Why isn't everything expanding in an expanding universe?

[zanick]

We do know that can't be the source of expansion, since such a mass imparts exactly zero acceleration on the particles contained inside. This result has been known for hundred of years. Google 'shell theorem'.

good point... kind of like if there was a bubble directly in the center of the earth, then if you stood there, you would just be floating?
So, the answer must be in another source of the force. (if the universe is still expanding as it appears to be)... and if there is this "force" , the force is not greater than the micro forces holding the small things together. because the universe is expanding and accelerating, then this force must be increasing, so eventually, everything will be expanding and ripped apart. what is it due to ? hmmmmm dark matter? forces we don't understand? how can the universe have accelerated expansion without a force to blame it on?

 

[wabbit]

Not dark matter - dark matter is actually the primary thing slowing down expansion, which would accelerate more without it. But dark energy if you want, that is just another name for this effect.

But it can also be seen as pure geometry - in the same way that gravity is said to be not a force in GR, but an effect of curvature, so is accelerated expansion, which is due to a particular kind of curvature.

 

[PeterDonis]

the answer must be in another source of the force

Expansion, by itself, does not require any force; it's just inertia. Things in the universe are flying apart. It doesn't take any force to keep them flying apart.

how can the universe have accelerated expansion without a force to blame it on?

Here you refer specifically to accelerated expansion, which, according to our best current model, is due to dark energy. That can be thought of as a "force", but only in the same way that any other kind of tidal gravity can be thought of as a "force". "Accelerated expansion" is just a name for a particular kind of spacetime curvature, i.e., tidal gravity, as wabbit says.

 

[newjerseyrunner]

I was under the impression that most physicists believed that dark energy came from a gauge boson like the electro-weak and strong forces rather than the curvature of spacetime? I remember watching a documentary on how the first began to try and find distant supernova to measure the expansion and in it the theory they laid out for dark energy was that most of it would come from a yet undiscovered unstable gauge boson that randomly pops in and out of existence from vacuum energy?

 

[PeterDonis]

I was under the impression that most physicists believed that dark energy came from a gauge boson like the electro-weak and strong forces rather than the curvature of spacetime?

No, that's not the right way to put it. The curvature of spacetime is there; it's been measured (that's what "accelerating expansion of the universe" means). The source of the curvature of spacetime--what produces it--is what is currently a matter of speculation: is it due to a cosmological constant, or a scalar field, or what?

 

[Chalnoth]

Deleted because I'm pretty sure what I wrote was incorrect.

 

[Chalnoth]

We do know that can't be the source of expansion, since such a mass imparts exactly zero acceleration on the particles contained inside. This result has been known for hundreds of years. Google 'shell theorem'.
edit:typo

There's been quite a bit of work to attempt to explain the accelerated expansion without a cosmological constant in terms of inhomogeneities. What they found is that if the Earth is almost identically at the center of a very large void, it is possible to observe an accelerated expansion as a result. But it turns out that this solution doesn't fit with observations.

 

[bcrowell]

There is a discussion of this topic here: http://physics.stackexchange.com/questions/70047/can-the-hubble-constant-be-measured-locally . I have a longer treatment in section 8.2.10 of my GR book http://www.lightandmatter.com/genrel/ . Both of those links have references to published papers.

 

[Quds Akbar]

Even if we were expanding in size, though that is not what I think, our instruments and labs would expand proportionally and we would not notice any difference, though I did not quite get your question or your point in Viewpoint 2. This was brought up in The Grand Design and it was concluded that objects do not increase in size as the universe expands using the balloon analogy. The distance between two objects may increase, but the size does not.

 

[Quds Akbar]

As far as I know, the universe at the smallest level does expand. A possible fate of the universe is called the big rip, where the expansion of space becomes so fast that atoms aren't able to keep themselves together.

Chronos, that's interesting, I never thought of that before, so why has the solar system not expanded? A gravitational orbit in an expanding space shouldn't remain stable should it?

But that happens when dark energy overcomes the gravitational force holding the objects together.

 

[wabbit]

I am no expert but my understanding is that a big rip is not a likely forecast, and not a consequence of the cosmological constant - dark energy need not overcome the gravitational force within bound systems, it separates such systems until each is isolated from all others.

For a simple system with a dominant central mass, there are stable orbits in a vacuum with cosmological constant, see e.g. Kagramanova & Lämmerzahl, Solar system effects in Schwarzschild-de Sitter spacetime

An interesting discussion of such a future is Krauss & Scherrer, The Return of a Static Universe and the End of Cosmology

 

[newjerseyrunner]

But that happens when dark energy overcomes the gravitational force holding the objects together.

Well, at the grandest scale that appears to already be happening. If dark energy continues to become ever stronger in proportion to gravity, then eventually galaxies will become unstable and start shedding mass. Solar system's orbits would start becoming unstable and eventually fly apart. If it continues on forever, eventually large stars will start shedding mass and explode. Planets will slowly evaporate, and eventually even black holes will vaporize or explode?

Question though, I've heard that dark energy is likely virtual particles popping in and out of existence because of random flux in vacuum energy. Because of entropy, shouldn't the vacuum energy density be steadily decreasing? Wouldn't this eventually slow the creation of random virtual particles? As the density of vacuum energy changes, shouldn't the ratio of the types of random virtual particles created from random flux change through time? Like very early on in the universe there was too much energy density to form any protons, then for a short period of time there was just energy energy density to create a ton of them, then it cooled further and the random creation of protons all but ceased. As entropy continues, shouldn't the amount of dark energy gauge bosons being produced wane, regardless of it's exact properties?

 

[wabbit]

Solar system's orbits would start becoming unstable and eventually fly apart

Would you have a reference supporting this ? Unless you are assuming an increasing cosmological constant, I don't understand how this is possible - if the ratio of $\Lambda$ to $G$ is constant, orbits are stable (actually, very slowly decaying, spiralling inward, due to gravitational radiation emission), and the local effect of the CC is forever as tiny as it is now. Kraus & Scherrer actually argue that given the low value of $\Lambda$ , this CC may even be undetectable locally. I am not sure I truly buy this last argument, but the general picture of an ultimate island universe seems convincing, and consistent with other arguments, be they from the Schwarzschild-de Sitter geometry or the shell theorem.

 

[Chalnoth]

Well, at the grandest scale that appears to already be happening. If dark energy continues to become ever stronger in proportion to gravity, then eventually galaxies will become unstable and start shedding mass. Solar system's orbits would start becoming unstable and eventually fly apart. If it continues on forever, eventually large stars will start shedding mass and explode. Planets will slowly evaporate, and eventually even black holes will vaporize or explode?

Question though, I've heard that dark energy is likely virtual particles popping in and out of existence because of random flux in vacuum energy. Because of entropy, shouldn't the vacuum energy density be steadily decreasing? Wouldn't this eventually slow the creation of random virtual particles? As the density of vacuum energy changes, shouldn't the ratio of the types of random virtual particles created from random flux change through time? Like very early on in the universe there was too much energy density to form any protons, then for a short period of time there was just energy energy density to create a ton of them, then it cooled further and the random creation of protons all but ceased. As entropy continues, shouldn't the amount of dark energy gauge bosons being produced wane, regardless of it's exact properties?

Sure but it's not likely that dark energy will become stronger.

 

[bcrowell]

Even if we were expanding in size, though that is not what I think, our instruments and labs would expand proportionally and we would not notice any difference[..].

No, this is discussed in the links in my #43, and in the references therein. Cosmological expansion, if not at a constant rate, causes local effects, such as the ones estimated in the Cooperstock paper and in the examples in my book, that are in principle detectable but in reality too small to measure. The Cooperstock paper is here: http://arxiv.org/abs/astro-ph/9803097v1

 

[PeterDonis]

If dark energy continues to become ever stronger in proportion to gravity

What makes you think it is? The average density of dark energy (which is constant as far as we can tell--see below) is becoming larger in proportion to the average density of matter and radiation; but that's not the same thing as dark energy becoming stronger in proportion to gravity. The gravity of a locally bound system, such as a solar system or a galaxy, is not determined by the average density of matter in the entire universe; it's determined by the density of matter in the locally bound system. The ratio of that density to the density of dark energy doesn't change as the universe expands; that's what "locally bound system" means.

shouldn't the vacuum energy density be steadily decreasing?

Not if it's a cosmological constant. In other models, it could change from place to place or change in time; but the change wouldn't necessarily be a decrease.

 

[newjerseyrunner]

Oh, I understand now.