predict that the rate of increase is increasing, but whether this will continue or not is unknown. IF the amount of space (distance) between two objects is increasing faster than the two objects are reducing that distance, and that remains true forever, then the two objects will never meet. If you are walking (against the flow) on a straight infinitely long moving walkway at a speed less than the speed of the walkway, you are "moving away" from fixed points in front of you, you will never reach them (unless something changes). To reiterate: objects in our local cluster of galaxies are NOT moving away from one another, more distant objects are moving away from us. So, we will not see (a cosmological) redshift from local galaxies, but will see red-shift with more distant ones. Space is expanding everywhere, distance isn't (in any meaningful way) if the objects are bound.

4. May 6, 2016

### rede96

I'm not too sure I would agree with it being a mater of taste. I would have thought there is a fundamental difference between the 'space' between two distant galaxies expanding, causing the galaxies to move apart and the distance between those two distant galaxies getting larger due to the two galaxies moving apart.

Again, this would imply that because the 'space' the photon is travelling through is growing, it stretches or red shifts the photon. In the same way a pen mark on a piece of elastic would grow if the elastic was stretched. But from what I remember that isn't what is going on with cosmological red shift.

Sure, I think for the purposes of this post I'd assumed the rate doesn't slow down in the future. So it makes sense to think if we are moving away from a distant source at a rate >c then a photon from that source would never catch up to us. However that statement causes confusion too. Because it's said that two distant objects can't move apart at speeds > c in a conventional sense as this would violate relativity. So I've heard it explained as the two distant objects are actually at rest wrt to each other, but the space grows between them. But what I thought I'd understood is that GR does allow objects at cosmological distances to move apart at speeds >c, it just doesn't allow things locally to pass each other at speeds >c.

So for me, I thought using the words 'space is expanding' is not only confusing but technically incorrect.

5. May 6, 2016

### Chalnoth

Nope.

The statement that the average distance between galaxies is increasing is the precise interpretation of the expansion.

General Relativity is interesting in that there are a lot of different ways of describing the exact same system that can sound very different from one another. You have to be really careful when making statements about how General Relativity describes the expansion (or other things) in order to avoid making ambiguous statements.

In this particular case, the ambiguity comes from time: before you can ask how far away two points are, you first have to find a way to synchronize your clocks between the two points. With an expanding universe, this is pretty easy: the same time at two different points is the time where they both measure the same CMB temperature. Once you've used the CMB to synchronize the clocks, the proper distance between any two points in the universe is well-defined, and the proper distance between any two points that are stationary with respect to the CMB increases with the expansion.

Other descriptions are used to try to get people to understand other interesting features of the expansion (as I mentioned), but they don't necessarily succeed. Many people on these forums tend to think that the "expanding space" description can be misleading. I don't know. Maybe it is, maybe it isn't. But either way, the "distance increasing" description is the most accurate way of describing it.

This isn't true in General Relativity, for the simple reason that in General Relativity, the relative velocity between two objects is only well-defined at a particular point. You can't actually subtract the velocity between two objects that are far away and get a single, definite answer. Because you can't get a definite answer for far-away objects, there can't be any speed limit for far-away objects.

The speed-of-light limitation in General Relativity instead is the statement that no object can outrun a light beam.

6. May 9, 2016

### rede96

Thanks for the further explanation. This is basically how I understood it. But this still doesn't answer the question about whether the distances are increasing due to the galaxies moving through space wrt each other or if the distances are increasing because space itself is expanding and moving the galaxies apart. That is where I read so many contradictory things about expansion.

Even watching Leonard Sussskind's lectures on Cosmology he explains expansion something like "As space expands, imagine little bits of space filling in the gaps" I'll need to find the exact quote, but that is roughly what he implied. Space itself grows, not just distances getting bigger.

Again still confusion here for me because if a distant galaxy emits a photon in our direction and we are far enough away (assuming no slowing down of expansion) then we will be moving away at a rate >c and thus the photon will never catch up to us. I think it is safe to say that is an accurate statement. Which then begs the question will we catch up with photons that had already passed us from that far away galaxy before our respective recession velocities were >c ?

7. May 9, 2016

### George Jones

Staff Emeritus

Care is needed with respect to the definition of "speed" in a cosmological setting.

8. May 9, 2016

### rede96

Ok, I didn't mention speed in the section you quoted so could you elaborate a little more please?

And here we have it! Another contradictory statement. You statement:
Chalnoth's statement:

9. May 9, 2016

### George Jones

Staff Emeritus
No, the posts by Chalnoth and me are not contadictory. I dd not say that there do not exist galaxies whose light we will never be able to observe, I said that if we can observer a galaxy now (or at any time in the past), then we always be able to observe that galaxy, even if its proper recessional velocity exceeds c.

10. May 9, 2016

### rede96

Ah ok, sorry I missed read that. Although that still confuses me.

EDIT: However on one of Susskind's lectures he does say that in many billions of years to come the only stars we will see will be from our own milky way. So that does contradict you.

11. May 9, 2016

### Chalnoth

"Space is expanding" is one way of looking at it, and it isn't inaccurate. It's just that it's not really a precise statement.

The way I like to think of it is this: imagine you're driving down a road, and pass a telephone pole. It's completely accurate to say either that the pole passed by you and that you passed the pole. There's no right answer between those two choices: it's just a difference in reference frame.

Saying that space is expanding is sort of like that. You could describe it that way, but you can just as accurately describe it as objects getting further apart.

This isn't entirely accurate, because the rate of expansion changes over time. A photon that started traveling in our direction, but was still carried away from us due to how fast the expansion was, might eventually start to gain ground as the expansion slows over time. In fact, most of the galaxies visible from Earth are in this situation, because the expansion rate in the past was much, much faster than it is now.

The picture here is that the photon leaves the galaxy, traveling in our direction. But the expansion is so fast that the distance between us and the photon still grows as it moves at speed c away from its source galaxy. Over time, the expansion slows down enough that the photon starts to gain ground instead, eventually reaching us.

Note that the rate of expansion is now decreasing very slowly, and seems to be approaching a constant (the cosmological constant). So the limit beyond which we can no longer see galaxies is a little bit further than the point at which their recession velocity reaches c.

12. May 9, 2016

### Chronos

See the article by Davis and Lineweaver ; http://arxiv.org/abs/astro-ph/0310808, Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe, for further discussion.

13. May 9, 2016

### George Jones

Staff Emeritus
I have not seen any of Susskind's lectures, but he could mean in a "for all practical purposes sense". From

http://arxiv.org/abs/0704.0221

The expansion of the universe in three ways diminishes the energy flux that we receive from distant galaxies.

1) a dimming due to increasing distance.

2) The energy of light is inversely proportional to its wavelength (energy of a photon is $E=hc/\lambda)$. As the light travels to us, the expansion of the universe expands the wavelength of the light by a factor of 1+z, where z is redshift.

3) Also, the expansion of the universe decreases the rate at which we receive photons, as compared to the rate at which photons left a source, by another factor of 1+z (gravitational time dilation).

This triple whammy means that we will lose contact in *practical* terms with most of the universe in the distant future.

14. May 9, 2016

### Staff: Mentor

I always thought the OP question was related to observational versus theoretical science. We can observe the light of distant galaxies, but not the intergalactic space. What happens in distant intergalactic space can be inferred but not observed. Therefore, if we stress what we know from observation, expansion of empty space is deprecated.

Where I think I sense disagreement among experts is if we consider a small region containing no mass, no photons,and no gravitational field. Does it expand? The answer would seem to depend on vacuum energy because to have a time evolution, the region needs a nonzero Hamiltonian. If I got it right, expert opinions on vacuum energy differ.

Another way to say it is that particles (massive and massless) and fields have properties. Take them away and we have nothing. It is meaningless to discuss the properties of nothing.

Is my description of the disagreement correct?

P.s. My spell checker just tried to correct "discuss" to "rickshaw". To me, that is an even deeper mystery.

15. May 11, 2016

### rede96

Thanks for the link. I've had a quick look but will review it in more detail over the weekend. However there is on thing that I'd like to mention, which comes from the quote below taken on page 5...

...in particular where it states that separation velocities are due to expansion of space and not movement through space. This implies to me that expansion is due to 'empty space' growing, or in other words, some physical framework which energy and matter ect are embedded, is growing. Moreover that dark energy therefore isn't a force that is 'pushing' galaxies apart, it is a force acting on the expansion of empty space, causing that to accelerate.

That does make sense to me, but it is different to many things I've read about expansion. This is fundamentally what I am trying to clear up.

16. May 11, 2016

### rede96

That is pretty much where my confusion is. As I posted above, the article Chronos referred to would seem to suggest there is 'something' expanding.

17. May 11, 2016

### Staff: Mentor

If there is no mass, no photons, and no gravitational field, then spacetime is empty and flat. So how could we possibly tell whether it was "expanding" or not? There is nothing to observe.

The way I would resolve the question raised by the OP is to say that the term "expansion", properly speaking, applies to a family of worldlines, not to "space". That is because "space" is not an invariant concept; what "space" is, and whether it is "expanding", depends on your choice of coordinates. But if we pick out a particular set of worldlines, then the question of whether those worldlines are "expanding" has an invariant meaning. Any set of worldlines has an invariant scalar associated with it called the "expansion scalar". If this scalar is positive, the set of worldlines is expanding; if the scalar is negative, it is contracting; and if the scalar is zero, the set of worldlines is not expanding or contracting.

When cosmologists say "the universe is expanding", what they actually mean, put in precise technical language, is that the expansion scalar of the set of "comoving" worldlines--that is, the worldlines of observers who see the universe as homogeneous and isotropic--is positive. That is an invariant statement and does not depend on any choice of coordinates, nor does it require one to say that "space" is expanding, or that it isn't, for that matter. Whether "space" is expanding is simply not a meaningful question as far as the physics is concerned.

The expansion scalar is actually one piece of a very useful mathematical tool in GR, called the "kinematic decomposition" of a set of worldlines (the more precise technical term is a "timelike congruence"). Some more information about that can be found here:

https://en.wikipedia.org/wiki/Congr...atical_decomposition_of_a_timelike_congruence

18. May 11, 2016

### Chronos

The concept of distance has no intrinic meaning under GR, which is a theory only of 'space-time coincidences'. This means all such measurement can only be local (at an event) . This destroys the notion of length and time co-ordinates as observables. To quote Einstein "All our space-time verifications invariably amount to a determination of space-time coincidences. If, for example, events consisted merely in the motion of material points, then ultimately nothing would be observable but the meeting of two or more of these points. Moreover, the results of our measuring are nothing but verifications of such meetings of the material points of our measuring instruments with other material points, coincidences between the hands of a clock and points on the clock dial, and observed point-events happening at the same event. The introduction of a system of reference serves no other purpose than to facilitate the description of the totality of such coincidences." So, in this light it is no more meaningful to say that space-time is expanding than it is to say the distance between far away galaxies is increasing. It is strictly a frame dependent convenience to characterize the relationship between non-local events and space-time. Some users prefer to talk in terms of an expanding coordinate system to characterize expansion. This too, however, is little more than a convenient illusion.

19. May 13, 2016

### rede96

For me personally, my question isn't about how we describe expansion. In the context of the questions I guess the words we use or choice of coordinates etc are for the most part irrelevant.

What I am asking is much more fundamental. Another way of putting this would be as the universe expands, is new 'space' being created? Are quantum fields, such as the quark field or higgs field etc being constantly created OR do they already exist and the universe is just 'moving' into those pre-existing fields?

20. May 13, 2016

### Staff: Mentor

I don't know what "new space being created" means.

I don't know what it would mean for new fields to be "created" vs. the universe "moving into pre-existing fields".

I understand that these seem to you like meaningful questions. But they don't correspond to anything in the actual models we use. That was part of my point.