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

[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?

 

[Doug Huffman]

The Robertson-Walker Scale Factor (a)t has a positive second derivative.
http://www.astro.ucla.edu/~wright/cosmo_03.htm

 

[wabbit]

Kiyal, you ought to visit http://www.phinds.com/balloonanalogy/

 

[Drakkith]

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.

 

[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

 

[Kiyal]

Kiyal, you ought to visit http://www.phinds.com/balloonanalogy/

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.

 

[wabbit]

"

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.

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.

 

[Kiyal]

. 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:)

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

 

[wabbit]

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

 

[phinds]

So distances among solar systems etc get further apart...

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.

 

[wabbit]

"Bound systems" include galactic clusters as well, so galaxies that are all in the same cluster are not affected by the expansion.

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.

 

[phinds]

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) within a galaxy, a cluster, etc... My guess is that the ratio of such pressures would give the "precise" equivalent of the rubber-strength to metal-strength ratio in the above example.

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.

 

[wabbit]

but I have no idea how one computes that in specific cases.

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.

 

[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).

 

[phinds]

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).

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.

 

[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.

 

[Blackberg]

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.

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)

 

[Doug Huffman]

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

 

[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.

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?

 

[Blackberg]

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

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

 

[phinds]

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.

Yes, that is exactly what I said in post #12

 

[phinds]

... negligible for a galaxy, bately noticeable for the largest clusters

I would say dense clusters, not large clusters, since it is the gravitational attraction of the outer galaxies to the cluster center that matters, and that depends more on density than size.

 

[wabbit]

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

Perhaps the distinction in a way similar to "where does the Earth atmosphere end"? There is no sharp limit - even if Earth were strictly alone in space, at some point random thermal movement gets some molecules kicked out (though one molecule kicked out might still come back after hitting another one)?

Edit: for a given galaxy at a given moment though, the distinction should be sharp, it is either bound to the cluster or not, depending if its velocity is lower or higher than the cluster escape velocity?

 

[wabbit]

I would say dense clusters, not large clusters, since it is the gravitational attraction of the outer galaxies to the cluster center that matters, and that depends more on density than size.

Agreed, a large but very dense cluster could be "tightly bound".

 

[phinds]

Agreed, a large but very dense cluster could be "tightly bound".

yes

 

[George Jones]

A technical reference on the effect of the universe's expansion on gravitationally interacting systems is "The influence of the cosmological expansion
on local systems" by Cooperstock, Faraoni, and Vollick,

http://arxiv.org/abs/astro-ph/9803097A reference (somewhat less technical than the reference above) on electrically interacting systems (e.g., atoms) is "In an expanding universe, what doesn’t expand?" by Price and Romano,

http://arxiv.org/abs/gr-qc/0508052

 

[wabbit]

I

A technical reference on the effect of the universe's expansion on gravitationally interacting systems is "The influence of the cosmological expansion
on local systems" by Cooperstock, Faraoni, and Vollick,

http://arxiv.org/abs/astro-ph/9803097A reference (somewhat less technical than the reference above) on electrically interacting systems (e.g., atoms) is "In an expanding universe, what doesn’t expand?" by Price and Romano,

http://arxiv.org/abs/gr-qc/0508052

Thank you very much, this is bound to be better than randomly guessing as I was doing :) . Reading this now.

Edit: also found : Valerio Faraoni, Audrey Jacques, Cosmological expansion and local physics (http://arxiv.org/abs/0707.1350).

 

[George Jones]

You're welcome.

this is bound to be better

Pun intended?

 

[wabbit]

Pun intended?

No, that was completely unintentional- thanks for the laugh !

 

[Blackberg]

Yes, that is exactly what I said in post #12

For a galaxy on a cluster threshold, someone most likely did try calculating the outgoing radiation pressure. The CBR (plus all stellar radiation) should apply a slight radiation pressure on everything. How does it compare to gravity? Can it account for at least some of the expansion?

If I'm going to bring anything to expansion theories today, it's probably limited to brownian diffusion (with unidentified particles) and radiation pressure lol.