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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?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.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.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.In a word yes.
Could you point me to where it says this?Um, no. As already explained, local objects do not expand in a universe that is (on average) expanding.
To the contrary it is gravity that is weak.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. Nothing to do with the strong force (gluons) holding protons & neutrons together and how would you know that the penny isn't expanding anyway? What would be your measuring stick? Relative to what? Because you would be expanding along with it! 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...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.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.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.Nothing to do with the strong force (gluons) holding protons & neutrons together and how would you know that the penny isn't expanding anyway?
Agreed. You should wait until you know more. There are plenty of books on the subject. :tongue: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,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.Could you point me to where it says this?
As Cosmic Novice said, your attack on Dave is a reflection of your own ignorance. Dave is a very helpful contributor on this forum and he DOES know what he's talking about.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.
The expansion refers to the space between galaxies. Within galaxies, gravity and electromagnetic force hold things together.
I think you meant to say '..but it can also be the dominant force, given...&c.'.It is pretty clear that while gravity is the weakest of the fundamental forces it can also be the strongest given certain circumstance.
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'.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.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.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'.
Actually it makes perfectly good sense.This makes no sense, as written.
Indeed they are. The dark energy "force" or whatever it is, is so incredibly weak that it is trivially easy for gravity to overcome it anywhere near large bodies.Whatever laws/processes of physics are at work between Galaxies are also at work between him and his monitor.
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?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
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?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, 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.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.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.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.