Expansion of space

Skeetss

I realize that because of gravity and the relatively high density of our place in the universe compared to the voids, cosmologists generally agree expansion isn't really significant, but I dont understand why this is so. If the universe is expanding, then why isnt the space between every atom expanding, making everything else expand? Is it because of gravity?

mathman

Atoms and matter in general are held together by the other three forces. Gravity and the expansion of space are so weak at the microscopic level that they can be safely ignored. In fact the expansion is only apparent between galaxies.

Skeetss

It is technically there though, correct? Just incredibly infinitesimal.

marcus

The point is moot, Skeets. The dimensions of an atom or crystal or planet orbit or galaxy or molecule stabilize. So there would be no change in the distance between the ends of a steel measuring rod.

Even if you could measure such an "incredibly infinitesimal" change in such a small distance.

Hubble law distance expansion involves what is called "proper" distances between observers which are stationary relative to the universe's ancient light. No doppler hotspot in their CMB sky. At rest relative to the background.

If you had a very very long steel rod and you made sure that one end was at CMB rest (which even in principle cannot be done with better than 0.001% accuracy but suppose) then the other end might be found to be slightly not at CMB rest. Because it was staying the same distance to its buddy.

But the current rate of expansion is only 1/140 of one percent per million years. So even with a very very long fixed length measuring rod the discrepancy is probably "incredibly infinitesimal".

Chalnoth

I think perhaps the best way to think of this as the expansion of space not being a cause of the matter expanding, but rather the other way around: expanding matter manifests itself as an expansion of space.*

So one way of looking at it is we start off with a cloud of gas that is expanding and cooling. This cloud of gas isn't uniform: some bits of it are denser than other bits. The bits that are dense enough manage to stop their local collapse and form bound objects like galaxy clusters and galaxies. The fact that these bound objects originally came from an expanding cloud of gas is completely incidental: they've collapsed now, and the matter within the object exists in more or less stable orbits around its center.

* The caveat here is that there is no one, unique, correct way of thinking about this. There are many correct ways of thinking about the expansion of space and matter's effect on the expansion. Ultimately you have to go to the math to see what the correct solution is, and then you can sort of go back and figure out the right way to think about the situation so that the correct solution seems intuitive. The correct solution here is obtained by taking a nearly-uniform expanding universe that has bits that are a little bit more dense than other bits, and see how that universe evolves in time. It does exactly what I said above: the parts that are dense enough collapse in on themselves and form stable objects. There just isn't any expansion at all within such stable objects.

Chronos

Expansion is simply not a factor at short distances. Many light years are required to gain one lousy meter of expansion. Local forces in the universe simply overwhelm the effect of expansion. At the atomic scale, it is immeasurably small. It's like firing a .22 at the moon, you are not going to knock it [or earth] out of orbit that way.

Matt Todd

The evolving universe you describe sounds very much in physical nature like a sponge expanding after compression. Initial 3D imaging of the universe appears sponge like, fillaments of galaxies with dark matter interspersed.
Trying to estimate the overall mass of the universe is guess work. Will the universe expand indefinitely, will it expand and become static, or will it expand and then contract?

Chalnoth

No, it's really not guess work at all. The overall mass is known today to within a couple percent. The primary uncertainty with regard to recollapse is the future behavior of dark energy, which is extremely hard to know. However, the most likely situation is one where dark energy is simply a cosmological constant, which means expanding forever.

Matt Todd

Very interesting. If dark energy is the unseen force driving everything apart, could dark matter work in an opposite way, like a cosmic glue?

phinds

Dark matter DOES work somewhat like "a cosmic glue" --- it holds galaxies together more than they would otherwise be held and it helps in their formation.

Matt Todd

I really need to stop painting houses and start studying :P

phinds

An analogy I like is that of an ant pushing on a tank. It is NOT going to have an infinitesimal effect, it is going to have zero effect because it cannot overcome the other forces in play to ANY degree. Even a man pushing on a tank would likely have zero effect unless the treads were incredibly well greased and the thing was pointed downhill.

edpell

On the steel rod getting bigger. It will not because it is self correcting. The lattice constant remains the same. If the atoms move apart due to Hubble expansion they will move back together due to electrostatic forces of the lattice.

Matt Todd

Would the same principles apply in a contracting universe?

Chalnoth

Until other matter runs into it, yes.

Matt Todd

Using the balloon analogy for an expanding universe, how then is it possible for galaxies to collide? Is the gravitational pull of a galaxy enough to influence another? Do dark matter and dark energy have an effect on expansion? Sorry, so many questions >.<

phinds

The balloon analogy is not useful for that question. When galaxies are close enough together, they attract each other gravitationally enough that they will eventually collide, as will the Milky Way with another galaxy in the Local Cluster some 4 or 5 billion years from now. The Local Cluster is all gravitationally bound and is not expanding (it's like one dot on the balloon)