Could the Universe Have Regions of Expansion and Collapse?

[coolcalx]

I understand that (from what we can tell) the universe is expanding at an accelerating rate. I'm just curious if it's possible that the universe is expanding in some areas, and receding in others. despite this going against the cosmological principle, is it possible that beyond our bubble of expansion, there is a bubble of collapse?

I'm just wondering if the universe is acting like entropy, in the sense that it could be expanding in some places, yet collapsing in others.

 

[Chalnoth]

I understand that (from what we can tell) the universe is expanding at an accelerating rate. I'm just curious if it's possible that the universe is expanding in some areas, and receding in others. despite this going against the cosmological principle, is it possible that beyond our bubble of expansion, there is a bubble of collapse?

I'm just wondering if the universe is acting like entropy, in the sense that it could be expanding in some places, yet collapsing in others.

In a way, this sort of kinda happens. Local regions which have a large overdensity of matter undergo collapse still. The accelerated expansion is only seen on very large scales (hundreds of millions of light years and greater).

 

[Chronos]

Yes, there are still regions of the universe that are locally collapsing, but, as Chalnoth correctly points out, expansion is only obvious on very large scales.

 

[twofish-quant]

Also you may get odd things happening outside of the cosmic horizon.

One limitation on the cosmological principle is the speed of light. Two pieces of the universe that aren't in communication can't exchange information to behave in the same way.

 

[phinds]

Yes, there are still regions of the universe that are locally collapsing, but, as Chalnoth correctly points out, expansion is only obvious on very large scales.

This does not seem incorrect, but it does seem like a somewhat misleading statement. The matter in some areas is coming closer together but that does not inhibit the expansion of space all around that region of matter, so the overall expansion is not affected.

Here is what I base that on, and I'm open to correction:

If you have galaxy A and galaxy B separated by 2 billion light years, and half-way between them there is a local cluster that is collapsing, the speed with which A and B are receding from each other is not affected by the existence of that cluster. Is this not correct?

If it IS correct, then I think you can see why I say your statement is somewhat misleading. If it's NOT correct, then of course ... (and I do see how it might not be correct)

 

[Naty1]

If you have galaxy A and galaxy B separated by 2 billion light years, and half-way between them there is a local cluster that is collapsing, the speed with which A and B are receding from each other is not affected by the existence of that cluster. Is this not correct?

I'd say it is NOT correct, but it's not an egregious error either. I don't like it on theoretical grounds:
Cosmological expansion is based on homogeneous and isotropic uniformity, the FLRW model. It simply does not apply to lumpy portions of the cosmos. If you want to disregard the collapsing interim galaxy, then you can approximate the model conditions if you consider that 'ok for my purposes' [accurate enough].

Thread here with some insightful discussion and details:

Cosmological expansion of Earth orbit
https://www.physicsforums.com/showthread.php?t=603429

 

[Naty1]

is it possible that beyond our bubble of expansion, there is a bubble of collapse?

Possible but not likely according the mainstream cosmology. I'm talking large scale 'collapse' here, like horizon to horizon type size.

Eternal inflation models, bubble universes, and multiverses [all generally synonomous terms] are alternative models from the FLRW model most cosmologists believe best respresents our universe. The FLRW model has lots of verifying, although often indirect, observational evidence; insofar as I know, the other models have none, but that does not entirely rule them out.

Summary information here:
Eternal inflation
http://en.wikipedia.org/wiki/Bubble_universe

 

[twofish-quant]

FLRW which assumes a perfectly smooth universe is like assuming that the Earth is spherical. The Earth isn't spherical, but it's close enough so that for a lot of things.

LCDM is like assume the Earth is ellipsoid. LCDM contains the effects of linear perturbations. The Earth isn't ellipsoid, but it's an even close approximation.

 

[clamtrox]

This does not seem incorrect, but it does seem like a somewhat misleading statement. The matter in some areas is coming closer together but that does not inhibit the expansion of space all around that region of matter, so the overall expansion is not affected.

Here is what I base that on, and I'm open to correction:

If you have galaxy A and galaxy B separated by 2 billion light years, and half-way between them there is a local cluster that is collapsing, the speed with which A and B are receding from each other is not affected by the existence of that cluster. Is this not correct?

If it IS correct, then I think you can see why I say your statement is somewhat misleading. If it's NOT correct, then of course ... (and I do see how it might not be correct)

You need to remember that GR is not a linear theory. That means, if you have a background solution which is expanding, and a local collapsing solution, the exact overall solution is not the combination of the two. I think it's very misleading to think that gravity and the expansion of the universe are somehow separate things, when in fact they are exactly the same thing (curving of the metric).

 

[phinds]

... if you have a background solution which is expanding, and a local collapsing solution, the exact overall solution is not the combination of the two. QUOTE]

I have no idea whether you are agreeing with me or disagreeing with me or neither.

 

[coolcalx]

FLRW which assumes a perfectly smooth universe is like assuming that the Earth is spherical. The Earth isn't spherical, but it's close enough so that for a lot of things.

LCDM is like assume the Earth is ellipsoid. LCDM contains the effects of linear perturbations. The Earth isn't ellipsoid, but it's an even close approximation.

what is the difference between these two? wiki has them both listed as the standard model of big bang cosmology :/

 

[Chalnoth]

what is the difference between these two? wiki has them both listed as the standard model of big bang cosmology :/

FLRW is a description of how a smooth universe that is the same in all directions expands (or contracts) over time depending upon the contents of said universe. FLRW makes no statements, one way or another, about those contents (except that they are smooth and the same in all directions). It merely presents a relationship between expansion and contents.

LCDM is explicitly about the contents of the universe: it is a universe where, besides normal matter, we have a cosmological constant (Lambda) and cold dark matter. This universe is approximately FLRW, but not exactly: matter in the universe tends to become rather clumpy with time. On large scales, on average, it is FLRW. But on smaller scales we have things like galaxy clusters, galaxies, stars, planets, and other objects that are much, much more dense than their surroundings.

 

[Naty1]

LCDM is explicitly about the contents of the universe:

yes, ours are included: LCDM is fine tuning where we inlcude in FLRW some observational evidence that more closely describes our universe. For example we derive some information from type 1A supernova [a standard brightness] observations.

Detailed parameters described here:
http://en.wikipedia.org/wiki/Lambda-Cold_Dark_Matter#Parameters

 

[coolcalx]

ahh okay thank you. I understood that the cosmological principle was only applicable on large scales, and that helps clarify it

 

[coolcalx]

sort of on topic, what are your thoughts on the possibility of a cyclic model for the universe? for example, the recent Baum–Frampton model

 

[Chalnoth]

sort of on topic, what are your thoughts on the possibility of a cyclic model for the universe? for example, the recent Baum–Frampton model

Personally, I think they're pretty highly unlikely, for various reasons. Most of the models I think are highly unlikely for entropy reasons: they require entropy to increase and then decrease periodically. This is just absurd.

The Baum-Frampton model is kinda neat in its own way, but is very absurd for a different reason: it requires an equation of state for dark energy that is generally expected to be fundamentally impossible.

My personal favorite model is this one:
http://arxiv.org/abs/hepth/0410270

But I wouldn't even suggest that it's more likely than not. Just that I think the model is beautiful.

 

[coolcalx]

My personal favorite model is this one:
http://arxiv.org/abs/hepth/0410270

I'll save this and read it after I start my astrophysics classes, so it make more sense to me haha

 

[audioloop]

ahh okay thank you. I understood that the cosmological principle was only applicable on large scales, and that helps clarify it

cp = two principles of spatial invariance. The first invariance is isomorphism under translation = homogeneity (uniformity would be independent of the location one chooses to make the observations)
invariance as isomorphism under rotation = isotropy (direction, such as North or South, can not be distinguished).

 

[bapowell]

Although the two principles are not independent: global isotropy implies homogeneity.

 

[enasset90]

Heck, what I want to know is this:
If during the first second of the "Big Bang" space grew larger than light could travel, how big could the universe be?

We have "figured out" the age of the universe, yet we have not "figured out" the expanse or breadth of it.
Ed

 

[phinds]

Heck, what I want to know is this:
If during the first second of the "Big Bang" space grew larger than light could travel, how big could the universe be?

We have "figured out" the age of the universe, yet we have not "figured out" the expanse or breadth of it.
Ed

It is likely that we will never figure out the expanse. It MAY have started off infinite in extent, in which case it still is (just bigger than it used to be), or it may have started off finite, in which case it still is (just bigger than it used to be). No one knows and I've seen estimates that put the minimum size as not much bigger than the observable universe (which I think is ridiculous) to dozens of orders of magnitude times the size of the observable universe.

 

[enasset90]

If I understand correctly, space is expanding at a given rate, faster than gravity is able to pull all things together. Yet, on a smaller scale, gravity is doing things, like keeping the Earth around the sun and causing Andromeda to collide with our galaxy, the Milky Way. So, things are being pulled together by gravity, but space (the distance between one atom to another) is growing longer...one year it is a yard, one million years later it is 1.001 yards. Sorry for all you EU guys... one year it is a meter, one million years later it is 1.0012 meters.
If this is correct, why couldn't the Professor speak this way?

 

[Chalnoth]

If I understand correctly, space is expanding at a given rate, faster than gravity is able to pull all things together. Yet, on a smaller scale, gravity is doing things, like keeping the Earth around the sun and causing Andromeda to collide with our galaxy, the Milky Way. So, things are being pulled together by gravity, but space (the distance between one atom to another) is growing longer...one year it is a yard, one million years later it is 1.001 yards. Sorry for all you EU guys... one year it is a meter, one million years later it is 1.0012 meters.
If this is correct, why couldn't the Professor speak this way?

On average, overall, the expansion is simply too rapid to pull things together. However, there are places in the universe that are more dense than average, dense enough that gravity is strong enough to pull the local matter together into clumps. So most of the stuff in the universe is getting further apart, but the universe is becoming more clumpy.

 

[phinds]

If I understand correctly, space is expanding at a given rate, faster than gravity is able to pull all things together. Yet, on a smaller scale, gravity is doing things, like keeping the Earth around the sun and causing Andromeda to collide with our galaxy, the Milky Way. So, things are being pulled together by gravity, but space (the distance between one atom to another) is growing longer...one year it is a yard, one million years later it is 1.001 yards. Sorry for all you EU guys... one year it is a meter, one million years later it is 1.0012 meters.
If this is correct, why couldn't the Professor speak this way?

Because it is not correct. NOTHING inside of gravitationally bound systems is being affected by dark energy. Atoms are NOT moving apart. If you put a ruler in deep space it will not change size.

A meter is a meter.

Thinks outside gravitationally bound systems DO get farther apart but that does not mean space, as a thing itslef, is expanding. See Metric Expansion (there's a link in this page: www.phinds.com/balloonanalogy and some further discussion of expansion)

 

[Chalnoth]

Because it is not correct. NOTHING inside of gravitationally bound systems is being affected by dark energy.

Pedantic correction:
Nothing inside of gravitational systems is being affected by the expansion. Dark energy still has an effect, but it's small. The expansion has no effect.

 

[phinds]

Pedantic correction:
Nothing inside of gravitational systems is being affected by the expansion. Dark energy still has an effect, but it's small. The expansion has no effect.

But if dark energy DOES have an effect, then the size of atoms would change, yes? Is this an interpretational statement or accepted fact?

 

[Chalnoth]

But if dark energy DOES have an effect, then the size of atoms would change, yes? Is this an interpretational statement or accepted fact?

Well, no. This can perhaps be understood by considering that the attractive gravitational force between two objects with a cosmological constant in the Newtonian limit:

F = {GMm \over r^2} - {\Lambda m c^2 \over 3} r

Here we imagine that M is some large mass (such as the Sun), m is the mass that is orbiting this large mass, and r is the distance from the center of this large mass. The force, then, is the force the orbiting mass feels towards the larger one. You can see that the cosmological constant reduces the attractive force of gravity by some small amount. For atoms, this would have the effect of making atoms ever so slightly larger than they otherwise would be (the difference really is utterly negligible, however). Even for something as large as the solar system, the difference is much too small to measure, because \Lambda is so tiny. The two forces only become comparable when the distance between objects is exceptionally large.

 

[phinds]

Well, no. This can perhaps be understood by considering that the attractive gravitational force between two objects with a cosmological constant in the Newtonian limit:

F = {GMm \over r^2} - {\Lambda m c^2 \over 3} r

Here we imagine that M is some large mass (such as the Sun), and m is the mass that is orbiting this large mass. The force, then, is the force the orbiting mass feels towards the larger one. You can see that the cosmological constant reduces the attractive force of gravity by some small amount. For atoms, this would have the effect of making atoms ever so slightly larger than they otherwise would be (the difference really is utterly negligible, however). Even for something as large as the solar system, the difference is much too small to measure, because \Lambda is so tiny. The two forces only become comparable when the distance between objects is exceptionally large.

OK, thanks for that correction. I've been saying here in several posts that it has NO effect inside galactic clusters, rather than negligible effect, so I'll change my tune.

Paul