1. May 21, 2005

### Zanket

Let a floating thread span the distance between two galaxies fast receding from each other due to space expanding between them. Eventually the thread must break. Expanding space forces apart adjacent particles of the thread all along the thread. The thread breaks at an arbitrary spot. Then any thread on Earth may break due to expanding space. There's a lot of fabric on Earth. Perhaps a thread somewhere spontaneously broke while you were reading this.

2. May 21, 2005

### Garth

Unless the thread itself expands with the universe.
(Not according to GR but SCC)
Garth

3. May 21, 2005

### matt.o

the space the Earth inhabits does not expand with the hubble flow, so the thread would not break on Earth. You never see galaxies stretching and breaking due to the expansion of the universe, do you?

4. May 21, 2005

### Garth

If you treat gravitation as a force then as it is ~10-40 the strength of the other forces we can safely conclude that the thread and physical rulers will not expand with the universe. The binding energies of the e-m and nuclear forces holding them together, and the local gravitational forces in the solar system and the galaxy holding the solar system and the galaxy together, are much stronger than those of the Hubble expansion. The Earth bound thread does not break, but the cosmological thread suspended between two galaxies does break.

If however we treat gravitation as space-time curvature, as indeed we do in GR, then the question arises as to whether this expansion applies to everything embedded in that space-time. So then, what expands with space-time?

As the Schwarzschild solution for gravitational orbits is embedded in that space-time should not its solutions co-expand? Also as the Bohr/Schrödinger/Dirac equations of atomic physics are also so embedded then should not their solutions, i.e. atoms, also expand? If so, as the thread is made of atoms it expands with the universe, so the Earth bound and now the cosmological threads do not break!

Furthermore, we might ponder that, if physical rulers expand with the universe, then there should be no expansion as measured by those rulers.

In such a case Hubble red shift would be interpreted as other than recession Doppler shift. (e.g. a variable mass effect).

If we ask whether there is any evidence for the solar system as a whole to so expand then such might be given by the intriguing Pioneer anomaly, which, interestingly, is of the same order as the Hubble acceleration cH.

So which is the more consistent approach in answering the ‘thread thread’ question, to treat gravitation as a force or as curvature?

Garth

Last edited: May 21, 2005
5. May 21, 2005

### Zanket

The Earth expands too, but gravity and other binding forces reel the pieces back in. The same with galaxies; they are not oases where expansion does not apply. A thread would break on Earth the same as in deep space.

Last edited: May 21, 2005
6. May 21, 2005

### Zanket

The location at which the thread between the galaxies breaks is arbitrary. The Earth is subject to cosmic expansion to the same degree as is an Earth-sized region in deep space. Then the Earth bound thread can also break.

Cosmic expansion is independent of gravity. Gravity and other binding forces work to counteract expansion (by reeling back in the pieces). In the expanding space paradigm, all space expands. Even you are expanding, but the binding forces counteract it.

7. May 21, 2005

### Zanket

There is a paradox of the expanding space paradigm here: If gravity and other binding forces work to counteract expansion of you or the Earth or a galaxy, then how can the thread spanning the galaxies break and the resulting ends fly apart as must happen? As far as I know, there is no answer.

8. May 21, 2005

### Garth

You have misunderstood what I was saying.
If we define atomic (rest) masses to be constant, as indeed is required by the conservation of energy-momentum, which is also consistent with the equivalence prinicple. Consequentially an atom, and the Earth, does not expand with the universe. That expansion is then interpreted as a real expansion and Hubble red shift is recessional Doppler shift in nature. A thread between two distant galaxies would break, but one strung between two poles on Earth would not.

Furthermore cosmological expansion is a prediction of Einsteins GR field equation, which is a gravitational theory. The standard version of the theory (without Dark Energy) predicts that the expansion should be decelerating 'because of the gravitational attraction between matter within it'. (BTW the fact that its not decelerating may be a more radical discovery than simply that of DE).

I was agreeing with you in proposing that such expansion might affect everything within the universe, but that is not the normal understanding of the cosmological solution of GR.

In that normal understanding the expanding universe is not modelled by an blown up balloon with dots painted on it, as such dots themselves would expand too. Rather it is normally modelled by a balloon with coins glued onto it. The balloon expands but the coins, galaxies, do not.

As I have said I question this understanding.

Garth

Last edited: May 21, 2005
9. May 21, 2005

### Zanket

That is what books say. What I’m saying is that it’s inconsistent. Books say, to paraphrase one for example, “A galaxy is held together by its own gravity and is not free to expand with the universe. Similarly, the Solar System, Earth, an atom, or almost anything is held together by various forces in some sort of equilibrium and cannot partake in cosmic expansion.” This does not imply that these things are in expansion-free zones. The space in which things exist does expand. But the binding forces of the things reel the separating pieces back in. In your sentence the only differences between the two threads is the length and the presence of the Earth. Neither is relevant. The thread between the galaxies breaks at an arbitrary location, so one meter of thread is as good as a megaparsec. (I string it between galaxies only to make it obvious that the thread will break. Nothing about the length of the thread makes it break.) And nothing about the Earth prevents the thread from breaking, because the Earth itself is not prevented from breaking. The paradigm just implies that the pieces of the Earth will be reeled back in by gravity and other binding forces. Since the Earth expands only something like a millimeter per century, this reeling back in, which happens continuously, is imperceptible.

The expanding space paradigm (that space itself expands) arose circa 1930, after GR. GR's expansion is just a free-rise between pairs of objects, like throwing a ball up in the air.

A book of mine says, "Slowly [circa 1930] emerged the idea that the universe consists of expanding space! The lesson we must learn from general relativity is that space can be dynamic as well as curved." Seeing as how GR came about in 1915, expanding space is presumably not predicted by GR. I'd like to know more about why books attribute the exanding space paradigm, newly emerged in circa 1930, with a theory of 1915.

I say that the expansion does affect everything within the universe, and then binding forces reel the pieces back in so that, for example, the galaxies do not expand.

A more accurate model would have the coins continuously expanding and then immediately contracting. When I drop an apple from a meter above ground, its fall to the ground is delayed by cosmic expansion, but negligibly so. Gravity overwhelms the cosmic expansion. The higher the height I drop it from, the more significant the delay, until, at some height, cosmic expansion overwhelms gravity and the apple recedes from the Earth instead of falling toward it.

Can you elaborate? What is SCC?

Last edited: May 21, 2005
10. May 21, 2005

### Mike2

So here's an idea: Expansion and being reeled back in sounds like some sort of uncertainty principle. If objects cannot be located with absolute certainty, then maybe there some sort of minimum spacetime "energy".

11. May 21, 2005

### Zanket

Good thought, but the paradigm doesn't suggest that. The apple example above shows that the "reeling in" is just gravity overwhelming the expansion. Maybe a better example is when you throw an apple up. Cosmic expansion keeps the apple in the air a bit longer (that is, longer than expected when gravity alone is considered), but isn't significant enough to keep the apple from coming back down. Likewise, as cosmic expansion continuously expands every part of your body, even at the subatomic level, the binding forces continuously pull the parts of you back to where they were. You expand only at a rate of something less than a nanometer per century, so the binding forces have easy work to keep you in shape.

12. May 22, 2005

### pervect

Staff Emeritus
Calculate the actual force (acceleration) on a kilometer long thread due to the expansion of the universe

For standard Friedman-Walker cosmologies, the acceleration / unit distance turns out to be -q H^2, where H is hubble's constant, and q is the "deceleration parameter", another (rather hard to measure) constant.

For the details see

To put it in perspective, this number is

3.12 × 10-33 m / s-2 per kilometer (also worked out in this thread) with soem reasonable assumptions for q and H.

There's another thread where this got brought up, I pointed out that the tidal forces due to the gravity of the Earth (or the moon) are MUCH stronger than this *extremely* weak tidal force.

So if you aren't worried about the moon breaking threads here on earth (the moon pulls more strongly on threads closer to it than it does on threads further away, stretching them), you should be even less worried about the expansion of the universe breaking them - at least with the current values of the various constants involved. The force/ unit distance can evolve with time, in some sceneraios with a non-zero cosmological constant it can eventually become significant.

13. May 22, 2005

### chronon

Zero - the so called expansion of space has no physical effect. (Of course a non-zero cosmological constant is a different matter)
-q H^2 is negative, so the thread would contract. This is simply the effect of gravity due to the other matter in the universe.

See http://www.chronon.org/articles/stretchyspace.html

14. May 22, 2005

### Zanket

I read the other thread you gave, thanks. Regardless of the smallness of the force on the thread spanning the galaxies, it must break eventually. It cannot stretch forever. Right? And when it breaks, it breaks at an arbitrary spot and the new ends must fly apart. Right? And then there is a paradox, as to how gravity can keep a galaxy together against cosmic expansion when the much stronger forces besides gravity that holds the thread together are not enough to keep it together.

And the Earth is not an expansion-free zone, so a thread on Earth can spontaneously break due to cosmic expansion and its ends can fly apart.

15. May 22, 2005

### Zanket

How can that be? It does stretch things, right? Do you mean almost zero?

I read this, thanks. (I had also previously read Ned Wright’s stuff and other books on this.) It seems to me that the main reason you want to think in terms of stretching space is because things in stretching space break eventually, whereas things moving apart do not. That’s a big difference. The way I see it, the expanding space paradigm implies that the galaxies (and all other material things) are continuously stretching or breaking apart due to cosmic expansion, and continuously being kept together or put back together by gravity and other binding forces. That is, these two sets of forces are in equilibrium. And that leads to the paradox mentioned above.

16. May 22, 2005

### Zanket

Also, is it true to say that the expanding space paradigm (created circa 1930) was shoehorned in to GR (1915)? My understanding is that expanding space in GR was originally just a cosmos mostly filled with objects free-rising from each other. The inclusion of the expanding space paradigm to GR adds stretching space, so that objects that look to be free-rising from each other either are, due to peculiar velocity, or are stationary with space stretching between them. Can someone give the history on this?

17. May 22, 2005

### Mike2

How much would the plank length have stretched in 13.7Gyrs? Yet if particles were strings and the expansion of space does not make particles any larger so that the physics would have changed in that time, then doesn't this prove that particles are not extended objects but are singularities instead? Thanks.

18. May 22, 2005

### Zanket

If I understand your question, I think the paradigm would say that a smallest-length-object would not stretch. It would slip on the expanding space to always maintain its length. In other words, this object is infinitely rigid hence non-stretchable.

19. May 22, 2005

### Janus

Staff Emeritus
Does it? Assumimg that the string is of uniform strength along its entire length, then it will break at its midpoint. The molecular bonds of the string try to hold it together, thus the expansion force felt at the ends is transmitted down the thread by these bonds, as will be the force acting at any point of the string. Each point will be subject not only to its own force due to expansion, but also that transmitted to it by the points 'outward' from itself, which it in turn transmits down the line to the next point. The midpoint will feel the result of all the expansion force working along the entire length of the thread, and being the point where the greatest amount of force is felt, will be the point at which the thread would break.
Imagine a thread hanging from an anchor point. each point of the string not only has to support its own weight, but the weight of all the thread below it. the anchor point has to support the whole weight of the string, and if the string gets long enough, this is th point where it will break. In your example, the 'anchor point' is the middle of the thread.
But any thread won't break due to expanding space, only one long enough where the cumulative expansion force acting along the whole length exceeds the strength of the molecular bonds at the midpoint of the string.

20. May 22, 2005

### Mike2

I have trouble with strings because it is not clear whether strings are spacetime itself of lower dimension embedded in the background of higher dimension or if they are something else. If they are embeddings, then they would stretch with the background, right, with all the physics changing with it. But if the physics does not change (e.g the tension, etc), then strings are not embeddings but are themselves different than lower dimensions embedded in higher dimensions.

My personl view is that everything that exist must ultimately be describible in geometric terms, manifolds within manifolds. Otherwise we are dealing with something that cannot be explained as having an origin describible in terms of mathematics. Yet it seems intuitive that if everything arises smoothly from a singularity, it must be explainable in terms of smooth geometry. It would not be possible to impose some arbitrary function on this geometry from the outside. All would have to result from the growth of the singularity in some smooth predictable way.

21. May 22, 2005

### Hurkyl

Staff Emeritus
No. That space would globally expand or contract fell naturally out of the math. Einstein introduced the Cosmological Constant precisely so that he could have an adjustable parameter that would eliminate that effect (when set to the right thing).

And even that wasn't "shoehorned". IIRC, it's essentially a constant of integration.

Also, you seem to have a fairly fundamental misconception about the effects of tension. If you grab the ends of a string and tug on them without breaking its elastic limit, you can tug for all of eternity with a constant force, and the string will never break. It will stretch once so that its intermolecular forces balance your tension force, and then it will stay in equilibrium until the applied tension is changed.

22. May 22, 2005

### chronon

No, I mean zero. If you ignore any cosmological constant/dark energy (i.e. the situation up until the 1990's) then there is nothing driving the galaxies apart. They are moving apart because they started out that way. The only force between them is gravitational, which causes deceleration.
No the reason for the expanding space paradigm was because the natural time coordinate chosen by cosmologists is the proper time, which is not compatible with special relativity. Expanding space was invented so that objects wouldn't seem to be moving apart faster than light. (My impression was that this paradigm started when it became common to detect objects beyond the Hubble sphere, that is in the last 30 years).

What GR introduced was the idea of spacetime as something real, which could be curved. However, it can't be said to expand, as that is a time based notion.

23. May 22, 2005

### Zanket

Space expands everywhere in the paradigm, so the stretch force is equalized everywhere. There’s no excess anywhere to be transmitted. The thread is not being stretched from any particular direction. It is being stretched throughout from within. Tell me, where would an infinitely long thread break? It has no midpoint. If it breaks, it breaks at an arbitrary point. And if it doesn’t break, then you must explain how it can stretch forever. That seems to address the rest of your post.

24. May 22, 2005

### Zanket

25. May 22, 2005

### pervect

Staff Emeritus
I went through the math not that long ago in that thread with Hellfire - there is in theory a physical effect due to the expansion of the universe. However, the effect is so small that it is not experimentally detectable as you can see by the numbers that I quoted.

The reason there is a physical effect is that the components of the Riemann tensor are not zero. Consider for instance the "flat" FRW metric

ds^2 = a^2(t)*(dx^2+dy^2+dz^2) - dt^2

The first clue that this space-time isn't flat is in the Christoffel symbols. They are a bit numerous to list, but consider for instance one example:

$$\Gamma_{txx} = -a(t) \frac{\partial a}{\partial t} = - a \dot{a}$$

This is easy to compute directly because we are in a coordinate basis:
(see MTW pg 210, for instance)

$$\Gamma_{abc} = \frac{1}{2}(g_{ab},c+g_{ac},b - g_{bc},a)$$

The comma notation is convenient, a comma means an ordinary partial derivative, hence the meaning of $$g_{ab},c$$ is $$\frac{\partial g_{ab}}{\partial c}$$

So to work our our example

G_txx = .5*(g_tx,x + g_xt,x - g_xx,t)

But g_tx = g_xt = 0, so the only non-zero component is g_xx,t, which is $$2 a \dot{a}$$. Thus $$\Gamma_{txx} = -a \dot{a}$$ as stated.

The fact that the Christoffel symbols are not identically zero should be enough for you to realize that the (t,x,y,z) coordiante system is NOT an inertial one! This means that pseudo-forces exist.

The detailed computation of the Riemann confirms that space-time isn't flat and gives the magnitude of the stretching components of the Riemann tensor. The thread also explains why $$\Gamma^x{}_{txt}$$ represents the tidal force.

As this has previously been done on the other thread, and is rather long, I won't repeat it here. You also might find it convenient to look up the Riemann for the FRW metric in a textbook, MTW gives the Einstein tensor on pg 728 for starters (you can figure out the Riemann from the Einstien, but it takes more work).

Note that q is a negative number for the numbers quoted, as far as sign issues go.