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Hi. So if the space in the Universe is expanding, does that mean that the space existing between me and my monitor, or the space between object A and object B is also expanding in some sense?
Perhaps with a qualification... If I understand it right, there are some closer galaxies in our region which form a thing called the "Local Group", which are called that because they are close enough that their gravity keeps them together. Is that right?mathman said:The expansion refers to the space between galaxies. Within galaxies, gravity and electromagnetic force hold things together.
This is correct. Basically, using General Relativity we can both predict that overall there will either be expansion or contraction (empirically, it's expansion), but that locally dense regions (whether a solar system, a galaxy, a galaxy cluster, or larger object) will tend to stay stable.narrator said:Perhaps with a qualification... If I understand it right, there are some closer galaxies in our region which form a thing called the "Local Group", which are called that because they are close enough that their gravity keeps them together. Is that right?
In a word yes.Gytax said:Hi. So if the space in the Universe is expanding, does that mean that the space existing between me and my monitor, or the space between object A and object B is also expanding in some sense?
Um, no. As already explained, local objects do not expand in a universe that is (on average) expanding.bluey said:In a word yes.
Chalnoth said:Um, no. As already explained, local objects do not expand in a universe that is (on average) expanding.
DaveC426913 said:The expansion "force" is extremely weak - far weaker than gravity. Anything gravitationally bound will overcome it. The only circumstances where objects are weakly bound enough for the expansion to overcome gravity are in the vast open spaces between galactic clusters.
Imagine a bunch of pennies glued to a balloon. If you inflate the balloon, you would not expect the pennies to tear apart into dust. It is apparent that the forces holding a penny together vastly outstrip the strength of the glue.
True. And the cosmological expansion is weaker.bluey said:To the contrary it is gravity that is weak.
An extremely weak force can produce quite an effect when it accumulates. With mass, like gravity. With distance, like CE.bluey said:When I hear of galaxy's speeding away from us at near the speed of light powered by inflation I think of an extremely strong force.
Not relevant to the current discussion.bluey said:Nothing to do with the strong force (gluons) holding protons & neutrons together and how would you know that the penny isn't expanding anyway?
bluey said:Actually I think there are just too many assumptions on this subject right now and I think it would be wise to wait for the evidence to come in on dark energy ect...
bluey said:I have been interested in this subject for over 60 years and I have read a lot of books on it over that time I think that my conclusion of the assumptions is as good as anyone elses,maybe you need to read more on the subject because there are a lot more alternative views to yours or mine.
Unfortunately, I don't know of any popular sources that describe this, but it comes from the perturbation theory expansion of a homogeneous, isotropic universe, and is in any standard, modern cosmology textbook.bluey said:Could you point me to where it says this?
bluey said:I have been interested in this subject for over 60 years and I have read a lot of books on it over that time I think that my conclusion of the assumptions is as good as anyone elses,maybe you need to read more on the subject because there are a lot more alternative views to yours or mine.
mathman said:The expansion refers to the space between galaxies. Within galaxies, gravity and electromagnetic force hold things together.
Cosmo Novice said:It is pretty clear that while gravity is the weakest of the fundamental forces it can also be the strongest given certain circumstance.
Gytax said:Hi. So if the space in the Universe is expanding, does that mean that the space existing between me and my monitor, or the space between object A and object B is also expanding in some sense?
Well, no, the effects of the expansion are simply not there when you have a gravitationally-bound system. If you place a gravitationally-bound system, such as a solar system, inside a space-time which is, overall, expanding, its behavior just doesn't change.cmb said:I think the most informative answer, at this level of question, would be to say; 'yes, the same physics is at work, but the effects of the expansion of space are so small on the you-monitor scale that it would be impossible to discriminate them, by orders of magnitude, over local effects'.
No. It's not true.cmb said:I think the most informative answer, at this level of question, would be to say; 'yes, the same physics is at work, but the effects of the expansion of space are so small on the you-monitor scale that it would be impossible to discriminate them, by orders of magnitude, over local effects'.
cmb said:This makes no sense, as written.
Whatever laws/processes of physics are at work between Galaxies are also at work between him and his monitor.
phinds said:The result between a guy and his monitor would be infinitesimal but non-zero if it were not for gravity, but because of gravity, it is zero
Chalnoth said:If you place a gravitationally-bound system, such as a solar system, inside a space-time which is, overall, expanding, its behavior just doesn't change.
cmb said:How much gravity do you need, then, for this effect to go from non-zero to zero? Any gravity? Are you saying there are spaces with no gravity acting, whatsoever, in it?
Gravity extends to infinity. All paricles in the universe are attracted to all other particles. They experience gravitational effects, though it is not enough to overcome their outward movement. It is at this point that cosmological expansion overwhelms gravity.cmb said:Ah, I see. So the Universe is not gravitationally-bound, and the Galaxies have no forces tending them to collapse back together. Is that right?
I don't entirely like this analogy, though, because it is gravity that is both holding galaxies together and affecting how quickly they move away from one another.DaveC426913 said:cmb, glue a penny to the ceiling. By your logic, gravity would pull on the penny regardless of the penny's internal forces, and eventually stretch the penny toward the floor.
No. Gravity is acting on the penny's lower surface but we do not witness the penny stretching to the floor as gravity works on it (yes, even if we wait a very, very long time). The penny's internal forces easily overcome gravity. It does not mean gravity does not apply, it simply means it is overwhelmed.
Chalnoth said:This means that some bits, having more matter, are better at slowing down the (local) expansion than other bits. If it turns out that there is enough local matter, then the mutual gravity is enough to not only slow down the local expansion, but cause the local system to collapse in on itself, forming a gravitationally-bound system.
Chalnoth said:I don't entirely like this analogy, though, because it is gravity that is both holding galaxies together and affecting how quickly they move away from one another.
cmb said:OK, I'll buy this proposition, but will you allow me to test it first?
The argument, as I understand what you are saying, is;
1) that there is no expansion of space within the physical limits of a gravitationally bound system,
2) that a gravitationally bound system is one in which the matter therein has enough 'gravity' to pull all the matter back, back to within that bounded system, and keep it like that*.
*(notwithstanding any processes that imparts enough energy to particular masses that may cause them to escape the system)
Please help me clarify these statements, or let me know if they are correct (wrt your proposition).
DaveC426913 said:Would you agree that there is a gravitational force from the Earth present within the volume of a copper penny? That the gravity of the Earth is not somehow cancelled out inside the penny?
The force is there, pulling on the atoms, yet the penny does not disintegrate. Why?
That's more or less accurate.cmb said:OK, I'll buy this proposition, but will you allow me to test it first?
The argument, as I understand what you are saying, is;
1) that there is no expansion of space within the physical limits of a gravitationally bound system,
2) that a gravitationally bound system is one in which the matter therein has enough 'gravity' to pull all the matter back, back to within that bounded system, and keep it like that*.
*(notwithstanding any processes that imparts enough energy to particular masses that may cause them to escape the system)
Please help me clarify these statements, or let me know if they are correct (wrt your proposition).
This is getting a bit pedantic, but I don't think it's accurate. The fundamental problem here is that we don't have two opposing forces. We just have one: gravity. I don't think it makes sense to think of the expansion as a force (even though yes, you can write it in as a pseudoforce). I think it makes far more sense to think about the expansion as being due to the initial conditions, and the gravity on the components of the universe determining how that expansion changes over time.DaveC426913 said:No, you misunderstand. The role of gravity in the two cases is not the same. In that sense it is a confusing analogy, using gravity in both cases, but using them in completely different ways.
Universe:
Gravity is the coalescing force,
cosmological expansion is the expansive force.
The coalescent force easily overcomes the expansive force.
Balloon analogy:
Atomic bonds of copper is the coalescing force,
gravity is the expansive force.
The coalescent force easily overcomes the expansive force.
I am simply trying to point out to cmb that, when there are two opposing forces, one can overwhelm the other such that the smaller one has no measureable effect.
This does not follow. In fact, the average spatial curvature does not change with time at all. It does change in relative importance as other parts of the universe do (or do not) dilute as the universe expands. But it remains the same value for all time.inf. improb. said:The more drastically the fabric of space-time is curved as a result of gravity, the less the "stretching" of space will affect that area of space-time. If you imagine that fabric stretching to infinity, however, you will quickly see that the curvature is neccessarily flattened over time.