B Do Atoms Expand with the Universe?

[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.