This is a question I thought of about the expansion of space. Any astrophysicists around to answer this for me?

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Please understand that im 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.
 

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  • #2
Ibix
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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...
 
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  • #3
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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 👌
 
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Ibix
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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.
 
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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.
 
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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 modelled 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 tendancy 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.
 
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  • #7
timmdeeg
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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

 
  • #8
hmmm27
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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.
 
  • #9
PeterDonis
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"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.
 
  • #10
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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. Im learning very fast that 1 answer = 2 more questions
 
  • #11
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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 modelled 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 tendancy 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.
 
  • #12
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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 👌
 
  • #13
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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 😂
 
  • #14
timmdeeg
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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.
 
  • #15
PeroK
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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.
 
  • #16
PeroK
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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.
 
  • #17
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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?
 
  • #18
timmdeeg
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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.

 
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  • #19
timmdeeg
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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.
 
  • #20
PeterDonis
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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.
 
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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. Im a basic man haha
 
  • #22
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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?
 
  • #23
Ibix
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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 in to 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.
 
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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.
 
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  • #25
PeroK
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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"?
 
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