Question about the expansion of space.

[2sin54]

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?

 

[mathman]

The expansion refers to the space between galaxies. Within galaxies, gravity and electromagnetic force hold things together.

 

[narrator]

The expansion refers to the space between galaxies. Within galaxies, gravity and electromagnetic force hold things together.

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?

 

[Chalnoth]

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?

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.

 

[bluey]

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?

In a word yes.

 

[Chalnoth]

In a word yes.

Um, no. As already explained, local objects do not expand in a universe that is (on average) expanding.

 

[DaveC426913]

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.

 

[bluey]

Um, no. As already explained, local objects do not expand in a universe that is (on average) expanding.

Could you point me to where it says this?

 

[bluey]

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.

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...

 

[DaveC426913]

To the contrary it is gravity that is weak.

True. And the cosmological expansion is weaker.

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.

An extremely weak force can produce quite an effect when it accumulates. With mass, like gravity. With distance, like CE.

Nothing to do with the strong force (gluons) holding protons & neutrons together and how would you know that the penny isn't expanding anyway?

Not relevant to the current discussion.

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...

Agreed. You should wait until you know more. There are plenty of books on the subject.

 

[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.

 

[Cosmo Novice]

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.

Bluey,

You really should listen to what people here have to say. Your 60 years of reading is pretty irrelevant as much of the current cosmological consensus has come in recent years.

It is pretty clear that while gravity is the weakest of the fundamental forces it can also be the strongest given certain circumstance.

It is also pretty clear (from the years of evidence and reams and reams of data) that expansion on a cosmoligcal scale only affects bodies of mass that are not gravitationally bound strongly enough - such as expansion on scales larger than the local group.

We could all do with reading more but remember some of the people who comment on this forum are experts in there field and they will echo the points that are being made.

 

[Chalnoth]

Could you point me to where it says this?

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.

 

[phinds]

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.

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.

By the way, when I say "ignorance" I do not mean to be rude and I certainly do NOT mean "stupid". We're all ignorant on lots of stuff but on this particular subject, Dave's level of ignorance is way less than yours.

 

[cmb]

The expansion refers to the space between galaxies. Within galaxies, gravity and electromagnetic force hold things together.

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.

I do not see how you could possibly dispute this, else questions like 'where is the boundary of a Galaxy' and 'how small can a Galaxy be before it is counted as being 'between' other Galaxies', and such like.

Laws of physics are Universal, not Galactic. This is harking back to days before the 'crystal spheres' was blown up as explaining why astronomical bodies hang in the sky without falling down! Whaat? The same gravity is at work here on Earth as it is in the heavens?? Preposterous!

I suspect visitors to Westminster Abbey are currently wondering what that scraping noise is - as Newton rolls in his grave!?

 

[cmb]

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 you meant to say '..but it can also be the dominant force, given...&c.'.

It cannot be both the weakest and the strongest, simultaneously!

 

[cmb]

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?

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'.

 

[Chalnoth]

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'.

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.

Dark energy does change things slightly, because dark energy actually does add an extra repulsive force between objects based upon distance. But its effect is so small for gravitationally-bound systems that we just don't care for most situations (though it does have a significant impact on the sizes of systems that eventually do become bound, so that our most sensitive proposed test of dark energy is observing how systems join together over time to form larger bound systems).

 

[DaveC426913]

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.

If it were true, that means that the expansion would also be happening at larger (but still sub-intergalactic) scales. It would mean we would detect that the galaxies themselves are expanding along with space. They're not. Which also means anything smaller (more strongly bound) than galaxies are also not expanding.

The expansion only takes over where gravity is virtually zero to many decimals - in the voids between whole clusters of galaxies.

[D'oh. Chalnoth beat me.]

 

[phinds]

This makes no sense, as written.

Actually it makes perfectly good sense.

Whatever laws/processes of physics are at work between Galaxies are also at work between him and his monitor.

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.

Your saying that this doesn't make sense, or implies different laws of physics is the same as saying that a kid pushing on a tank doesn't move the tank but that If the kid were pushing on his little red wagon, he would move the wagon so you object that he should be able to move the tank.

Different level forces produce different results.

The fact that far distant galaxies are receeding from us FTL makes lots of folks think that dark energy is very strong and would have a noticible local effect. Not true.

I heard it best said once like this: Even though the universe is expanding, it's still going to be hard to find a parking place. Sounds silly, but the point behind it is this. If you magically draw parking space lines in intergalactic space, how long does it take before there's room for another car? The answer is billions of years. BUT when you add up all those unbelieveable scadzillions of parking space sized areas over billions of light years, the result is amazing.

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

 

[cmb]

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

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?

 

[cmb]

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.

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?

 

[DaveC426913]

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?

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.

Likewise, I take you back to the pennies-on-a-balloon model. By your logic, the expanding balloon would stretch out the pennies as it acted upon every copper molecule that was glued to the balloon. Why does the penny not stretch? Because the forces binding copper atom to copper atom are far stronger than the glue binding copper atom to balloon.

 

[DaveC426913]

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?

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.

 

[Chalnoth]

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.

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.

The basic, basic picture here is that overall, you have a big universe that is, on large scales, smooth and uniform. The mutual gravity of all of the matter in the universe wants to slow this expansion down.

Now, in slightly more detail, some bits of the universe are more dense than other bits. It isn't entirely smooth. 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. The overall, large-scale expansion still goes on, but the local gravity is enough to stop it in certain parts of the universe.

Of course, as I mentioned earlier, dark energy changes this somewhat as it adds an extra repulsive force, and this does have an impact on how big things can be and still end up bound together. But this effect is small on the scale of galaxy clusters and even smaller on anything smaller than a galaxy cluster.

 

[cmb]

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.

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).

 

[Calimero]

Cmb, I think that you take "expansion of space" too literally. Throw a ball and, voila, space between you and ball is expanding. It is just matter of coordinate interpretation.
Things are just little more complicated when you add dark energy into the picture. Regardless of its true nature it would need to have negative pressure, acting repulsively. Some structures can resist that pressure, some large enough can't.
Dark energy, in principle, could be detected in lab, here on Earth. Trouble is that you can't turn it off, or shield against it, so you don't have reference point against which to measure.

 

[DaveC426913]

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.

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.

 

[DaveC426913]

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).

You are taking it too literally.

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?

 

[cmb]

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?

I do agree, I do agree, and because the binding forces between the atoms is stronger than the gravitational field, respectively.

But I do not see the connection between those answers, and the points 1 and 2 I have asked to clarify.