# Does matter follow the expansion of space?

Non-physics person here. I am trying to understand the expansion of space. I've read about the redshift of light and the balloon model (2 points on expanding balloon move away from each other).

The detection of the expanding universe depends on light. But what about matter? Does matter follow the expansion of space? Or does it "slide" so that space expands and whizzes by the matter, but the matter remains in the same distance and configuration relative to other matter?

I'm thinking of a very smooth disc of ice and hockey pucks on top. Suppose the disc somehow expands. Due to it being very smooth, the expansion doesn't move the pucks. They remain in the same relative positions but the disc underneath it expands and points on the ice whiz by the pucks.

Can someone explain how matter fits into space expansion and its detection?

The FRW metric which describes the expansion of spacetime only holds in areas with homogeneous mater density. Where there is a galaxy or something of the like, the spacetime expansion does not work the same way because matter is not distributed evenly. The spacetime dynamics around a galaxy are described by other equations that don't necessarily have expansion terms. For instance, look at the schwartzchild metric, there is no $a(t)$ in that metric, and it is invariant under time translations.

Here is a nice article written for laymen that expands more on the topic. But the very rough idea is that the space between galaxies expands but the space inside them does not.

edit: also I guess this is a duplicate of this, but the answers to that question require a bit more physics background to understand.

• I think you may have misunderstood what the OP was asking. I think they were effectively asking whether noninteracting particles participate in cosmological expansion, not whether gravitationally bound systems expand. – user4552 Apr 27 '18 at 17:11
• Hmm... that might be the case. Since OP admitted to being a layman, I assumed they were asking about how expansion effects atoms/molecules/planets/etc... (which is a typical early source of confusion). In any case, I think the statement about the space between galaxies, and your answer as well, essentially make clear the idea that, at a large enough scale, the space between matter is inflating because the matter distribution can be seen to be roughly homogeneous at that scale. Thanks for the input! – Bobak Hashemi Apr 27 '18 at 20:10
• The [nice article] that Bobak provided explains: "We cannot observe space and neither can we observe space-time. We merely observe how space-time affects matter and radiation, which we can measure in our detectors." So there are precisely 2 factors. In part 1 of the diagram in that article, I read that "Matter dilutes as the Universe expands". That is the thing I was wondering about. Part 2 of the diagram explains that "Radiation dilutes and redshifts as the Universe expands". Was wondering if it's possible that radiation can dilute but matter not dilute hence the "ice disc" model question. – J. Doe Apr 28 '18 at 18:04

Yes, matter does follow the expansion of space, at least on the average. That is, we can model the universe to be made out of particles of dust (relativists actually do call it "dust"), where each dust particle is actually something like a galaxy or maybe a cluster of galaxies. The galaxies do get farther apart as the universe expands.

The way this enters into the cosmological models is really that we assume a high degree of symmetry. We assume that the universe is homogeneous and isotropic, which seems to be true observationally. If you try to cook up a model in which there is expansion, and the expansion is observable, but the matter is not getting dispersed, you end up violating homogeneity or isotropy.

To start with, there was the original Big bang model ,which postulated an expansion of the universe in the sense of an explosion. As in an explosion the fragments follow their trajectories, it was posited that the particles, after being created, followed their trajectories, and then the forces between particles, much stronger than the impulse of separation, took over, creating clusters of matter which evolved into the observable universe.

The four forces of nature are strong enough to attract and keep matter together, even when an accelerating expansion of the universe was detected.

The currentBig Bang.model with accelerated expansion,seen in the changing opening angle.

This accelerated expansion of the universe also does not expand bound states, as atoms, molecules , cups and tables, planetary systems, galaxies or even cluster of galaxies,bound systems.

The expansion of space does not take place on bound systems , because ,as a force, accelerated expansion is very very very weak. Even weaker than gravity.

After all, we discovered expansion and accelerated expansion because clusters of galaxies and galaxies were moving away from each other . If all matter were expanding uniformly, we would never know about an expansion.