B Do Atoms Expand with the Universe?

[Tayler]

Please understand that I am a professional car detailer, not an astrophysicist. Hence the question. If space is expanding, and atoms contain space. Would that mean that the atoms themselves would also be expanding? Making not only the universe expand but also everything contained in the universe i.e matter? Either atoms have 0.00% space inside of them or we are all expanding at the same accelerated rate of the expansion of space, or the universe.

 

[Ibix]

Welcome to Physics Forums.

If space is expanding, and atoms contain space. Would that mean that the atoms themselves would also be expanding?

No, although it's a common enough misconception. Bound systems like atoms and galaxies, held together by their electromagnetic and gravitational interactions, do not expand. It's only distant galaxies that are moving away from each other.

There are quite a lot of caveats to that "space is expanding" thing. I sometimes think it would be better if we didn't describe things that way because it misleads people like it misled you. But I can't think of a better way of saying it without just spouting jargon...

 

[Tayler]

So the space in and around such bound systems are being stretched and pulled away from them without effecting the mass of the object and all within the system stay in close proximity due to the force of gravity. Anything outside of the systems gravitational pull is expanding with space as there's no "tether" ie gravity.. oh well the milky way is big enough haha. Thanks for your reply Ibix

 

[Ibix]

So the space in and around such bound systems are being stretched and pulled away from them without effecting the mass of the object and all within the system stay in close proximity due to the force of gravity

Not exactly. Nothing's being stretched or pulled.

We see the cosmic microwave background, the last glow of the Big Bang, looking (nearly) the same in all directions so we argue that we are (nearly) at rest. But it's fairly straightforward to work out that those distant galaxies that are moving away from us will also see the cosmic microwave background looking the same in all directions. So they are at rest too - but they are moving away from us. The easiest way to conceptualise this is to describe the space in between as expanding, but that doesn't mean that there's something literally stretching.

Heavy duty maths is required for a good answer.

 

[Tayler]

So basically I need to expand my mind first to understand this "expansion" of space haha
Thankyou for your answer ibix how long have you studied this category of physics? If you don't mind me asking.

 

[Will Learn]

Hi Tayler.

So the space in and around such bound systems are being stretched and pulled away from them without effecting the mass of the object and all within the system stay in close proximity due to the force of gravity.

I've seen issues like the one you raised discussed elsewhere. Sometimes an approach like this one can help.

It's not quite as if every region of space must be expanding. There are some parts of the universe where the sort of expansion that you're talking about simply does not have to happen. The little region of the universe around that atom you're talking about can be one of those little patches.

On the largest of scales, the universe is well approximated by a model of spacetime that includes a "scale factor" which evolves with time - so we can say "space expands" if we want to keep it simple. That model is only useful on the largest astronomical scale. Small little patches of the universe are not well modeled by this solution. In some small patches of the universe there doesn't have to be any time dependant scale factor in the solution - so space does not have to expand.

There is a tendency to jump between Newtonian ideas of gravity and the main alternative which is General Relativity and sometimes technical names for things make more sense in one than the other. That's unfortunate but it's just the way it is. A "gravitationally bound system" is one of those terms that pulls the reader toward Newtonian ideas where gravity is a force that can bind things together instead of just letting them slip away with space that is dragged out from under them. However, the interpretation with general relativity can be different: A gravitationally bound system can be one where the distribution of matter and energy is suitable to ensure that in that little patch there just isn't any expansion of space.

None the less, your concern about the space between and within atoms wasn't silly. Ibix has already mentioned that there is another force that can help: There are electromagnetic interactions that can bind atoms and molecules together. But if you just want to see some Popular science videos, you should search for something called the "big rip". This is where the expansion of space becomes so importat that no region of the universe could remain bound and all forces can be overwhelmed. Molecules and atoms will be ripped apart as you may have imagined. Don't lose sleep, its PopSci and Wow factor, we really don't know if the universe will expand like that. My money would be on perfectly stable gravitationally bound regions of the universe persisting for far longer than human beings will.

Best wishes to you.

 

[timmdeeg]

So basically I need to expand my mind first to understand this "expansion" of space haha

Expansion means growing distances between comoving objects or sloppy between galaxies which are far away from each other. Now you can ask why these distances are growing and the answer is ambiguous and leads to the question in Ned Wrights FAQ "Are galaxies really moving away from us or is space just expanding?", see http://www.astro.ucla.edu/~wright/cosmology_faq.html#MX .
The answer is:
This depends on how you measure things, or your choice of coordinates. In one view, the spatial positions of galaxies are changing, and this causes the redshift. In another view, the galaxies are at fixed coordinates, but the distance between fixed points increases with time, and this causes the redshift.

You may like to read "Expanding Space: the Root of all Evil?

https://arxiv.org/pdf/0707.0380.pdf

​

 

[hmmm27]

I've been wondering the same thing (I think). I'd put it as "Does the expansion of space influence bound systems".

Like, if I dangle an iron nail from a magnet, it doesn't mean that gravity has disappeared ; just that the magnetic force overcomes the gravity. If it was a weak magnet the nail would drop off.

 

[PeterDonis]

"Does the expansion of space influence bound systems".

No.

f I dangle an iron nail from a magnet, it doesn't mean that gravity has disappeared ; just that the magnetic force overcomes the gravity. If it was a weak magnet the nail would drop off.

The expansion of space is not a force, so this analogy is not valid.

 

[Tayler]

No.The expansion of space is not a force, so this analogy is not valid.

From what I've read it's dark energy that creates the force of expansion. But then I've also read dark energy is a theory and may not even exist. I am learning very fast that 1 answer = 2 more questions

 

[Tayler]

Hi Tayler.

I've seen issues like the one you raised discussed elsewhere. Sometimes an approach like this one can help.

It's not quite as if every region of space must be expanding. There are some parts of the universe where the sort of expansion that you're talking about simply does not have to happen. The little region of the universe around that atom you're talking about can be one of those little patches.

On the largest of scales, the universe is well approximated by a model of spacetime that includes a "scale factor" which evolves with time - so we can say "space expands" if we want to keep it simple. That model is only useful on the largest astronomical scale. Small little patches of the universe are not well modeled by this solution. In some small patches of the universe there doesn't have to be any time dependant scale factor in the solution - so space does not have to expand.

There is a tendency to jump between Newtonian ideas of gravity and the main alternative which is General Relativity and sometimes technical names for things make more sense in one than the other. That's unfortunate but it's just the way it is. A "gravitationally bound system" is one of those terms that pulls the reader toward Newtonian ideas where gravity is a force that can bind things together instead of just letting them slip away with space that is dragged out from under them. However, the interpretation with general relativity can be different: A gravitationally bound system can be one where the distribution of matter and energy is suitable to ensure that in that little patch there just isn't any expansion of space.

None the less, your concern about the space between and within atoms wasn't silly. Ibix has already mentioned that there is another force that can help: There are electromagnetic interactions that can bind atoms and molecules together. But if you just want to see some Popular science videos, you should search for something called the "big rip". This is where the expansion of space becomes so importat that no region of the universe could remain bound and all forces can be overwhelmed. Molecules and atoms will be ripped apart as you may have imagined. Don't lose sleep, its PopSci and Wow factor, we really don't know if the universe will expand like that. My money would be on perfectly stable gravitationally bound regions of the universe persisting for far longer than human beings will.

Best wishes to you.

Thankyou mate, actually found a video with Neil deGrasse Tyson explaining exactly what I was trying to understand.

 

[Tayler]

Expansion means growing distances between comoving objects or sloppy between galaxies which are far away from each other. Now you can ask why these distances are growing and the answer is ambiguous and leads to the question in Ned Wrights FAQ "Are galaxies really moving away from us or is space just expanding?", see http://www.astro.ucla.edu/~wright/cosmology_faq.html#MX .
The answer is:
This depends on how you measure things, or your choice of coordinates. In one view, the spatial positions of galaxies are changing, and this causes the redshift. In another view, the galaxies are at fixed coordinates, but the distance between fixed points increases with time, and this causes the redshift.

You may like to read "Expanding Space: the Root of all Evil?

https://arxiv.org/pdf/0707.0380.pdf

​

Thanks mate, that looks great! Will definitely be looking at this tonight

 

[Tayler]

I've been wondering the same thing (I think). I'd put it as "Does the expansion of space influence bound systems".

Like, if I dangle an iron nail from a magnet, it doesn't mean that gravity has disappeared ; just that the magnetic force overcomes the gravity. If it was a weak magnet the nail would drop off.

Yeah, its a crazy thought! I've been thinking about it for years but the big rip theory definitely gives you a bit of enlightenment. But on the same hand makes you ask more questions

 

[timmdeeg]

From what I've read it's dark energy that creates the force of expansion.

More precisely the accelerated expansion of the universe creates tidal forces which affect the largest known structures called superclusters. These are "stretched" over time in contrast to smaller clusters of galaxies which are gravitationally bound systems and thus resist "stretching".

If such clusters aren't "stretched" then its obvious that galaxies, solar systems, molecules and atoms aren't "stretched" too.

 

[PeroK]

More precisely the accelerated expansion of the universe creates tidal forces which affect the largest known structures called superclusters. These are "stretched" over time in contrast to smaller clusters of galaxies which are gravitationally bound systems and thus resist "stretching".

If such clusters aren't "stretched" then its obvious that galaxies, solar systems, molecules and atoms aren't "stretched" too.

Unless the physical parameters that underlie the laws of physics are changing with time then all systems will behave in the future as they behave today. The solar system, for example, is a matter dominated system and there is no ongoing expansion in its evolution. Its dynamics remain independent of cosmological time.

The universe as a whole, however, is a fundamentally different system from the solar system, galaxy or galaxy cluster. The Friedmann equation that governs its evolution implies expansion - with or without dark energy.

One cannot apply the Friedmann equation to every subsystem of the universe.

Moreover, as the universe as a whole expands it becomes more and more vacuum dominated. Without dark energy that would imply a slowing expansion. With dark energy we have an accelerated expansion.

In any case, the critical difference between bound systems and the entire universe is an equilibrium between matter and vacuum in one case; and, a changing balance between matter and vacuum on the other.

 

[PeroK]

PS in order to get the "big rip", therefore, we need the energy density of the vacuum to change over time. This hypothetical increasing vacuum energy density changes the dynamics of bound systems over time.

 

[Tayler]

My money would be on perfectly stable gravitationally bound regions of the universe persisting for far longer than human beings will.

Thats another good topic. Exactly how far do you think we will advance as a civilisation? Because if we were able to inhabit our whole local cluster, wouldn't there be enough stuff to sustain life just about forever and therefore survive as long as the universe allows us?

 

[timmdeeg]

The universe as a whole, however, is a fundamentally different system from the solar system, galaxy or galaxy cluster. The Friedmann equation that governs its evolution implies expansion - with or without dark energy.

According to the late-time integrated Sachs-Wolf effect superclusters are getting stretched out over time. As I understand it this requires a positive value of $\ddot{a}/a$ and in this sense the Friedmann equations. If correct we wouldn't observe this effect in case the universe was matter dominated and thus expanding decelerated.

​

​

 

[timmdeeg]

Moreover, as the universe as a whole expands it becomes more and more vacuum dominated. Without dark energy that would imply a slowing expansion. With dark energy we have an accelerated expansion.

Which will go over into exponential expansion in the very far future.

 

[PeterDonis]

From what I've read

What have you read? Can you give any specific references?

it's dark energy that creates the force of expansion.

This is not correct. Expansion, by itself, is just inertia; comoving objects are expanding (moving apart) now because they were moving apart in the past.

Dark energy causes the expansion to accelerate, but it is not necessary for just expansion.

I've also read dark energy is a theory and may not even exist.

"Is a theory" is a meaningless comment, because the word "theory" has so many possible meanings. If you are suspicious of anything that is "a theory", you should avoid driving over bridges because engineers use structural theory to design bridges.

It is true that there are some outlier viewpoints in cosmology that question whether the expansion of the universe is actually accelerating, but I don't think any of those viewpoints have much traction. "Dark energy" is basically just another way of saying "the expansion is accelerating", so if the expansion is in fact accelerating, then dark energy exists. We don't fully understand at a microphysical level where dark energy comes from, but we don't need to know that to observe that the expansion is accelerating.

 

[Tayler]

What have you read? Can you give any specific references?

Should of said "from what I've watched". Documentaries and the like.
I listen to a few reputable scientists including Niel deGrasse Tyson. Kurzgesagt on YouTube is fascinating. I am a basic man haha

 

[Tayler]

This is not correct. Expansion, by itself, is just inertia; comoving objects are expanding (moving apart) now because they were moving apart in the past.

Dark energy causes the expansion to accelerate, but it is not necessary for just expansion."Is a theory" is a meaningless comment, because the word "theory" has so many possible meanings. If you are suspicious of anything that is "a theory", you should avoid driving over bridges because engineers use structural theory to design bridges.

It is true that there are some outlier viewpoints in cosmology that question whether the expansion of the universe is actually accelerating, but I don't think any of those viewpoints have much traction. "Dark energy" is basically just another way of saying "the expansion is accelerating", so if the expansion is in fact accelerating, then dark energy exists. We don't fully understand at a microphysical level where dark energy comes from, but we don't need to know that to observe that the expansion is accelerating.

Thanks for the correction mate. You make a very valid point! I could definitely work on my terminology haha maybe the word hypothetical would have been more fitting?

 

[Ibix]

Thanks for the correction mate. You make a very valid point! I could definitely work on my terminology haha maybe the word hypothetical would have been more fitting?

It's a more philosophical point than that. Everything is a theory, at some level. The question is how much evidence and further explanation we can provide. For example, if you want to know why you don't fall through the floor I can talk about matter being made of atoms and objects as being collections of atoms bound together, and explain that you don't fall through the floor because of repulsion between the electron clouds of the atoms of your feet and the floor. I can relate all the bits of that claim to a great many observations related to electromagnetism and atomic physics that all are consistent with this idea. This is a very well understood concept that links to other parts of physics in a great many ways.

Dark energy, on the other hand, is just this: the expansion of the universe is accelerating because of the presence of dark energy. What is dark energy? It's the stuff that makes the expansion of the universe accelerate. We know no more about it than that. It's effectively a placeholder for our ignorance - we can't make our models match our measurements unless we add some stuff with certain properties. Now we have to go out and see if we can find any corroborating evidence. So, at the moment, it's an extremely weakly supported theory that doesn't really link into much else we understand. It's even possible it doesn't exist, and that this is all the beginnings of evidence for a new theory of gravity (although I don't know of any serious contenders).

how long have you studied this category of physics? If you don't mind me asking.

About ten years, somewhat haphazardly in my spare time. Note that I started from the relatively high level of having a physics degree (and a doctorate in a totally unrelated branch of physics), which helped a lot with the maths.

 

[MacWylie]

The bottom line is somewhere there has to be new space being added to the universe in a totally inexplicable mechanism you can’t keep claiming never ending points of view as sleight of hand to get around the reality if the universe is getting bigger then space is being created period.

 

[PeroK]

The bottom line is somewhere there has to be new space being added to the universe in a totally inexplicable mechanism you can’t keep claiming never ending points of view as sleight of hand to get around the reality if the universe is getting bigger then space is being created period.

Is that a quotation from "The Armchair Book of Physics" aka "How to understand the universe without really trying"?

 

[Ibix]

The bottom line is somewhere there has to be new space being added to the universe

No. That's a fundamental misunderstanding of what "expanding space" is supposed to be describing.

 

[berkeman]

Thread closed temporarily for Moderation and cleanup...

 

[berkeman]

After a bit of thread cleanup, the thread is reopened. Thanks for your patience.

 

[Tayler]

It's a more philosophical point than that. Everything is a theory, at some level.
Dark energy, on the other hand, is just this: the expansion of the universe is accelerating because of the presence of dark energy. What is dark energy? It's the stuff that makes the expansion of the universe accelerate. We know no more about it than that. It's effectively a placeholder for our ignorance - we can't make our models match our measurements unless we add some stuff with certain properties. Now we have to go out and see if we can find any corroborating evidence. So, at the moment, it's an extremely weakly supported theory that doesn't really link into much else we understand. It's even possible it doesn't exist, and that this is all the beginnings of evidence for a new theory of gravity (although I don't know of any serious contenders).

The way I perceive dark energy i think is close to what you just said. Like a caveman trying to understand what makes things fall down.

About ten years, somewhat haphazardly in my spare time. Note that I started from the relatively high level of having a physics degree (and a doctorate in a totally unrelated branch of physics), which helped a lot with the maths.

Thats awesome mate!

 

[Tayler]

The bottom line is somewhere there has to be new space being added to the universe in a totally inexplicable mechanism you can’t keep claiming never ending points of view as sleight of hand to get around the reality if the universe is getting bigger then space is being created period.

But wouldn't that mean new energy is also being created? From what I think i understand you can't add or take energy from the universe.

 

[Tayler]

The bottom line is somewhere there has to be new space being added to the universe in a totally inexplicable mechanism you can’t keep claiming never ending points of view as sleight of hand to get around the reality if the universe is getting bigger then space is being created period.

I think that violates the first law of thermodynamics.

 

[berkeman]

I think that violates the first law of thermodynamics.

Don't worry -- you were trying to figure out what a forum troll was saying. That distraction has been removed.

 

[PeterDonis]

wouldn't that mean new energy is also being created? From what I think i understand you can't add or take energy from the universe.

It's not that simple; energy conservation in GR does not mean quite what you think. See this article:

https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/

 

[Tayler]

It's not that simple; energy conservation in GR does not mean quite what you think. See this article:

https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/

Thanks Peter

 

[Tayler]

So if space isn't expanding inside of bound systems, what is happening to the particles in space that aren't in a bound system? Could they possibly be tearing apart or even expanding? Is it possible that the expansion has any effect on such particles or other matter?

 

[timmdeeg]

No. That's a fundamental misunderstanding of what "expanding space" is supposed to be describing.

The usual answer is "expanding space" means increasing distances.

One could argue that in an expanding universe the volume enclosed by 4 galaxies is growing and therefor additional space is created. So on one hand the (proper) volume is growing measurable in m³ but on the other the volume is growing according the growing scalefactor $a$ which is dimensionless however.

In other words GR doesn't predict growing (in the sense of additional) space but a consequence of the growing $a$ seems to be that any arbitrary volume enclosed by comoving points is growing. Isn't this a contradiction in itself and how do you resolve that?

 

[timmdeeg]

So if space isn't expanding inside of bound systems, what is happening to the particles in space that aren't in a bound system?

In this case these particles would be comoving and thus the proper distance between them would increase.

 

[Tayler]

In this case these particles would be comoving and thus the proper distance between them would increase.

So basically, in a square metre of space, the concentration of particles will decrease over time. Correct?
But would there be any significant change to the particle itself?

 

[Will Learn]

Hi again Tayler. Hope you are well.

what is happening to the particles in space that aren't in a bound system?

would there be any significant change to the particle itself?

Well, possibly. The space could be expanding as discussed earlier. However, all ordinary matter tends to create solutions to the Field Equations of General Relativity where space does not expand. It may even contract. So just by having that bit of matter which is your atom (various protons, neutrons and electrons) in a small region of space will tend to stop that little patch of space expanding.

It has previously been mentioned that we don't know much about "dark energy". However, even if something was driving the expansion of space where your atom is located, there are some other forces that can take over and pull all the sub-atomic particles through space to keep them together. I think it was Ibix who first mentioned electromagnetic interactions. There is also something called the strong force which holds sub-atomic particles together. It is extremely unlikely that all these forces can be overwhelmed.

 

[timmdeeg]

So basically, in a square metre of space, the concentration of particles will decrease over time. Correct?
But would there be any significant change to the particle itself?

Yes, within a volume measured in cubic meters the concentration of comoving particles would decrease. Counting them a certain time and then later at a certain time would reveal that. "At a certain time" is important because we talk about proper distances.
No, the particles would remain unchanged.

 

[Ibix]

So basically, in a square metre of space, the concentration of particles will decrease over time. Correct?
But would there be any significant change to the particle itself?

Cubic meter, but yes.

Nothing happens to the particles themselves. In a sense, the "expanding universe" is just like a cloud of particles expanding as if from an explosion (but note that there isn't a single point source for all matter in the universe). They just keep moving the way they do because there are no forces on them. But all that mass curves spacetime, and that has more complex effects.

The key problem with "space being created" is that general relativity models everything as spacetime. That's a 4d structure, and "the universe, now" is a 3d slice through it. "The universe, a second later" is a different 3d slice through spacetime. So it isn't that "space was created", but that you're thinking about successive slices through spacetime that have different scales.

 

[Tayler]

Cubic meter, but yes.

Nothing happens to the particles themselves. In a sense, the "expanding universe" is just like a cloud of particles expanding as if from an explosion (but note that there isn't a single point source for all matter in the universe). They just keep moving the way they do because there are no forces on them. But all that mass curves spacetime, and that has more complex effects.

The key problem with "space being created" is that general relativity models everything as spacetime. That's a 4d structure, and "the universe, now" is a 3d slice through it. "The universe, a second later" is a different 3d slice through spacetime. So it isn't that "space was created", but that you're thinking about successive slices through spacetime that have different scales.

Sorry haha I did mean m3.

So from what you just said I imagine spacetime is like a loaf of bread where each slice is bigger then the last and each slice contains the entire universe and is a different point in time. Each slice gets bigger but its contents stay unchanged. So hypothetically if you could weigh the entire universe at any given time or say, a slice from the first few minutes after the big bang and then a slice 2 billion years later, they would be the same weight, correct?

 

[Ibix]

So from what you just said I imagine spacetime is like a loaf of bread where each slice is bigger then the last and each slice contains the entire universe and is a different point in time. Each slice gets bigger but its contents stay unchanged. So hypothetically if you could weigh the entire universe at any given time or say, a slice from the first few minutes after the big bang and then a slice 2 billion years later, they would be the same weight, correct?

More or less, except all the slices are infinitely large as far as we know. So "space expanding" means that the distance between any two objects is growing at the same rate everywhere rather than that the slices are getting further apart. And we don't know any way to define "a total mass of the universe", so whether or not it's changing isn't known. With dark energy, whose density doesn't change and normal matter and radiation whose density does change, and curved spacetime to make the maths extra fun it's not the straightforward question you might assume.

 

[Tayler]

normal matter and radiation whose density does change,

Change how and from what?

 

[Will Learn]

I'm loving this discussion. You're inquisitve, Tayler.
How deep do you want go?

1. The argument about space being created (I think that was MacWylie who first mentioned it) is often presented as if space is being "injected" in from elsewhere. Between any two (distinct) points in space, more space is constantly being injected in. It's also possible not to worry about it and look at things the other way (Timdeeg was one of the first to mention this), space is just space but the scale factor changes. You might like to think of it as if the co-ordinate system is getting updated and modified as time progresses. Two points in space that are said to be "co-moving" just do have an increasing distance of separation as time progresses. The arguments are two sides of the same coin and I expect there are many other ways of looking at the situation.

I prefer thinking about the expansion of space with the idea of an evolving scale factor. It avoids all problems of considering space as "stuff" that has to be created from somewhere. Space doesn't have to be "stuff" it's just distance. There is some space between A and B just means there is some distance between A and B.

2. About the changing mass of time slices through spacetime:

So hypothetically if you could weigh the entire universe at any given time or say, a slice from the first few minutes after the big bang and then a slice 2 billion years later, they would be the same weight, correct?

This is complicated and I think Timdeeg has typed an answer while I was doing this one.
Start by replacing "weight" with mass. This is a minor technical change but it will help us. Mass is an intrinsic property of matter, while weight changes according to the gravitational field you're in. I don't think we could keep a total weight of the universe constant but there's some hope of keeping constant mass.
Next consider that mass is just a concentrated form of energy, so we'd be better off considering the total energy in the time slice. Mass can certainly change as time passes, nuclear reactions convert mass into energy, for example. Total energy at least has a better chance of being conserved.
Finally, see some comments that were mentioned earlier (I think it was Peter Donis who pointed at an article in the Preposterous universe blog).

Energy isn’t conserved; it changes because spacetime does. See, that wasn’t so hard, was it?

[Taken from https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/ ]​

So, ummmm... no. Hypothetically, the stuff in each time slice is different in many ways. However, the changes are predictable. A human being studying physics could identify a place in one time slice that would correspond to a place in another time slice. Your analogy about expanding slices of bread is quite good for this. Some things change their properties but that is also in a predictable way (e.g. blue light may change into red light), you could identify it as being the same light just at a later time.

Study physics and tell me the correct answers as they stand in 10 years time.

 

[timmdeeg]

@Ibix would you mind to comment on post #36? I'm not sure if I'm missing something.

 

[PeroK]

So basically, in a square metre of space, the concentration of particles will decrease over time. Correct?
But would there be any significant change to the particle itself?

Yes and no. In principle, yes; in reality, a cubic metre of intergalctic space is likely to contain no particles.

The important thing you are (still) missing is that expansion occurs on a cosmological scale, where galaxies themselves are considered as the particles (think big!). You can't apply the same equations and get expansion on the scale of anything smaller.

You should consider the galaxies and galaxy clusters as the point particles. The universal expansion drives these further apart.

The universal expansion simply does not apply even within galaxy clusters (although you may need to take the vacuum energy into account when looking at the dynamics). And the dynamics within a galaxy are almost totally matter (and dark matter) dominated.

You must somehow lose the idea that you can apply universal expansion to every cubic metre of space.

 

[PeroK]

PS in terms of cosmology, the basic unit of length is not the metre but the megaparsec, which is about 3 million light years.

 

[Ibix]

@Ibix would you mind to comment on post #36? I'm not sure if I'm missing something.

The first two paragraphs seem fine. The last seems to be rather confused.

There is a special class of observer in FLRW spacetimes - the so-called co-moving observer. These are people who see, and have always seen, the cosmic microwave background as isotropic. Since they see it as isotropic they have a decent claim (or as decent a claim as it is possible to make) to be "at rest with respect to the matter in the universe". (We're doing about 600km/s compared to co-moving observers, so we see the CMB blueshifted in one direction and redshifted in the other.)

But all of the matter, including all other co-moving observers, is moving away from them. So if all co-moving observers are what we define as stationary for the purposes of this discussion, yet they are moving apart, one way of conceptualising that is to say that there's more space between them - "space is expanding". The scale factor of the distance increase is $a$, as you say.

The direct measurable fact is that if a co-moving observer bounces a series of radar pulses off another one, the return time increases. Any (almost any?) sane coordinate system is going to take this fact and give you that the volume between four tetrahedrally arranged co-moving observers is increasing. It's the interpretation of this as "expanding space" that I think is a bit awkward. But I can't think of a better way to say it...

Does that help?

 

[Ibix]

Change how and from what?

Stuff is moving apart, so the density now is lower than it was yesterday (edit: on average, and very very slightly on that timescale). Radiation also redshifts (edit: ditto). That's all I meant.