# Why are the more distant galaxies/stars moving faster away?

Tags:
1. Mar 22, 2015

### Kiyal

The universe is expanding: Why is it that the further galaxies and stars are away from us they appear to be moving away at a much faster speed than ones closer to home?

2. Mar 22, 2015

### Doug Huffman

3. Mar 22, 2015

### wabbit

4. Mar 22, 2015

### Staff: Mentor

Because that's precisely how expansion works! If you were to bake delicious raisin bread, as the bread rises the raisins on the outside of the dough would be moving away from the raisins on the opposite side of the dough faster than they are moving away from the raisins near the center. In other words, the closer two raisins are to each other the slower they move apart.

This is because this type of expansion causes distances to increase at a rate, not a speed. This rate can be measured in percentage increase over time. For example, if our raisin bread takes 1 hour to rise fully and increases to twice its original size, then the rate of expansion was 100% per hour, meaning that the distance between each raisin increased by 100% over the course of 1 hour.

Similarly the expansion of the universe causes galaxies further away from each other to increase in distance faster than galaxies closer to each other.

5. Mar 22, 2015

### Quarlep

Hubble Law says V=HD V speed of object H hubble constant D is distance from galaxies so If make D bigger it means V will be bigger.Thats the why further objects seems much faster than the near ones. If you want more info look this video

Just watch first 15 minute

6. Mar 22, 2015

### Kiyal

This helped me a lot, thank you!

So gravity and electromagnetism holds together atoms and solar systems because the slow expansion of space can't overcome the forces of gravity/electromagnetism? So distances among solar systems etc get further apart, though we don't as we're held together by said forces? I think this is making sense, if I'm on the right tracks.

7. Mar 22, 2015

### wabbit

"
Yes. It is as if you tried stretching a metal coin by attaching a rubber band to each side and pulling on that rubber band - only weaker, empty space is not as strong as rubber:)

It's not really that slow though, its about 7% per billion years for large scale distances - but, if my understanding is correct, about 0% within a galaxy due to the gravitational binding.

Edit: just to be clear, distances "between solar systems" don't increase if they are both part of the same galaxy.

Last edited: Mar 22, 2015
8. Mar 22, 2015

### Kiyal

Oh wow, that makes a lot of sense! I like that analogy with the rubber band and the coin!

9. Mar 22, 2015

### wabbit

I like it too, which is why I stole it from phinds :)

10. Mar 22, 2015

### phinds

As wabbit already pointed out, solar systems do not get farther apart because they are part of the same galaxy. It goes even farther than that though. "Bound systems" include galactic clusters as well, so galaxies that are all in the same cluster are not affected by the expansion.

11. Mar 22, 2015

### wabbit

Right. To get a quantitative estimate of the relative "strength of expansion" vs "strength of gravitational binding" I was trying, following one of marcus' many enlightening calculations, to compare the corresponding pressures: the negative pressure of expansion turns out to be about 2×10-15 atmospheres IIRC - but I couldn't find an estimate of the average positive pressure of gravity (or equivalently the average energy density including matter and radiation but excluding DE) within a galaxy, a cluster, etc... My guess is that the ratio of such pressures would give a "precise" equivalent of the rubber-strength to metal-strength ratio in the above example.

Last edited: Mar 22, 2015
12. Mar 22, 2015

### phinds

Yeah, I've never gotten into the details at that level. It seems that some galactic clusters, very large ones but with relatively low density, ARE affected at the outer galaxies because their gravitational attraction to the cluster center is not enough to overcome expansion, but I have no idea how one computes that in specific cases.

13. Mar 22, 2015

### wabbit

My guess here is that it just amounts to dividing the mass of the cluster (converted to energy through E=mc2) by its volume, but perhaps that is (far) too simplistic.

14. Mar 22, 2015

### Blackberg

If we drop ping pong balls in an ocean, they will drift apart. Wouldn't that be an acceptable analogy?
The ping pong balls being galaxy clusters. The water being "expanding space".
(water molecules may be playing the role of a particle someone else may be able to identify).

15. Mar 22, 2015

### phinds

No, not at all, because the analogy fails completely to show that normal galactic clusters do NOT drift apart and ones where the outer galaxies do drift off don't have the rest of the galaxies drifting apart.

16. Mar 22, 2015

### Doug Huffman

Susskind uses two flowing water metaphors, one of a uniform manifold of pipes supplying water to a shallow pond, the surface of which is everywhere flowing away from itself, and another of a single drain hole to illustrate the effects of the event horizon as a limit.

17. Mar 22, 2015

### Blackberg

At some sufficiently large radius for a galaxy cluster, I would expect gravity to be sufficiently weak for the outer galaxies to drift away with expanding space. (drifting would break apart a sufficiently large ping pong ball, or maybe an onion rather, which has many layers)

Last edited: Mar 22, 2015
18. Mar 22, 2015

### Doug Huffman

Beggars the meaning of galaxy cluster. Either they are gravitationally bound or not.

19. Mar 22, 2015

### wabbit

Sorry this is wrong wrong wrong. Let me try again I don't know if I'll get this right this time either, but at least there should be one error removed.

The "expansion pressure" I mentionned is another name for the cosmological constant, which explains the acceleration of expansion, not expansion itself. Expansion per se has no associated energy density or pressure.

So, tentatively:

Bound systems should be completely unaffected by (unaccelerated) expansion, because expansion doesn't pull at all - it is a form of relaxation, not a response to a force.

But, they are affected by accelerating expansion, i.e. by the negative pressure of the cosmological constant, and a possible guess is that measuring the ratio of their own energy density to that of the CC, might quantify this: negligible for a galaxy, bately noticeable for the largest clusters.

Is that closer to being correct?

Last edited: Mar 22, 2015
20. Mar 22, 2015

### Blackberg

I see. The assumption seems fair to me that they lie on a threshold between a gravity well and space expansion.