# If Universe is Expanding at an Accelerated Rate Why

1. Oct 4, 2011

### ronholton

Why are objects still the same distance apart? The sun is still roughly 92,900,000 away and the last time I checked Proxima Centauri was still about 4.2 years from us. I know this is a dumb question, but I am just a layman, although a fascinated layman.

2. Oct 4, 2011

### Lost in Space

As I understand it, the expansion of space can only be detected from very large intergalactic distances so local expansion would be hard to detect. Expansion is measured by the amount of redshifted light received from galaxies etc which are very distant. And as this expansion is only apparent at great distances in terms of the visible light from all objects at those distances being significantly redshifted, Proxima Centauri, or for that matter the Sun, are inside the galaxy and very close to us, so the expansion would be infinitesimally small.

3. Oct 4, 2011

### DaveC426913

Not "hard to detect" - it's non-existent. Gravitationally-bound objects such as the solar system are not expanding at all.

The expansion of space applies an extremely weak effect upon objects. Gravity in all but the most distant of objects can easily overcome it. Only galactic clusters have weak enough gravity between them such that the expansion of the universe will act to spread them.

The venerable balloon analogy illustrates:

Glue a bunch of pennies to a balloon. Blow up the balloon. The pennies will spread apart.

Will the pennies themselves expand? Of course not.

The force (glue) trying to pull the pennies apart is easily overwhelmed by the cohesion of the penny's structure (gravity in a galaxy or solar system).

Last edited: Oct 4, 2011
4. Oct 4, 2011

### Lost in Space

Thanks Dave. So does this mean that the expansion of space is not uniform? Won't the mass in the universe affect how space expands even if there's not enough of it to halt the expansion? For example, will the intergalactic voids be like bubbles pushing the galaxies on their edges further apart, and will bigger voids expand faster than smaller ones?
Our local group of galaxies is being pulled towards the Virgo supercluster so am I right in assuming that what you say about the weakness of gravity being such that the expansion of space overcomes the gravitational effect of individual and clusters of galaxies must be on a far larger scale than those kind of distances?
If dark energy is responsible for the increasing rate of the expansion of space is it in turn increasing? I understand that the law of conservation of energy isn't violated with regard to this as the theory is that the vacuum is made of virtual particles that constantly appear and disappear. What puzzles me however, is that although these particles are virtual, surely the total energy in the system is increased as the volume of the vacuum increases during expansion, even if most of this energy is dark?

5. Oct 4, 2011

### DaveC426913

Galaxy superclusters are indeed forming into filament-like structures on cosmological scales, like when you pull taffy apart.

[PLAIN]http://www.acru.ukzn.ac.za/images/sims.jpg [Broken]

Well there's still intrinsic velocities - just like Andromeda is currently moving toward us.

Last edited by a moderator: May 5, 2017
6. Oct 5, 2011

### ronholton

Thank you. I understood some of this. The pennies on the balloon analogy helped a lot.

7. Oct 11, 2011

### andiamaj1431

i see where you are coming from with this question, but gravity will hold us and our neighboring stars in the same place that they are, yes we will constantly move but not away from each other, our galaxy is bound by gravity, so as distant galxies or clusters will be moving away from us we will stay where we are in our own galaxy

8. Oct 11, 2011

### andiamaj1431

so basically, anything headed toward us will keep coming toward us right? so does that mean that its the fabric of space itself moving away from us? this universal expansion thing has me kinda confused on some things

Last edited by a moderator: May 5, 2017
9. Oct 11, 2011

### phinds

The fabric of space is not moving away from us, space is expanding everywhere except inside of gravitationally bound systems such as our galaxy. This causes a mutual moving apart between us and everything that is not gravitationally bound to us, so you COULD look at it as the expansion causing things to move away from us but I don't see that as being the same as saying the fabric of space is moving away from us.

10. Oct 19, 2011

### andiamaj1431

thank you, this information has helped me to understand more clearly

11. Oct 19, 2011

### phinds

Good.

I should also add, lest I create confusion, that no matter WHERE you are in the universe, everything would be moving away from you that was not locally gravitationally bound. So while expanding space does make everything move away from us, it's more correct to say that it makes everything move away from everthing else (always excepting gravitationally bound systems).

12. Oct 30, 2011

### bigmig

I need some clarification on this. I understood that the 'fabric' of space is expanding - like the 2D baloon suface etc, but in bound systems like galaxies or planets (or us!) the attractive forces easily overcome expansion. The accelration due to expansion is a very small fraction of the attractive force of gravity never mind nuclear forces.
Not to mix analogies but would the effect be something like a flowing river where anything not anchored will be swept along but things anchored (by such as gravity) will remain still, relative to the water flow? So in bound systems the 'space' is expanding like water flowing past anchored objects.
Otherwise it would be that gravity or other attractive forces actually prevent local space from expanding - which seems nonsensical (although we get used to things not making everyday sense in cosmolgy/quantum mechannics.)
I've read some good SA and other articles which touch on this but don't quite get to the points/questions I have raised here.

13. Oct 30, 2011

### phinds

I don't have an answer, but I must say, it seems to me like a very good question. On the other had, I think the analogy is weak (although I completely understand the logic of it) because the river is flowing and I can't see the expansion of space as a "flow" because it happens everywhere at once ... except perhaps inside of gravitationally bound systems, which of course is the heart of your question.

I look forward to some of the more knowledgeable members giving us a better understanding of this.

Thanks for the question.

Last edited: Oct 30, 2011
14. Oct 30, 2011

### bigmig

Yes, perhaps I should have used a more 3D analogy like the rising of bread dough, where here you could imagine a cubic lattice (L1) of side say 2cm embedded within the dough ball. As the dough expands through and around, the lattice would remain at 2cm due to the connecting rods. And if you placed another lattice cube; L2, initially at say 3cm from L1 then the distance between L1 and L2 would increase by the rate of expansion of the dough, but both cubes would remain the same size.

15. Oct 30, 2011

### phinds

Right ... this is the same analogy I've heard before. Same with the pennies in the baloon analogy ... they get farther apart but not bigger.

I accept that gravitationally bound systems don't get bigger due to expansion but the question is WHY do they not get bigger? Is it that the gravitational field suppresses the creation of new space in the region? Seems weird (not impossible, of course, but weird). Or is it that new space IS created but it "flows" outward (yes, now I'm using the term I told bigmig didn't seem right) without taking any of the gravitationally bound object with it?

Last edited: Oct 30, 2011
16. Oct 30, 2011

### DaveC426913

I don't know why the pennies on a balloon analogy doesn't make this perfectly intuitive. You have no trouble conceptualizing why the balloon's expansion does not rip the pennies to pieces.

17. Oct 30, 2011

### phinds

I thing my problem is that I can envisage the pennies haveing fixed locations relative to a spot on the expanding balloon but what I can't seem to get anyone to tell me is, does the suface of the ballon remain unchanged under the penny as the rest of the ballon expands or does that part of the skin expand as well?

18. Oct 30, 2011

### marcus

Phinds,
we are using words to translate a math model and my words may not be the right ones for you. If they don't work, sorry. there can be several different translations of something in a foreign (math) language. Try several and choose the one best for you.

My take is this: there is no rubber. Geometry is not a material.

The balloon model is ONLY to help you visualize a pattern of expanding distances.
It is not meant to make you think there is a material called space analogous to rubber. In fact the basic "covariance" principle of GR implies that points of space cannot have objective physical reality.

GR is only about geometry. It describes how geometry evolves with time and in interaction with matter. We have no right to presume that geometry has to be the usual static Euclid setup. In fact GR explains why in our neighborhood it IS NEARLY like that. GR provides a reason why it is almost the usual static schoolbook Euclid setup.
It is both our law of gravity and our law of geometry.

So there is no rubber "under the pennies". There is no rubber. It is all a web of geometric relations. That web can change. It is just information. Not material.

The very simplest version is what is called the Friedmann model which is a simplified universe with uniformly distributed matter. It is a good approximation. In that model the distances that grow are ones between observers who are at rest relative to CMB.

Now I'll take a chance and say something a bit risky. IF the CMB rest criterion were a bit more uniform and precise and stable than it really is we might find that the galaxies in a cluster of galaxies were just a little farther out from the center of mass than their orbital speed would suggest. As if the cluster had been stretched out. As if there really was "rubber under the cluster".

We might even find that expansion of distance in our own galaxy has an effect. Someone will correct me if I'm wrong. We might find (if we could measure that accurately) that the rim stars are just a tiny tiny bit farther from center, given their speed, than they Newtonly ought to be. Their distance has stabilized at a slightly stretched out value.
An observer on a rim star would be able to detect that even after compensating for circular orbital motion he was still not at CMB rest, but was falling in towards Center just fast enough to compensate for the increase of distance from center.

But the expansion effect is only 1/140 of one percent per million years! So this infall speed would be undetectably small. And it would not change his actual distance from Center. It would only be a speed measured relative to CMB.

However in reality the CMB has that random fluctuation element you see in maps of it, so no such very precise measurement could be made. So it is a what-if kind of thing, to take with a grain of salt if at all. In that very limited sense one might want to imagine that there is rubber under the pennies, since you asked

Last edited: Oct 30, 2011
19. Oct 30, 2011

### phinds

Marcus, thank you. That does make sense. I was particularly helped by:

I take it then that the geometry of gravitationally bound systems is not affected by whatever it is ("dark energy") that causes the expansion between objects that are not gravitationally bound.

EDIT: by the way, I'd like to thank you again for all the time you spend helping folks like me to understand this stuff to the extent that we CAN understand it. It's got to be a sometimes frustrating chore.

20. Oct 30, 2011

### marcus

I appreciate that new people take a serious interest in asking questions getting straight on stuff so they feel like they understand and then helping newer newcomers. For me the PF Cosmo forum is basically a pleasure. I only sweat something if I want the work out.
We are very lucky now, at this period, we have a bunch of experienced people all of whom have knowledge and like to explain stuff clearly (brian powell, ben crowell, chalnoth, redbelly, etc etc I forget who all) and we also have helpful newer people.

I don't know what the experience would be like if we didn't have these good conditions. But why think about it? Anyway it is not a frustrating chore for me at least knock on wood. You just do what you feel moved to do and it seems to work out.