Expansion of the Universe

Paul Martin

Galaxies in our expanding universe have been likened to dots painted on an expanding balloon and to raisins in an expanding lump of dough. Brian Greene likened them to pennies glued onto an expanding balloon to make the point that the galaxies themselves don't necessarily expand along with the universal expansion. Whether or not the galaxies themselves expand is less interesting to me than the question of whether all matter participates in the expansion or not: Do the diameters of atoms, for example, expand at the universal expansion rate?

Based on information given in Brian Greene's "The Fabric of the Cosmos", I have calculated that if atoms, and thus all objects made of atoms, increased in size right along with universal expansion, they would expand at the rate of .0000052 inches per year per mile of diameter! That would be easy to overlook. (From pages 46 and 229, stretching speed per mile of separation = 5.5 million mph / 100 million light years = 9.27X10-15 mph/mile).

It seems to me that if my calculations are right, that rate is so slow that it would have been unnoticed in our experiments. My questions are

1. Are my starting assumptions correct?
2. Are my calculations correct?
3. Has anyone actually tried to measure atomic expansion?
4. What were the results?

Of course, if all matter expands at the same rate as the universe, then so too do all length-measuring instruments and all standards of length, which would beg the question of what we mean by 'expansion' in the first place.

Paul

EL

Actually, this was somewhat discussed here quite recently.

Jimmy Snyder

"Beg the question" does not mean the same thing as "raise the question".
http://www.worldwidewords.org/qa/qa-beg1.htm

DaveC426913

So, despite the fact that Greene points out that galaxies do not expand, you are concluding that smaller things - such as the things galaxies are made of - do?


Here's the thing: the expansion of space is not some weird metaphyscial phenomenon that ignores normal physics. It is simply a force, and a very weak force at that - weaker than gravity. It is overwhelmed by all the more powerful forces, and even by the smaller forces (such as gravity). The reason galaxies are not subject to it is because gravity easily overwhelms this tiny force.

You are pulled upward by the gravitational force of the Moon overhead, but surely you don't think that, if given enough time, you would eventually float up to the Moon do you? No, the Moon's pull on you is overwhelmed by the Earth's pull on you.



The force causing the expansion of the universe is so weak that the only place it can manifest is in in the gaps between galaxies, where even gravity is too weak. So, galaxies move apart from each other, but are not pulled apart.

Paul Martin

Thank you. I learn something every day.

Paul Martin

I did not come to that conclusion. I was simply raising the question. I suppose I should accept whatever Greene points out as a fact, but I can't help wondering about some things.

It sounds to me like the question is still unresolved.

EL

Try the link in my first post.

Paul Martin

I did. That's where I got the impression that the question is still being disputed.

Anyway, you guys didn't answer any of my questions numbered 1-4.

matt.o

question 1; no, they are not correct. The atoms don't expand at the rate at which the Universe expands. Why bother with the rest?

Jimmy Snyder

Here is a another thread on a similar topic:

Local effects of Hubble flow.

http://www.physicsforums.com/showthread.php?t=99140

Chronos

DaveC gave the best explanation, IMO. 'Dark energy' [the force driving expansion of the universe] is so pathetically weak that even gravity [which is incredibly weak compared to the other fundamental forces of nature] looks like a 500 pound gorilla by comparison. The dark energy effect [expansion] is only noticable at cosmological distances.

EL

I find this use of "Dark energy" somewhat strange.
Dark energy is not needed "to drive the expansion", at least if we are talking about the "ordinary" kind of dark energy. Even if there was no dark energy in the universe it would still expand (but not accelerate).

In fact I don't like calling the expansion of the universe "a force" at all.

Garth

There are two questions that recur on GA&C or S&GR:

1. "Do atoms expand with the universe?" and

2. "May it be that the universe is static and atoms are shrinking within it?"

These are not trivial questions, in fact they are probing the field of conformal gravity theory. If atoms, and hence steel metre rulers, expand with the universe then there would be no detectable expansion. This question itself then raises the further question: "How are measurements across the deep reaches of astronomical and cosmological space and time to be made?" It also raises the further question: "What actually is it that is being measured as cosmological red shift?"

A measurement is a comparison of the observations of a set of events with some defined standard units in Mass, Length and Time, so what standards are to be used and how are they to be transported to the far reaches of the cosmos? How do you know that the standard units themselves are not going to change for example?

You need a conservation principle, something that is not going to change, in order for a consistent comparison to be made.

In the standard model this principle is the Principle of the Conservation of 4stress-energy-momentum subsumed by the Principle of Equivalence. Atomic rest masses, and therefore their sizes, are constant by definition by this principle. Cosmological red shift is thereby interpreted as a Doppler recession effect and the universe expands around a fixed ruler.

In conformal gravity theories, in which the metric is a conformal transformation of the GR Robertson-Walker metric, something changes over time and space. It could be particle masses, the gravitational constant, the speed of light, the fine structure constant, Planck's constant or a combination of these. As a matter of pragmatism some of them have to be held constant and one or more of the others allowed to vary. In such theories the universe might well be static with shrinking rulers, rather than an expanding universe with fixed rulers. (As an after note I of course have to point out that Self Creation Cosmology has two conformal frames, in which these two possibilities are respectively realised.)

I hope this helps.

Garth

pervect

To paraphrase what Garth says a bit, to measure distances at all you need a standard of measurement, i.e. a ruler.

By definition, a ruler, being the standard way that distances are measured, will not change its length. The sole purpose of a ruler is to define the notion of distance. Without a definition of distance, it cannot be measured or even talked about.

Because rulers _define_ distance, they do not change due to the expansion of the universe (or anything else).

Sometimes people use different standards for measurement for conveinence in formulating a theory, this can cause a lot of confusion. To actually compare theories, it is important to use a common standard of measurement. Otherwise one is comparing apples to oranges.

The current standard for distances and times is the SI standard, based on the cesium atom as a clock, and the speed of light as a conversion factor from time to distance.

For intelligibility, it is probably a good idea to use the standard notions of time and distances when comparing theories. If the theory meets scientific standards of testability, a conversion from theory-convenient measures into standard measures should be possible. If the theory is not well enough formulated to allow a clear conversion from theory-convenient measures into standard measures, it's probably not a testable theory.

Even with the best of intentions, sometimes sublte points still remain, such as the notion of simultaneity. To measure cosmological distances, one needs to define the time at which the distance is measured, and the set of points (the path) along which the distance is measured. This path or set of points is assumed to all be at 'the same time'. Different notions of simultaneity are possible, and can give different distance measures.

phcatlantis

We could simply note that expansion is a phenomona observed only to the precision of the large scale structure of the cosmos and is characterized as a consequence of a model at that scale. An FLRW model makes no claim to accuracy when we choose to do physics on scales where the assumption of homogeneity and isotropy breaks down. It's tempting to resort to rubber sheets and balloons, and I fear Dave's explanation feeds more temptation to think of some fifth fundamental force when there is no observation to support that hypothesis as of yet.

Paul Martin

Yes. That does help. Thank you very much. I had almost given up on getting anything helpful.

Garth

Paul, I'm glad to have helped.

The most significant, for me, high school physics experiment that I carried out was a simple one: the coefficient of expansion of different liquids. We had a conical beaker with a bung with two holes. We inserted into one a thermometer and into the other a glass tube.

Different liquids; water, alcohol and liquid parafin, were heated over a bunsen burner and then the teacher repeated the experiment with mercury.

Graphs were plotted of the temperature against the distance the liquid expanded up the tube. A simple experiment you might think, so why did I find it so significant?

The graphs were all nearly straight lines, except that for mercury, which was exactly straight. Why was mercury alone straight?

We guessed at many different possible reasons: because it was a metal? a good conductor of heat? or possibly electricity? because it was so much denser than the other liquids? because its freezing point was much lower than the others? because the teacher had performed the experiment more carefully than us students?

It took a little while for us to realize the real reason; we were using mercury thermometers!

You have to ask the question when defining a standard and method of measurement: "How is the measurement to be made, what standard are you comparing it with and how might that itself change?"
Ideal rulers "do not change due to the expansion of the universe (or anything else)", real rulers actually do change, for example a steel ruler will expand with temperature.

As I said, in order to measure the universe you have to find a conservation principle: something that you believe does not change, in order to have a standard against which the comparison may be made.

In GR that conservation principle is the Conservation of Energy-Momentum:

[tex]T^{\mu}_{\nu;\mu} = 0[/tex],

which leads to constant atomic (rest) masses, therefore constant atomic sizes and therfore a constant proper length of a steel ruler.

If this does not hold then the way we measure the universe would significantly change.

Note that in the GR field equation:

[tex]R^{\mu\nu} - \frac{1}{2}g^{\mu\nu}R = 8 \pi GT^{\mu\nu}[/tex]

the left hand side is identically divergent free by the Bianchi identities, however, on the right hand side, whether the Equivalence Principle ([itex]T^{\mu}_{\nu;\mu} = 0[/itex].) holds, or not, depends on whether G is constant or not.

So, is G absolutely constant? The whole field equation depends on it! Yet this is a question of experimental verification and not just a matter of definition!

Garth