Universe Expansion: Why Isn't Space Between Atoms Expanding?

[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 don't understand why this is so. If the universe is expanding, then why isn't 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]

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

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 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 don't understand why this is so. If the universe is expanding, then why isn't the space between every atom expanding, making everything else expand? Is it because of gravity?

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]

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.

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]

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?

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]

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

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]

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 don't understand why this is so. If the universe is expanding, then why isn't the space between every atom expanding, making everything else expand? Is it because of gravity?

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]

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.

Would the same principles apply in a contracting universe?

 

[Chalnoth]

Would the same principles apply in a contracting universe?

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]

Using the balloon analogy for an expanding universe, how then is it possible for galaxies to collide?

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)

 

[alexg]

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 >.<

Gravity overwhelms the force of expansion on scales up to 200 million light years. The local galactic supergroup is gravitationally bound, so there's no expansion between the galaxies in the group.

 

[Matt Todd]

Ah okay, It's making sense now.

 

[edpell]

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 >.<

If the galaxies were not moving they would not collide. The galaxies have some velocity relative to each other. If they are headed toward each other they collide. Also as you note they gravitationally attract each other.

Dark matter act just like normal matter it is gravitationally attractive. Dark energy is supposed to be net repulsive but you need to ask others for details.

 

[Wentu]

So many threads on this topic and i still have very confused ideas.
Using the ("useless") balloon metaphor, are physical objects better described by dots or ants on the surface of the balloon? Because if i think of dots, when I double the surface of the balloon, the dot's surface also doubles while the ants do not, they remain the same size.

I think it makes a lot of difference to say "material objects do not expand/stretch with the expansion of space" and "material objects expand/stretch but so little that it is AS IF they didn't". Which one of the two is correct?

Thankx

Wentu

 

[Mark M]

So many threads on this topic and i still have very confused ideas.
Using the ("useless") balloon metaphor, are physical objects better described by dots or ants on the surface of the balloon? Because if i think of dots, when I double the surface of the balloon, the dot's surface also doubles while the ants do not, they remain the same size.

I think it makes a lot of difference to say "material objects do not expand/stretch with the expansion of space" and "material objects expand/stretch but so little that it is AS IF they didn't". Which one of the two is correct?

Thankx

Wentu

Wentu, the ants analogy is a better way of thinking about it. The distance between points increases with expansion, not their size.

 

[phinds]

So many threads on this topic and i still have very confused ideas.
Using the ("useless") balloon metaphor, are physical objects better described by dots or ants on the surface of the balloon? Because if i think of dots, when I double the surface of the balloon, the dot's surface also doubles while the ants do not, they remain the same size.

I think it makes a lot of difference to say "material objects do not expand/stretch with the expansion of space" and "material objects expand/stretch but so little that it is AS IF they didn't". Which one of the two is correct?

Thankx

Wentu

As Mark said, using the way you have chosen to think about it, your ant analogy is correct and your dot analogy is not.

Systems that are bound by gravity or the weak force or the strong force are not affected AT ALL by dark energy. It DOES exist, as far as is known, at local scales but it has no effect.

It is like an ant pushing on a house. It's not that the ant has a tiny effect towards moving the house, it's that is has NO effect at all.

 

[Wentu]

I am sorry, so sorry to iterate for the milionth time the same questions but it would be so nice to grasp this concept of expansion in a more concrete way.
Marcus often says: one of the best aspect of the balloon metaphor is that the small coins on the balloon stay at the same latitude longitude coordinates and this means there is no real motion as we are used to think. Now... let me try to explain my excruciating pain in visualizing this:
If coins keep their size while the balloon inflates, then the coordinates of the center of the penny do not change but the coordinates of the rim of the coins do change and they become closer to those of the center of the coin. Is this an aspect of the metaphor that is misleading? Do galaxies shrink? From what I read, no.

Another issue. Please tell me if this picture i am trying to make of the whole situation is correct:
in a flat space the distance between the points (x,y,z) and (x',y',z') would be SQRT(A*(x-x')^2 + A*(y-y')^2 + A*(y-y')^2 ) with A = 1.
Is the expansion of the universe something like "A is increasing" ? So the two object are preserving their coordinates, they are in the same points, but nontheless their distance is increasing? Or am I completely off road?

Thank You very much

Wentu

 

[phinds]

then the coordinates of the center of the penny do not change but the coordinates of the rim of the coins do change

No, they do not. ALL of the points on the coin remain fixed relative to each other. The coin is an indivisible unit as far as the expansion is concerned and does NOT itself change shape in any way. That's why the suggestion is to think of solid coins on the balloon, not circles drawn on the balloon.

Systems which are bound by one of the standard forces (gravity, weak force, strong force, electromagnetic force) such as atoms, people, galaxies are completely unaffected by whatever it is that causes the expansion.

The dark energy that causes the acceleration of the expansion is so incredibly weak on small scales (galaxy size is small in this case) that it has no effect.

 

[JamesOrland]

Someone should correct me here if I'm wrong, but what you should think of when it's said that space is expanding is that the distance between any two given galaxies is, on average, expanding. Every galaxy is moving away from each other. Within a given galaxy, nothing changes at all. But the distance between, say, Andromeda and the Milky Way is getting larger.

 

[alexg]

Someone should correct me here if I'm wrong, but what you should think of when it's said that space is expanding is that the distance between any two given galaxies is, on average, expanding. Every galaxy is moving away from each other. Within a given galaxy, nothing changes at all. But the distance between, say, Andromeda and the Milky Way is getting larger.

Andromeda and the Milky Way are approaching each other, and will collide in about 4.5 billion years.


On any scale less than the local galactic supergroup (200 million lys) the effect of gravity overwhelms the force of expansion. It's only when you get outside the supergroup that you see expansion.

 

[Naty1]

"If the universe is expanding, then why isn't the space between every atom expanding.."

Everyday matter is held together by forces that are incomprehensibly large compared with any local cosmological expansion. Like you or me trying to pull apart a steel rod. And the gravitational forces holding our solar system and our galaxy are likewise much stronger.

In contrast space, or space time, naturally contains a bit of cosmological constant, negative gravitational pressure, so it naturally expands.

 

[JamesOrland]

Andromeda and the Milky Way are approaching each other, and will collide in about 4.5 billion years.


On any scale less than the local galactic supergroup (200 million lys) the effect of gravity overwhelms the force of expansion. It's only when you get outside the supergroup that you see expansion.

Right. My bad. So replace the relevant parts in what I said with 'superclusters' or whatever could be called the minimum particle subject to expansion :P