Does expansion exist on the micro level as well as the galactic level?

[Axuality]

Does expansion exist on the micro level as well as the galactic level?

I've never read about it, but it occurs to me that there can be no uniqueness about where the expansion of the universe exists.

I have to envision an atom as a 'kind' of solar system, and a molecule therefore as a 'kind' of galaxy, and an ore sample of this molecule as a galactic cluster. With this simplistic view in mind, it seems to me that the space between ALL particles, not just large objects, must be increasing- and frankly, that the particles themselves (since they are not "solid" objects, but rather consist of "smaller" entities (smaller expanding "universes")) must be expanding also.

Why have I read nothing of this? I'm not well-read admittedly.

By the way, if this were the case, wouldn't we feel the expansion of the physical object beneath our feet as a force pushing up against the bottom of our feet? Come to think of it, our feet would also be expanding down toward the ground, thereby creating some sort of feeling.

 

[Janus]

Expansion does not extend down to the microscopic level. In fact, it doesn't affect things smaller than clusters of galaxies. The reason for this is that the forces holding these objects together (gravity, electromagnetic and nuclear forces) are stronger than universal expansion.

 

[Axuality]

Expansion does not extend down to the microscopic level. In fact, it doesn't affect things smaller than clusters of galaxies. The reason for this is that the forces holding these objects together (gravity, electromagnetic and nuclear forces) are stronger than universal expansion.

Are we accepting that clusters of galaxies are expanding along with the space between them or not?

You implicitly say that stronger forces stop the expansion on levels smaller than clusters. You mean that these forces stop THINGS from moving.

When I stand on the surface of the earth, is it not said that I am accelerating at 32 ft/sec/sec, although I'm not MOVING toward or away from the center of the earth? Can there not be accleration or even expansion without movement?

It does not make intuitive sense to me that at some arbitrary level, expansion would no longer exist.

We may have to discover that at some relevant frame of reference, the forces of which you speak are not stronger than one another.

 

[Chronos]

In the present epoch, as Janus noted, it safe to say expansion does no operate at levels below intergalactic space. The future is less certain. Google on 'the big rip' for discussions.

 

[bwana]

...
It does not make intuitive sense to me that at some arbitrary level, expansion would no longer exist.

I agree and have puzzled about this contradiction as well. I explain it thusly:

Hubble parameter measures expansion of space and it is greater at increasing distances. Since these distances are HUGE and measured in light years, the information provided by these observations are VERY OLD. I conclude that the universe is no longer expanding since we do not see a Hubble shift in our neighborhood, although we could detect it.

here is what i wrote:
https://www.physicsforums.com/showthread.php?t=335946
http://forums.anandtech.com/messageview.aspx?catid=50&threadid=2320348&enterthread=y

 

[Janus]

Are we accepting that clusters of galaxies are expanding along with the space between them or not?

You implicitly say that stronger forces stop the expansion on levels smaller than clusters. You mean that these forces stop THINGS from moving.

When I stand on the surface of the earth, is it not said that I am accelerating at 32 ft/sec/sec, although I'm not MOVING toward or away from the center of the earth? Can there not be accleration or even expansion without movement?

It does not make intuitive sense to me that at some arbitrary level, expansion would no longer exist.

We may have to discover that at some relevant frame of reference, the forces of which you speak are not stronger than one another.

The forces of which I speak all get stronger as the distance between the objects get closer together. The expansion of the universe does not. So yes, there is a boundary between expansion and nonexpansion. it is that point where galaxies are close enough for their mutual gravitational attraction is greater than their tendency to move apart. Galaxy clusters do not expand because the individual galaxies are close enough to each other for gravity to hold them together. Galaxy clusters move apart because they are not close enough to each other.

An analogy: You are standing on a tile floor in your stocking feet. The floor has 1ft square tiles. The floor starts to expand (the 1 ft tiles eventually become 2 ft tiles, etc.)

You will see a person standing a few tiles away recede from you. (as the tiles expand, the distance between the center of the tiles increases. )

Now imagine a third person standing next to you, and you are holding hands. The person few tiles away will still recede from you, but the person standing next to won't. This is because your grip is stronger than the friction between both of your stockings and the floor. You are like two galaxies in a local cluster gravitationally bound to each other. The tiles still expand under your feet, you just don't move apart with them.

 

[bwana]

---

You are like two galaxies in a local cluster gravitationally bound to each other. The tiles still expand under your feet, you just don't move apart with them.

If gravity has this effect, then measurements of the Hubble parameter in the vicinity of intense gravity wells (black hole, quasar,etc) should be 0. So, 2 galaxies that are very far away might have a certain redshift but a supermassive black hole next to a different galaxy in another part of the universe (but just as far away) should have much less redshift. Has this been seen?

 

[herbert]

Expansion does not extend down to the microscopic level. In fact, it doesn't affect things smaller than clusters of galaxies. The reason for this is that the forces holding these objects together (gravity, electromagnetic and nuclear forces) are stronger than universal expansion.

Whilst i appreciate that i am not at the high level of a PF mentor,

Surely the above cannot be correct.
If so how did inflation come about?
Space must expand at every level.
However, we have gravity too. this causes things to collapse.
There is a battle between expansion and gravity/electrical forces
On the microscopic, gravity and electromagnetic forces win and so whilst space itself expands the particles, planets etc don't.
One has to get far away and into what is called the 'Hubble flow' before expansion effects are greater than 'local effects'.
For instance the andromeda galaxy is close by and will eventually collide with our own. Gravity wins because the expansion effects are smaller that gravity effects.
BUT... all space exapnds no matter on what scale.

 

[herbert]

If gravity has this effect, then measurements of the Hubble parameter in the vicinity of intense gravity wells (black hole, quasar,etc) should be 0. So, 2 galaxies that are very far away might have a certain redshift but a supermassive black hole next to a different galaxy in another part of the universe (but just as far away) should have much less redshift. Has this been seen?

Intense gravity wells etc have gravitational redshift. this will be added on to cosmological redshift.
IE a photon of light escaping a black hole, gains lots of gravitational potential energy. Since E = hf, its energy reduces, frequency reduces wavelength increases.
Gravitationally redshifted.
Not zero.

 

[Chronos]

Gravitational redshift is a non contributor to H0. The vast majority of photons generated by black holes, etc., originate too far from the gravity well to be noticeably redshifted.

 

[PRDan4th]

If Newton were alive today and knew what we know, I think he would have added another term to his gravity equation. This would be a negative term (repulsive) and have a very very small constant times distance squared and perhaps independent on their masses. This term applies all the way from short distances to extremely long distances but is so small that it is not detectable at distances shorter than inter galactic distances. Perhaps something like:

F=G*(m1*m2)/r^2-R*r^2 and may have a m1*m2 relationship as well.

R would be the expansion or repulsive constant.

If this were the case at some distance the R term would balance the G term and gravity force F would be zero, beyond that distance expansion would overwhelm attraction.

Does this make any sense?

PRD

 

[herbert]

Gravitational redshift is a non contributor to H0. The vast majority of photons generated by black holes, etc., originate too far from the gravity well to be noticeably redshifted.

But surely one needs a 'standard candle' to determine Ho?
Cepheid variables work as do supernovae 1a.
Is there a standard candle black hole? ie a black hole that is equally bright (or dark?) no matter where it is in the universe?
Otherwise, surely it is not a matter of the photons 'not being noticeably redshifted' but that they just cannot be used?

 

[sylas]

Does expansion exist on the micro level as well as the galactic level?

No.

To a first approximation (and I must thank https://www.physicsforums.com/member.php?u=51971" as a major help for my understanding this) expansion simply means that things are moving apart from each other. It isn't a force pushing things apart. The universe expands because things are moving apart from each other ... not the other way around.

That's the first approximation. The second approximation is that there are actually forces that act to slow down or to increase the rate at which things are moving.

Gravity works to pull things together, and hence to slow down expansion.

Dark energy works to push things apart, and hence to speed up expansion. As I understand it, "dark energy" is just a name we give for the apparent existence of something we don't really understand but which is the cause of expansion to be speeding up.

But on sufficiently small scales (a cluster of galaxies or less) you can just consider the nice simple model you are all used to, of a bunch of stuff moving around in space, and subject to gravity.

A galaxy isn't expanding, because its constituent parts are not moving apart from each other. No matter what motions you see elsewhere, the galaxy is all hanging together very nicely, and there is no particular "force" to counteract that, or to pull it apart. (Dark energy might in principle; but in the present epoch dark energy is far too weak to pull apart a galaxy or a galactic cluster.)

That's the bottom line. The only thing that might push things apart is a "dark energy", and what we infer about dark energy now is that it is way WAY too weak to matter at small scales. Apart from that, there's nothing at all about the expansion of space that would lead one to expect anything to be pulled apart, on any scale, unless it is already moving apart right now.

Cheers -- sylas

 

[herbert]

No.

To a first approximation (and I must thank https://www.physicsforums.com/member.php?u=51971" as a major help for my understanding this) expansion simply means that things are moving apart from each other. It isn't a force pushing things apart. The universe expands because things are moving apart from each other ... not the other way around.

Don't think so, Space expands carrying all with it. The expansion we see now is just the dying embers of inflation and its scalar fields.
On top of that we have the peculiar motion of the heavenly bodies within it.
On the small scale, gravitational effects are far bigger than expansion of space effects so they dominate.
But on the large scale expansion effects dominate and things move away from each other.
Its a good thing that objects themselves don't get any bigger as otherwise we would not see the expansion.
If distances got bigger and our 'ruler' got bigger at the same rate then we would measure the distances as constant. Its like a thermometer. The only reason the mercury rises is that mercury expands but the glass doesn't (hardly). If everything expanded then the mercury level would stay the same.
Congratulations on the edit Sylas

 

[Axuality]

Don't think so, Space expands carrying all with it. The expansion we see now is just the dying embers of inflation and its scalar fields.
On top of that we have the peculiar motion of the heavenly bodies within it.
On the small scale, gravitational effects are far bigger than expansion of space effects so they dominate.
But on the large scale expansion effects dominate and things move away from each other.
Its a good thing that objects themselves don't get any bigger as otherwise we would not see the expansion.
If distances got bigger and our 'ruler' got bigger at the same rate then we would measure the distances as constant. Its like a thermometer. The only reason the mercury rises is that mercury expands but the glass doesn't (hardly). If everything expanded then the mercury level would stay the same.
Congratulations on the edit Sylas

I was under the impression that the expansion of galaxies is NOT measured by rulers, but by the red shift. In which cases it would be allowable for the rulers to be getting bigger at the same rate as distances.

 

[Axuality]

Expansion does not extend down to the microscopic level. In fact, it doesn't affect things smaller than clusters of galaxies. The reason for this is that the forces holding these objects together (gravity, electromagnetic and nuclear forces) are stronger than universal expansion.

I should ask, how do we know that the universe is "expanding"? Is it not the red shift? And of course we make the assumption that red shift means that things are getting more meters apart. But the red shift would still be observable if all things, including space, were expanding, would it not?

And are we certain that what you call the strength of "universal expansion" is not greater in small distances (close quarters), than it is at the galactic cluster level?

 

[Chronos]

Redshift tells us the universe is expanding. Relativity tells us it is not a classical doppler effect. Spectral line shifts do not lie. We do not see discordant redshift in spectral lines. That tells us it is a global [spacetime] effect.

 

[v2kkim]

Another way to look at the expansion effect in small scale is to start with Earth and sun orbit. Th expansion and the gravitation will happen at the same time, though gravitation is much larger. Here, discrete modeling may help, that is say the sun-earth distance increases a little instantly due to expansion, then the Earth orbit will change slightly and as a result the sun-earth distance increase will be diminished considerably after a while (actually the Earth orbit will be slightly more elliptical.). and this diminishing effect will be greater in a mutibody case like in a galaxy.
I think mathematically we can make equation and even can find a solution for a simple case, and the result will be in line with above rough thinking.
In much smaller scale like our body cells, the dominant force is electric or inter-atomic force which will adjust each atomic motion continuously, diminishing expansion effect considerably.
This is a more detail thinking of the expansion effect to small scale.

 

[Axuality]

Redshift tells us the universe is expanding. Relativity tells us it is not a classical doppler effect. Spectral line shifts do not lie. We do not see discordant redshift in spectral lines. That tells us it is a global [spacetime] effect.

d

I'm not disagreeing with you and let me ask you:

Does what you're saying agree or disagree with the proposition that while according to the red shift the universe is expanding, it is also possible that at the same time all objects and the space in between them are expanding also?

 

[v2kkim]

d

I'm not disagreeing with you and let me ask you:

Does what you're saying agree or disagree with the proposition that while according to the red shift the universe is expanding, it is also possible that at the same time all objects and the space in between them are expanding also?

The explanation by advisor and mentor helped me a lot to understand. On this Axuality's question, I 'd say the answer is yes in infinitely short moment.. i. e. the expansion applies even in subatomic scale, but this expansion does not accumulate as time goes on in short distance.
In long distance beyond galaxy cluster, the expansion accumulates as time goes on. Thanks.

 

[Flatland]

If expansion does indeed occur on microscopic scale, then shouldn't we detect a small repulsion force between bonding atoms and molecules?

 

[v2kkim]

If expansion does indeed occur on microscopic scale, then shouldn't we detect a small repulsion force between bonding atoms and molecules?

In principle you may be right , something reflecting space expansion effect in atomic scale,
on which I can just speculate based on my limited knowledge of quantum physics. The basic equation of quantum mechanics is Schrodinger wave equation, and well maybe you can try yourself, because: First I do not know that much though I used that equation a lot, secondly the expansion magnitude is really really small in atomic scale. Roughly the expansion magnitude is (expansion rate* distance), and do some math on the distance ratio of Galaxy cluster to galaxy cluster vs 1E-10 meter, so practically impossible to detect something at atomic level, and scientist are busy to deal with many other important subject.
But I respect your thinking, and you may try something theoretically at least. Let me know if you can develop some new idea of new equations.

 

[sylas]

If expansion does indeed occur on microscopic scale, then shouldn't we detect a small repulsion force between bonding atoms and molecules?

Expansion isn't a force. It's just things moving away from each other. What forces do is change the rate at which things move away from each other. On large cosmological scales, forces can work to accelerate or decelerate expansion. But expansion itself doesn't need a force, in the same way that ordinary motion of individual objects is maintained without any force.

Small things are not expanding. They've already been pulled together and are no longer moving apart. Small, in this case, is much bigger than microscopic. For example, galaxies are not expanding. All the stuff in a galaxy has been pulled together and is no longer moving apart.

Cheers -- sylas

 

[Flatland]

I never meant that expansion was a force, just that matter on the microscope scale needs to resist the expansion of space, otherwise atoms and molecules would be pulled apart.

 

[sylas]

I never meant that expansion was a force, just that matter on the microscope scale needs to resist the expansion of space, otherwise atoms and molecules would be pulled apart.

Um... if it is not a force, then what is "pulling"? Expansion doesn't "push", or "pull" or "force" things. It is a description of motions, not a tendency towards certain motions.

As with motion in local space, expansion of things proceeds by inertia. Once you pull things together, you've already altered the expansion, and (in the case of our galaxy, let alone atoms and molecules) they simply aren't expanding any more. "Expansion" isn't a force, or a "pull" or anything else like that which needs to be resisted.

Cheers -- sylas

 

[bapowell]

Perhaps it's easiest to think in terms of energy/matter distributions here. Einstein's Eqs give us an expanding universe, the Friedmann Robertson Walker Universe, when the energy density (the source in Einstein's Eq) is uniform and isotropic. This is approximately true of the universe only on the largest scales (ie the scale of galaxy clusters). On smaller scales, the universe is far from homogeneous. The spacetime metric in such regions is not of the FRW form, and is not expanding. This is essentially equivalent to many of the posts above that say that strongly bound systems (solar systems, galaxies, atoms) don't expand.

As far as inflation goes, there's nothing wrong with space expanding on all scales. The energy density during inflation was necessarily homogeneous on all scales!

 

[Ich]

The spacetime metric in such regions is not of the FRW form, and is not expanding. This is essentially equivalent to many of the posts above that say that strongly bound systems (solar systems, galaxies, atoms) don't expand.

It's equivalent to these posts, but wrong - or, at least, not the whole story.
Even if the universe were perfectly homogeneous, and could be decribed by the FRW metric down to the smallest size, two test particles at rest wrt each other won't follow the expansion.
Their bahaviour is completely independent of H, and is dictated only by the energy and pressure density between them.
In a "perfect" LCDM universe, right now, they would actually slowly start accelerating outwards. But a few billion years ago, they would fall inwards, despite H being much bigger then.

 

[George Jones]

It's equivalent to these posts, but wrong - or, at least, not the whole story.
Even if the universe were perfectly homogeneous, and could be decribed by the FRW metric down to the smallest size, two test particles at rest wrt each other won't follow the expansion.
Their bahaviour is completely independent of H, and is dictated only by the energy and pressure density between them.
In a "perfect" LCDM universe, right now, they would actually slowly start accelerating outwards. But a few billion years ago, they would fall inwards, despite H being much bigger then.

For such observers, the universe does not obey the cosmological principle, i.e., from the point of view of at least one of these observers, the universe is not isotropic.

Are you saying that this effect is more relevant for "holding galaxies together" than the stuff that bapowell wrote? If so, why?

The stars in a galaxy cannot be approximated as test particles as they clearly make a non-negligible contribution to the stress-energy tensor for the spacetime region of the galaxy. Consequently, a region of spacetime large enough to contains a few galaxies but not much larger is not homogeneous, so this region won't be decribed by a FRW metric, and there is no reason to expect that a galaxy will expand.

 

[Ich]

For such observers, the universe does not obey the cosmological principle, i.e., from the point of view of at least one of these observers, the universe is not isotropic.

Yes. In FRW coordinates, at least one of them has nonzero peculiar velocity. That's an initial condition.

Are you saying that this effect is more relevant for "holding galaxies together" than the stuff that bapowell wrote? If so, why?

There's no question that galaxies are tightly bound and can withstand any "drag" from the expansion with a safety margin of many OOM.
The questioni is: is there such a "drag effect" from expansion in the first place?

What sylas is saying, and what I'm supporting, is: expansion is an initial condition rather than an ongoing effect. If two objects have relative velocity, they will move apart. If not, they won't. You don't need the gravitation of a galaxy to justify that, that's simply inertia.

Of course, if there is matter or pressure between the objects, its gravitation will change their velocity.

Consequently, a region of spacetime large enough to contains a few galaxies but not much larger is not homogeneous, so this region won't be decribed by a FRW metric, and there is no reason to expect that a galaxy will expand.

The point is: even if the region were perfectly homogeneous, and described by the FRW metric, is there a reason to expect that the things (say, test particles) in that region have a tendency to "expand" (i.e. change distance) when they're not starting with relative velocities?
This question can be decided by looking at the local background density at any time, without referencing to H.
At any given time, it is irrelevant whether the rest of the universe (or even the local background) is expanding or contracting, as long as the background's kinetic energy may be neglected.

 

[JDStupi]

I must say that I know nothing, expecially compared to some of the well-informed responses posted as, I would say, successfull refutations of the matter expanding idea, but here is my two cents: to OP, let's assume that the deeper knowledge of astrophysics/galactric dynamics that these posters display is uknown (it is to you, and it is to me), but we do still know some properties of matter! True, matter is not definatley "point-particle like" as is commonly conceptualized, but what is it? It is the interaction of the electromagnetic (and other) forces on the small scale, and they occupy volume because of the properties of electrons (which is generalized to a class of particles) on the small scale. Using that information, we may be able to infer that matter would not "get bigger" because if the space inside it was expanding (assuming it was strong enough to break the incredibly strong electromagnetic forces) the "electron cloud" spread out through space's radius would get bigger, thus decreasing the electromagnetic forces between the atoms/molecules by Coulomb's law, and so rather than get "bigger" if anything matter would simply break apart starting with the largest molecules with the weakest bonds, something maybe like some type of weak molecular solid. This is unobserved, and seems unlikely, expecially when considering that space or, in a loose way, distance from an atom's nucleus,c an define the quanitzed energy level of a particle. What would happen there? There would be some funky things going on with energy. Adn we can also consider the yet unkown intimate relationship between space/matter.

 

[sylas]

What sylas is saying, and what I'm supporting, is: expansion is an initial condition rather than an ongoing effect. If two objects have relative velocity, they will move apart. If not, they won't. You don't need the gravitation of a galaxy to justify that, that's simply inertia.

Exactly. The issue is also considered in this paper:

• Davis, T.M. et. a. (2003) Solutions to the tethered galaxy problem in an expanding universe and the observation of receding blueshifted objects, in Am. J. Phys, Vol 71, p 358, arXiv.org:astro-ph/0104349

Cheers -- sylas

 

[cepheid]

Now imagine a third person standing next to you, and you are holding hands. The person few tiles away will still recede from you, but the person standing next to won't. This is because your grip is stronger than the friction between both of your stockings and the floor. You are like two galaxies in a local cluster gravitationally bound to each other. The tiles still expand under your feet, you just don't move apart with them.

I didn't read much past this post by Janus, so I'm not commenting on the rest of the thread. I just wanted to say I thought this was a really great analogy. This is the crux of the answer to the OP.

 

[sylas]

I didn't read much past this post by Janus, so I'm not commenting on the rest of the thread. I just wanted to say I thought this was a really great analogy. This is the crux of the answer to the OP.

One of the problems with simple answers is that always go wrong at some point. Janus' analogy is good for sorting out some issues, but it still might give the misleading impression that there's actually a force involved, and that if you let go hands, you'll start moving with the floor again.

To get a better idea, you'd have to imagine everyone initially moving with the floor... and then the floor becomes completely frictionless. Everyone then just keeps drifting, and because there's no acceleration involved in moving with the floor, everyone stays above the same spot on the floor as all the tiles continue to expand.

NOW reach out and grab hold of someone else. You lock positions with them... and then you can release hands... and you will STAY locked in position with them, although the both of you are still expanding away from everyone else.

Cheers -- sylas

 

[twofish-quant]

One other way of thinking about it is to point out that the basic calculation that gives an expanding universe assumes that everything in the universe is this nice homogenous gas and there are no lumps in the universe. If you start with this assumption, then you get this nice uniform expansion. Now at large scales, the universe is close enough to this approximation, but as small scales, these assumptions no longer work.

One other analogy is the equations that people use to model the universe basically assume that the universe consists of a "gas" of galaxies. If you look at the air around you, it seems to be a continuous fluid, and for a lot of calculations you can assume that it is a continuous fluid, but if you look at very small scales (i.e. the individual atoms) the assumptions you are making break down.

The other thing is that I think the answer changes if there is a cosmological constant.

 

[Flatland]

Um... if it is not a force, then what is "pulling"? Expansion doesn't "push", or "pull" or "force" things. It is a description of motions, not a tendency towards certain motions.

You seem to be arguing semantics over here. And how can it be a description of motion? Nothing actually moves during the expansion of space.

As with motion in local space, expansion of things proceeds by inertia. Once you pull things together, you've already altered the expansion, and (in the case of our galaxy, let alone atoms and molecules) they simply aren't expanding any more. "Expansion" isn't a force, or a "pull" or anything else like that which needs to be resisted.

Cheers -- sylas

how do you alter expansion by pulling things together?