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Eric12407
2006-Feb-28, 10:59 PM
Hello Experts ...

I'm aware that the space between galaxies is expanding and that this is true from whatever galaxy you happen to be on ...so an observer in any galaxy sees the same phenomenon .. all galaxies receding ...

What I was wondering was ... why this is at the galaxy level of matter only.

Since this is an attribute of the universe ...when and why did it decide to form galaxies first and then start expanding in this way ..

How did any matter coalesce under this expansion?

Whats happening to the matter between the galaxies now ....?

If we travel between galaxies will we be ripped apart ... just joking ...

Also ... if I look at a galaxy billions of years away ... and an observer from there looks at me ... and I am receding from his perspective ... am I not going back into the Big Bang?

If they are all receding from each other at all times ... how could they have ever been reduced to the singularity?

grant hutchison
2006-Feb-28, 11:10 PM
Space expands everywhere, but gravity is more than able to pull things together despite the local expansion.
Scientific American ran an article in March last year which addressed this very topic. The expansion of local space exerts a very slight outward force on the Earth, about 10-30 as strong as the force of gravity at the Earth's surface. So the Earth is infinitesimally bigger because of the expansion of the Universe.

Grant Hutchison

Ken G
2006-Mar-01, 02:23 AM
My guess is, it is not the "expansion of space" that is responsible for this "force", but rather, the fact that the components of the solar system still at some tiny level "remember" that they formed from material that was initially flying apart, rather than from stationary gas as is normally assumed. The expansion of space picture is a convenient way to express this initial condition, but my money says you can deconstruct it into a more conventionally physical explanation. But in any event, note that 10 to the minus 30 is so tiny that no remnant of the assumptions needed to describe motion in the solar system survives at that level.

Ari Jokimaki
2006-Mar-01, 07:05 AM
Space expands everywhere, but gravity is more than able to pull things together despite the local expansion.
So matter doesn't affect expansion in any way?

If matter doesn't affect expansion on small scale, why it affects expansion on large scale (i.e. dictates what kind of geometry universe has)?

grant hutchison
2006-Mar-01, 09:59 AM
So matter doesn't affect expansion in any way?

If matter doesn't affect expansion on small scale, why it affects expansion on large scale (i.e. dictates what kind of geometry universe has)?I'm not sure that this follows. The fact that matter is able to hold itself together on galactic scales despite the expansion of space doesn't say anything about how matter affects the expansion of space. Does it?

Grant Hutchison

Ari Jokimaki
2006-Mar-01, 10:58 AM
I'm not sure that this follows. The fact that matter is able to hold itself together on galactic scales despite the expansion of space doesn't say anything about how matter affects the expansion of space. Does it?

Grant Hutchison
I'm not sure. I'm just thinking that if the amount of matter in the universe might be able to stop the expansion of the whole universe (in closed universe case), then it should manifest itself somehow in smaller scale also, for example so that matter concentration of certain density might slow or even halt the expansion in that place.

Ken G
2006-Mar-01, 01:56 PM
I'm not sure. I'm just thinking that if the amount of matter in the universe might be able to stop the expansion of the whole universe (in closed universe case), then it should manifest itself somehow in smaller scale also, for example so that matter concentration of certain density might slow or even halt the expansion in that place.
It does. If you take the picture that gravity affects space, then the space inside the solar system is not expanding, because the gravity prevents it. What is happening to space is not necessarily uniform, it is generally expanding on large scales but is all choppy on small scales. I think you can also take the approach Grant explained of just letting all space expand uniformly, and treating local gravitational perturbations in the dynamics of the matter.

grant hutchison
2006-Mar-01, 07:37 PM
OK. I missed a subtlety when I first reported Lineweaver & Davis's article from the March 2005 SciAm. What they actually said was:
In fact, in our universe the expansion is accelerating, and that exerts a gentle outward force on bodies. Consequently, objects are slightly larger than they would be in a non-accelerating universe, because the equilibrium among forces is reached at a slightly larger size.They go on to discuss the potential for a "big rip" if the acceleration continues to increase: the expansive force from accelerating acceleration could eventually overwhelm not just gravity, but the forces between and within atoms.
What's interesting about this (to me, at least!) is that photon wavelengths would expand even in a steadily expanding Universe, but that material objects (which have a specific size because of force equilibriums) require an accelerating expansion of space before they experience an expansive force, and an accelerating acceleration is required to make material objects grow with time (as they adjust to an increasing expansive force).
The whole article, Misconceptions about the Big Bang is well worth reading, I think. Charles H Lineweaver has a link to it from his home-page. (I'm not providing a direct link because I have some worries about copyright issues: the same article doesn't seem to be publicly available on the SciAm website.)

Grant Hutchison

Ken G
2006-Mar-02, 05:25 AM
This is an important distinction. I must say I don't really understand the big rip, is that supposed to come from a cosmological constant? I shouldn't have thought that a cosmological constant would do any more to matter than it is doing already, effectively weakening the binding of matter due to the antigravity of the vacuum. Note that it is not necessary to describe that in terms of anything happening to space, merely that space itself is associated with antigravity. Maybe the big rip requires some other type of antigravity?

grant hutchison
2006-Mar-02, 09:28 AM
Wouldn't a Big Rip require a non-constant cosmological constant, if it is generated by the sort of "accelerating acceleration" Lineweaver and Davis describe? So the antigravity effect would become increasingly powerful as time passed.

Grant Hutchison

Ken G
2006-Mar-02, 01:24 PM
That's what I'm wondering. But note accelerating acceleration isn't very descriptive-- a cosmological constant will induce exponential expansion as it takes over, so the second derivative of the scale parameter will be proportional to the scale parameter and will increase. However, the physical acceleration should be of the scale of c over the time constant, so that's constant. Maybe you need the actual scale of the acceleration to increase, not the second time derivative of the scale parameter, to get a big rip?