B Do Atoms Expand with the Universe?

[timmdeeg]

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...

That's the sore point I tried to address. We agree that the proper volume increases and at the same time we agree that no additional space is created. That's why I asked in #36 "Isn't this a contradiction in itself and how do you resolve that?". You seem to call that "rather confused" but what's the difference saying its "a bit awkward"?

I think I'm missing something. What am I missing?

It is clear that the views whether space expands vs. galaxies are moving away depends on the chosen coordinates, s. the FAQ in Ned Wrights tutorial I mentioned earlier. This clarifies that GR doesn't predict that space expands in the sense that additional space is created.

 

[Will Learn]

Hi Timdeeg.

I'm not sure who you're asking, looks like you targeted Ibix.
If you're throwing the question out to anyone...
1. That Ned Wright's FAQ doesn't tell you how they define space.
2. How are you defining space? Is it just some length (or length³) in the 3-D spatial manifold?
3. You've mentioned changing co-ordinates to fit two views on something else. Are you happy to change co-ordinates here to fit two different views about the creation of space vs. an alteration in the metric?

 

[Tayler]

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.

So "stuff" meaning matter? And are you saying the density of the space its contained in is moving apart, or of the physical object itself?

 

[PeterDonis]

all ordinary matter tends to create solutions to the Field Equations of General Relativity where space does not expand.

This is not correct. There is an expanding FRW spacetime solution with nothing but ordinary matter in it.

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.

This is wrong. Just having "matter" is not sufficient to create a bound system (unless you consider a proton or neutron to be a bound system since each contains quarks).

Also, "space expanding" is just a heuristic description, not an exact representation of the actual physics. "Space" is not a thing. This is an area where you can't just wave your hands and use heuristic reasoning if you want to get things right. You need to look at the math.

 

[PeterDonis]

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?

In a closed universe with a finite spatial volume, yes, there is a finite conserved quantity that more or less corresponds to what you are calling the "weight" of the universe.

In a spatially infinite universe, however, this "weight" is infinite, which is another way of saying "not well defined".

 

[Tayler]

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:

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.

Why thankyou will. As deep as I need to get a decent idea of the universe! I'm sure I'll learn a good .00000001% in a few years

 

[PeterDonis]

A human being studying physics could identify a place in one time slice that would correspond to a place in another time slice.

Sort of. The notion of "place" that we intuitively use is not well defined in an expanding universe. It is possible to use comoving worldlines to mark out "places" that can be compared at different times, but this implies that two objects that both "stay in the same place" also move apart (since "expanding universe" just means that comoving worldlines move apart with time). So one has to be careful with intuitive reasoning like this.

 

[PeterDonis]

are you saying the density of the space its contained in is moving apart, or of the physical object itself?

Neither. "Space" is not a thing, and "the space it's contained in" is not a meaningful concept.

 

[Tayler]

In a closed universe with a finite spatial volume, yes, there is a finite conserved quantity that more or less corresponds to what you are calling the "weight" of the universe.

In a spatially infinite universe, however, this "weight" is infinite, which is another way of saying "not well defined".

I think I'm starting to understand somewhat!

 

[timmdeeg]

1. That Ned Wright's FAQ doesn't tell you how they define space.

Yes, but that is not the intention of this FAQ

2. How are youdefining space? Is it just some length (or length³) in the 3-D spatial manifold?

I would say space is nothingness which has geometric properties.

3. You've mentioned changing co-ordinates to fit two views on something else. Are you happy to change co-ordinates here to fit two different views about the creation of space vs. an alteration in the metric?

I have mentioned said FAQ. It is agreed that expansion of the universe means increasing distances between galaxies which are far away from each other so that local peculiar velocities don't play a role. Ned Wright's FAQ explains that there are two views to understand the increasing distances. In one view, choosing FRW-coordinates, the galaxies stand still and space expands, in another view choosing other coordinates the galaxies move away from each other. The point is that neither view can be "true" physics. True physics is what we observe: increasing distances.

 

[PeterDonis]

I would say space is nothingness which has geometric properties.

You could use this as a heuristic definition of spacetime, but not space, since "space" is coordinate-dependent.

 

[timmdeeg]

You could use this as a heuristic definition of spacetime, but not space, since "space" is coordinate-dependent.

I see, thanks.

 

[Will Learn]

Hi.

There is an expanding FRW spacetime solution with nothing but ordinary matter in it.

Agreed. More than one, I would think. An Einstein-de Sitter universe comes to mind first of all - but if you add an inflationary phase at the beginnning of a matter dominated universe then presumably all such universes begin with $\dot {a}$ > 0.

Just having "matter" is not sufficient to create a bound system

It's not "sufficient" - but it tends to cause deceleration of expansion and favours contraction (I hope). This worries me if it's wrong and I can only apologise to everyone, especially Tayler who was seeking advice.

 

[phinds]

It's not "sufficient" - but it tends to cause deceleration of expansion and favours contraction (I hope). This worries me if it's wrong and I can only apologise to everyone, especially Tayler who was seeking advice.

no, I'm pretty sure matter ONLY decelerates expansion if it IS a bound system. Just random bits here and there woluld BE a bound system if there was enough mass for gravity to act against expansion/acceleration but otherwise just the presence of mass is not enough.

 

[PeroK]

It's not "sufficient" - but it tends to cause deceleration of expansion and favours contraction (I hope). This worries me if it's wrong and I can only apologise to everyone, especially Tayler who was seeking advice.

That's definitely not right. A matter-dominated universe may expand indefinitely, albeit with a declerating expansion. We need the density to be greater than the critical density for the universe eventually to start contracting.

https://astronomy.swin.edu.au/cosmos/C/Critical+Density

 

[PeterDonis]

More than one, I would think. An Einstein-de Sitter universe

Also has a cosmological constant/dark energy in it.

The model I was referring to is the expanding FRW model with just matter (i.e., perfect fluid with positive energy density $\rho$ and zero pressure, and nothing else). Technically I suppose there are three such models, corresponding to the three possibilities for spatial curvature/closure (closed--will recollapse, flat--will expand forever, open--will expand forever).

 

[Tayler]

All you guys are amazing people! I definitely wish to study advanced mathematics to get my head around all the equations and have more of an understanding. This stuff is great! Love it!

 

[Will Learn]

(Einstein de Sitter Universe) ... Also has a cosmological constant/dark energy in it.

Thanks but that means I've been reading a bad set of on-line notes again.
Einstein de Sitter was defined with $\Lambda$ = 0

[University of St Andrews, http://www-star.st-and.ac.uk/~spd3/Teaching/PHYS3303/obs_cos_lecture6.pdf ]​

A matter-dominated universe may expand indefinitely, albeit with a declerating expansion

...and also @phinds with a similar comment.
Thanks for these replies. A universe may expand idefinitely, sure that's fine - but matter does tend to cause deceleration. That's not too worrying.

@Tayler
All the guys? What about the girls? Anyway, best wishes to you (and everyone).

 

[PeterDonis]

Thanks but that means I've been reading a bad set of on-line notes again.
Einstein de Sitter was defined with $\Lambda$ = 0

[University of St Andrews, http://www-star.st-and.ac.uk/~spd3/Teaching/PHYS3303/obs_cos_lecture6.pdf ]​

This might be a terminology issue. I was thinking of something more like the Schwarzschild-de Sitter model, but it does look like "Einstein-de Sitter" is also used to refer to the flat, matter-only FRW model. Sorry for the confusion on my part.

 

[PeterDonis]

matter does tend to cause deceleration

Yes, that's correct.

 

[Tayler]

@Tayler
All the guys? What about the girls? Anyway, best wishes to you (and everyone).

I meant everyone! Haha where I'm from the word "guys" is used not so gender specific.

 

[Carpe Physicum]

Great discussion and I really appreciate your patience with us laypeople. The thing that always confused me was that the expansion is always described as NOT being like a balloon expanding within a large room so to speak. The room/balloon/all-there-is itself is expanding. To me that means the "rulers" are expanding too. I put that in double quotes because obviously we're not talking actual rulers, but it does seem to boil down to distance. But if you and me are one mile apart, and we moved two miles apart BUT our rulers also doubled in size, then we haven't really moved apart in any way we can tell. Even measuring angles of a giant triangle to observe their change depends on rulers of some sort.

I think from the discussion above the fancy way of saying this is that the metric is expanding as well. Just seems like if by "expansion" we mean distance, and distance is something you measure with a "ruler/metric" (again the double quotes) then you can't say we're expanding but NOT inside a larger room. Either that or we ARE saying that there IS a larger room within which the universe is expanding. Or maybe even simpler - expansion implies changing distance which implies an unchanging "ruler", which is only possible with reference to something outside the thing that's expanding. Is it my concept of ruler/distance that's off?

 

[Ibix]

I really appreciate your patience with us laypeople.

You're welcome (can't decide if I actually mean "thanks" here... ). It's a part of why we're here.

Is it my concept of ruler/distance that's off?

I think you are overthinking it. A physical ruler, like you might buy in a stationery shop, does not expand because it is a bound system - the atoms are held together by electromagnetic forces. So if you could lay a lot of 1m rulers between here and a galaxy a few hundred megaparsecs away, you would find gaps opening between the rulers (or a gap appearing at the end between the last ruler and the galaxy or something - it depends a bit how you set up the rulers). That's a way to conceptualise what we're talking about here. Chunks of matter do not expand, and nearby galaxies (which are bound together by gravity) do not get further apart, but galactic clusters do.

 

[Carpe Physicum]

You're welcome (can't decide if I actually mean "thanks" here... ). It's a part of why we're here.

I think you are overthinking it. A physical ruler, like you might buy in a stationery shop, does not expand because it is a bound system - the atoms are held together by electromagnetic forces. So if you could lay a lot of 1m rulers between here and a galaxy a few hundred megaparsecs away, you would find gaps opening between the rulers (or a gap appearing at the end between the last ruler and the galaxy or something - it depends a bit how you set up the rulers). That's a way to conceptualise what we're talking about here. Chunks of matter do not expand, and nearby galaxies (which are bound together by gravity) do not get further apart, but galactic clusters do.

Okay that makes sense. And that doesn't seem so complex an idea that it couldn't be included in various books about the subject for laypeople.

Follow on - what would it look like IF the bound systems themselves were expanding too (but such that we could still use them for measurement) but some other aspect wasn't expanding? I think of how we say spacetime is warped by gravity. I believe that doesn't mean it's physically curved like illustrations depict, but that something underlying the physical space is different from places where gravity is relatively absent (worldlines I believe it is). So what about a black hole for example? Could that be the observable effect of a local portion of s-t itself NOT expanding (bound systems and all)? I guess this is speculation and not allowed. But I just throw it out there.

 

[Ibix]

Okay that makes sense. And that doesn't seem so complex an idea that it couldn't be included in various books about the subject for laypeople.

Unfortunately, the aim of popsci is more around entertainment and building up public enthusiasm about science than teaching useful models. To be fair, professionals spend literally years studying and learning about this stuff and there's only so much you can say about so much in a short book, but it is a problem.

Follow on - what would it look like IF the bound systems themselves were expanding too (but such that we could still use them for measurement) but some other aspect wasn't expanding?

That isn't a question that can be answered because you're asking about how the universe would be if it weren't like it is. Who knows? You would have to invent a complete mathematical implementation of a consistent system of physics that had the properties you want - which may not even be possible.

I believe that doesn't mean it's physically curved like illustrations depict,

It isn't a rubber sheet, no, and such pictures are deeply misleading. However the curvature is directly measurable, so it's probably not correct to say that it isn't physically curved, whatever it actually is.

I guess this is speculation and not allowed.

Probably.

 

[phinds]

what would it look like IF the bound systems themselves were expanding too ...

https://www.physicsforums.com/insights/how-to-avoid-breaking-physics-with-your-what-if-question/

 

[Prishon]

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.

The very fact that atoms don't expand along makes that we can perceive expansion. Otherwise all distances between particles would grow ar equal rates making an expansion look as non-existing. Only if there were an outside you could see it grow.