# Universe Expansion Question

1. Feb 28, 2009

### hagendaz

If the universe is expanding per the inflating balloon example, how come the planets within our own (and every other) solar system aren't spreading further appart.

2. Feb 28, 2009

### mathman

In a word - gravity.

3. Feb 28, 2009

### marcus

Nor is our galaxy spreading further apart. A system of several hundred billion stars orbiting the center of mass bulge.

Things in orbit may have already adjusted for any slight percentage increase in distances. In any case gravitationly bound systems are not pulled apart.

Think of things orbiting imperceptibly farther from center than their orbit speed would ordinarily allow. So that other things being equal they'd have a tendency to fall inwards to orbit closer to center. And it just compensates. Maybe it's an oversimplification but it helps me.

The percentage expansion rate is only 1/140 of one percent every million years.

The general rule of thumb is that distances between gravitationally bound objects don't increase. The only distances that increase, by Hubble law, are between widely separated pairs of objects which have no significant gravitational effect on each other and are not bound.

So even clusters of galaxies just go about business collectively orbiting each other and they are not spread apart.

I understand there are borderline cases where what looked like a wannabe cluster got pulled apart, it wasn't really gravitationally bound to begin with.

4. Mar 1, 2009

### ThomasT

By "pulled apart" do you mean that there are 'groupings' of individual galaxies that because of the proximity of the individual galaxies they would be expected to behave as a bound system, but are observed to be moving away from each other? If so, is this behavior primarily due to the velocities of the galaxies involved -- or is there more to it?

5. Mar 1, 2009

### hagendaz

If two rifles in deep space were fired exactly parallel to each other, but far enough apart that the bullets had zero gravitational pull on each other, would the effects of space expansion increase the distance between the two bullets, or would their forward momentum, as with gravity, be too strong a force for it to overcome? If it is unable to change the gap between the two bullets in the example above, how is it able to expand the distance of objects opposite sides of the universe...

I guess it sounds to me like a wind that can tremendous effect at pushing things forward in the direction they are already travelling and expanding the universe, but is too weak a force push anything sideways.

6. Mar 1, 2009

### Chronos

That depends, if spacetime is flat, they remain parallel and equidistant for eternity. If it is curved, they either more apart, or collide at some point in time [in minkowski space].

7. Mar 1, 2009

### v2kkim

For parallel running bullets, another factor other than the spacetime flatness should be the basic space expansion. Assuming gravitational interaction ignored, they will fall apart at the standard expansion rate of 1/140 % per million light yrs.

8. Mar 2, 2009

### Ich

Not if they were parallel to each other.

9. Mar 2, 2009

### v2kkim

Ich. I do not agree with you. The space expansion does not matter it is parallel or not, but it affects everything evenly.

10. Mar 2, 2009

### Ich

Space expansion per se (the first derivative of the scale factor $$\dot a$$) does not affect anything at all. It is merely a set of initial conditions, saying that at a certain starting time nearby objects have a (measurable) velocity v=H*d, if "d" is distance.
If we assume the bullets to be low-speed, so that we don't have to care about spatial curvature, they will stay at a fixed distance if there is no such transversal velocity component, i.e. if they're fired parallel. Only if the guns were comoving, i.e. moving apart from the start, the bullets' distance will increase accordingly.
If you add gravity to the picture ($$\ddot a \neq 0$$), their distance will change eventually, but not proportional to a if the guns were not comoving.

11. Mar 4, 2009

### v2kkim

What I am saying is the distance between bullets will increase due to space expansion.
But I do not care if you call it parallel or not. Basically the space expansion applies quite fairly to all objects, unless there are disturbing forces like gravitation or electric or other forces.

12. Mar 5, 2009

### Ich

Yes, and what I'm saying is that it will not increase.
Space expansion applies exactly to those objects that are subject to a certain set of initial conditions.
In the absence of disturbing forces, e.g. gravity or a cosmological constant, it is nothing else than good old velocity. If there is relative velocity initially, the distance of objects will incease with time (that's what velocity means). If there isn't initial velocity, objects will stay where they are.

13. Mar 6, 2009

### Dusty_Matter

I thought that the cosmological constant is the same as spacetime expansion, and that it was an ongoing condition. That all objects are under this albeit very small constant velocity force. So while it does not affect objects that are close in distance, or objects that are gravitationally bound to each other, the further apart things are, the more apparent this force becomes.
If this is true, then the two bullets fired in parallel with each other, if they were a million light years apart in distance, would they not feel this 1/140% of cosmological constant expansion, and therefore their distance between each other would actually be increasing with time?

14. Mar 8, 2009

### Ich

"expansion" usually means that objects are moving away from each other. They could do so wihout a cosmological constant or gravity.
A cosmological constant will accelerate expansion, gravity will slow it down. If those influences exist, the bullets will not generally stay parallel. But neither will they follow "the expansion", because they were not initially moving away from each other, contrary to the objects which we denote "comoving" with the expansion.

15. Mar 8, 2009

### Dusty_Matter

Okay, lets say that the bullets to be fired are not a million light years apart, but are now 13 billion light years apart, but they are still parallel when they are fired onto their paths. Are you saying that they still will not spread apart along with the expansion of the universe, but will stay parallel with each other?

16. Mar 8, 2009

### hagendaz

So does space expansion only act as a "tailwind", but is unable to expand the distance between two objects lateral to their path of motion? What then if two bullets were fired from opposite sides of the universe directly at each other? Would the expansion of space not increase the distance they would have to travel to hit each other?

17. Mar 9, 2009

### Ich

Whatever "parallel" and "when they are fired" means at these distances. Better stay close, so that the bullets can be described in a common nearly flat frame.
I'm saying that neither wil happen. They will eventually spread abart, but more slowly than nearby comoving objects.
Expansion is unable to alter the distance of any two objects, unless these objects are initially prepared with some relative velocity. (where "expansion" has the meaning I use in this thread.)
Yes, cosmological distance between observers at rest wrt each other would initially increase under some circumstances. But "cosmological distance" is a coordinate construct, not what one would call a "distance" in any realistic operational way.
For example, in flat, empty space, one has a definite notion of two observers moving parallel or being at rest wrt each other. If you apply the definition of "cosmological distance" here, it would still increase with time. Not because the objects are moving away from each other, but because cosmological distance's definition is different from the "normal" one.

18. Mar 9, 2009

### Dusty_Matter

I believe that when they talk about the universe expanding, a lot times they use the example of raisin bread rising in the oven to show how galaxies are spreading apart. They are in motion with space/time which is expanding. The galaxies are not flying apart due to any velocity on their part, but due to the expansion of the universe. If it was due to an intial explosion of some sort, then there would be a central location from which they are leaving. But galaxies are getting farther apart from each other in all directions. In other words the universe is getting less dense. Anything therefore in our universe that exists, is on a ride with the spacetime fabric of the universe which is expanding. Expansion does alter the distance between objects, otherwise they would be fighting against the growth of the universe.

19. Mar 10, 2009

### Ich

Yes, that's an usual picture. I dont'tlike it at all. The picture of space dragging things along with it, or the notion of motion through space as opposed to (and physically different from) motion of space, are awfully inadequate and misleading.
Yes, and every observer will think that the explosion happened exactly at his position. It's just a matter of coordinate choice:
In comoving coordinates ("public space"), everything is essentially at rest, and redshift is due to "stretching of space". In observer-centered coordinates ("private space"), things are moving away from the center, and redshift is due to their velocity and gravitational time dilation.
Both pictures cover different aspects of the same universe, and both should be contemplated.
There is a tendency to use exclusively the first set of coordinates, but describe it as if it were minkowskian coordinates. That's the source of a lot of misconceptions and bad wording in cosmology.

20. Mar 22, 2009