Please help me understand the expansion of space

Fraggle:

Actually, virtually all of the observed expansion is expansion of space. The remaining motions of galaxies are called "proper motions", and they can be in any direction. There's no particular preference for galaxies moving towards or away from each other when it comes to proper motions, although the gravitational attraction of one galaxy by another obviously plays a major role in local dynamics.

I'm not sure where you got this from. It is true, however, that there hasn't been enough time for galaxies to get as far apart as they now are since the big bang, even if they moved through space at the speed of light.

If it helps, you can think of space as being like a substance that the galaxies are embedded in, like raisins embedded in a loaf of bread. (This is only an analogy, though. Space isn't really a substance.) The expansion of space is like the baking of the bread. The raisins are carried along with the general expansion - they do not move through the bread under their own steam. (Note: this is not quite true of galaxies, which do have small proper motions, as discussed above.)

Also, I should say that not all scientists are poor communicators. There are many excellent communicators of science out there. When physicists talk about "expansion of space" they are using an accurate terminology which describes exactly what the physical theories say about what is happening. The problem in explaining the concept to the general public is that scientists are not "allowed" to refer back to the mathematics that really describes how space expands and in what sense. In the general relativistic equations that describe the expansion of space, there is indeed a single "scale factor" which determines in a particular way how big the universe is and the rate at which space is expanding. But that scale factor is a variable in a set of complicated tensor equations that operate in a four-dimensional spacetime. There are a lot of explanatory steps necessary to begin to get a vague idea of what is being discussed, and even if a good communicator does manage to convey a mental image to an untrained layman, they still won't have really connected the layman to the actual science. Hopefully, the idea will make some kind of sense, and that's the best we can hope for, unless you want to do an advanced physics degree.

This kind of explanatory problem is not confined to physics. It crops up in every specialised scientific field. If a medical researcher is publicising her latest work on retroviruses, she'll often have a hard time giving the general public a picture of what she has actually done. If she is a good communicator, she may be able to build up an analogy or mental picture of how these viruses work and so on, but there will inevitably remain layers of complexity that go unexplained in the press release.

ALL of the space expands. There are no exceptions. The space between galaxies expands, and so does the space between planets in the solar system, and so does the space between atoms in your leg.

The difference between galaxies moving away from each other and atoms in your leg moving away from each other lies in the fact that atoms in your leg are initially much closer to one another than two atoms in neighbouring galaxies. Therefore, their local gravitational attractions are much stronger. Moreover, and just as importantly, there are other significant forces acting to attact the atoms in your leg to each other - primarily electromagnetic forces. Since galaxies as a whole are electrically neutral, the only force that significantly affects the dynamics of galaxies is gravity. The same is utterly untrue for atoms in your body, for example, where electromagnetism is far and away the most significant force (ignoring the strong nuclear force that holds the individual atomic nuclei together).

So, short answer for why we don't see expansion on "small" scales: local forces, gravitational or otherwise, overwhelm the "small" space-expansion trend.

The size of an individual photon is not well defined. For many purposes you can consider it to be a point particle.

When considering the expansion of space, it can be better to think of the wave model of light. Light waves then exist as long trains of waves spread "embedded" in a region of space. As the space expands the waves stretch out. This is the cause of the red shift due to expansion of space.

It does stretch those little photons, in a sense, but only a little over a short time. Photons have to be travelling a long time (which means also a long distance) in order for the stretching of their wavelengths to become noticeable.

The stars within galaxies have relatively strong local gravitational attractions from neighbouring stars and from the central black holes in the galaxies.

 

Captain Kremmen:

There was a big initial "push" at the time of the big bang, followed by a period of very rapid expansion or "inflation". The rate of expansion thereafter decreased significantly, but has been gradually accelerating again in the post-inflationary period.

Initially, it wasn't. The current big bang model has been "patched" in order to solve a number of apparent problems with the simple model first proposed.

I'm not an expert on this, so I don't really understand what drove inflation. I believe that under the conditions of the early universe gravity was actually repulsive for a short time. But I could be wrong about that.

Yes. That's the situation we're currently in, at least as far as clusters of galaxies are concerned. The clusters themselves manage to hold together more or less against the current rate of expansion, but if things continue as they are then that won't always be true.

Yes. It's like repulsive gravity, in a sense.

 

The Wikipedia article on the Universe, for example. It says the radius is nearly 50KLY. That means that the most distant galaxies have been--er um what's a good neutral verb here--"traveling" at more than three times the speed of light.

I assume, then, that this required a major revision in the theory of Relativity? All those new tensors you're talking about?

Yeah, not a very good analogy at all. The bits of starch and protein that comprise the dough are still moving in the good old-fashioned Newtonian sense. A "substance" without mass can travel at the speed of light, even under the old rules. But nothing can travel faster than that. Is empty space, therefore, exempt from the rules that apply to "substances"? Even though it's not really empty but is jam-packed with electromagnetic waves that get mutilated in the process???

It took me six months to read and understand a book that explained relativity to 15-year-old students. I couldn't possibly remember all the steps and explain it to someone else, but everything made sense and I was satisfied at the end. I suppose I'd be willing to put in that much effort to understand expanding space. Although I'm not as smart as I was in 1959, at least not with that kind of thinking.

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The matter that comprised the early universe was expanding of its own accord, from sheer thermodynamics. (The temperature value had some huge number of zeroes, right?) It wasn't until the expansion had been going on for quite a while (and there's a Wikipedia article somewhere that charts the diameter of the universe over time) that the matter solidified into discrete chunks: stars, planets, etc. At this point some of those chunks were far enough apart that the strength of the Four Elementary Forces on them was very low (indeed gravity is now the only one that matters, and it attenuates quadratically), allowing the--er um now I need a noun Alex--phenomenon in question was able to start pulling them away from each other.

(Have I got this right?

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Fraggle:

Yes and no. The required mathematical apparatus to described an accelerating expansion of the universe was present in the theory of relativity when Einstein first formulated it. At the time, Einstein thought that he wouldn't need his "cosmological constant" term in the equations of general relativity, and famously called it his "biggest blunder". Now it turns out that we do need it after all.

On the other hand, inflation was not built into the initial general relativistic cosmological models. It had to be added later on - that part of the theory is only a couple of decades old, from memory.

Space itself is not a substance, which is why I said the analogy breaks down. The expansion of space itself is not bound by any speed-of-light speed limit.