Expansion of space & expansion of matter within space

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Main Question or Discussion Point

Space evidently is expanding, so we say that the very fabric of space time is expanding uniformly in all directions, so two questions

1)originally this expansion was driven by the energy content within the universe aka the dense hot matter , versus nowdays the acceleration is picking up speed and since visible and known matter doesn't account for that alone we plug in the terms dark matter and dark energy to our cosmological model as a explanation (although the wording dark indicates an unknown subject)

2) this was bit tricky for me so I decided to ask, if space is expanding the matter within the space should also expand should it not? Like for example everything from the very tiny such as subatomic level to the very large like galaxies and planets and stars should also increase in their size by some portion since space is expanding uniformly everywhere similarly right? Now do we not see this expansion in matter but see it through light/EM radiation simply because for systems such as galaxies and planets and stars and even atoms the other forces like gravity and electrostatic repulsion/attraction, EM fields etc are much more stronger than any effect from the expansion so these forces cancel out any potential effects?


thank you.
 

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  • #2
PeterDonis
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Space evidently is expanding
No, it isn't. Galaxy clusters in the universe are moving apart, but that's not the same thing as "space expanding".

so we say that the very fabric of space time is expanding uniformly in all directions
No, we don't. Spacetime doesn't "do" anything; it just is. It is a 4-dimensional geometric object.

originally this expansion was driven by the energy content within the universe
No, it wasn't, it was driven by the fact that the matter and radiation in the universe at the end of inflation was rapidly expanding to begin with.

nowdays the acceleration is picking up speed
No, the acceleration is positive, but constant.


since visible and known matter doesn't account for that alone we plug in the terms dark matter and dark energy to our cosmological model as a explanation
Dark energy is the name for whatever it is that is causing accelerated expansion. Dark matter is something different.

if space is expanding the matter within the space should also expand should it not
The question is based on a mistaken premise (that "space is expanding", see above). Also, the "expansion" of the universe--galaxy clusters moving apart--does not apply to gravitationally bound objects. So the Earth, the solar system, our galaxy, and the galaxy cluster that our galaxy is a part of, being gravitationally bound objects, are not expanding.
 
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@PeterDonis you yourself use the word accelerated expansion but since you also say the expansion is constant then from also the other sources which I read I can conclude that the expansion is constant but faster than it would be if we only accounted for the known forces in the universe, namely gravity, I guess this should be a less confusing statement?

Because when I read in literature the name accelerated expansion it gives the impression that something is "accelerating" aka becoming increasingly larger/faster would you agree?


As for expansion I get the fact that the space unlike a balloon can't expand into anything surrounding it and also unlike a balloon there is no more matter being pushed into it which drives the expansion because in a balloon we add matter/gas and energy to drive the expansion but in the universe the expansion is driven by what already is within the universe correct?

That being said , you note that instead of the fabric expanding galaxies and objects are moving further apart , but as far as I read this happens uniformly in all places so the space itself also has to become larger otherwise the galaxies moving apart would instead "pile up" at some outer (I assume unknown and unseen) theoretical regions of space, because the way I get this is that having more space between each object also means having more total volume.
 
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  • #4
timmdeeg
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@PeterDonis you yourself use the word accelerated expansion but since you also say the expansion is constant then from also the other sources which I read I can conclude that the expansion is constant but faster than it would be if we only accounted for the known forces in the universe, namely gravity, I guess this should be a less confusing statement?
Please read very careful, @PeterDonis said: "No, the acceleration is positive, but constant." He did not say the expansion is constant.
As for expansion ...
Expansion means that the distances between comoving objects are increasing. For more please read Are galaxies really moving away from us or is space just expanding
 
  • #5
PeterDonis
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you yourself use the word accelerated expansion but since you also say the expansion is constant
As @timmdeeg pointed out, I did not say "the expansion is constant".

in the universe the expansion is driven by what already is within the universe correct?
No.

space itself also has to become larger otherwise the galaxies moving apart would instead "pile up" at some outer (I assume unknown and unseen) theoretical regions of space
This is not correct.

the way I get this is that having more space between each object also means having more total volume
According to our best current model, the universe is spatially infinite so its total volume is infinite. That means it doesn't change with time--it was infinite before, it's infinite now, and it will be infinite in the future.

There is an FRW spacetime--the closed universe--in which each spacelike hypersurface of constant time has a finite total volume (each such hypersurface is a 3-sphere), so for that particular case, you can say that the total volume increases with time. However, that doesn't justify your arguments about "having more space" due to expansion, since your arguments, if they were correct, would not apply only to the case of the closed universe, they would apply to any FRW universe, including spatially infinite ones.
 
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phinds
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@artis your mistaken understanding of cosmology is COMPLETELY normal for folks who have gotten their "knowledge" from pop-science presentations. We constantly have to correct this kind of "knowledge". Pop-science presentations are ENTERTAINMENT, not education.

You can find hundreds of threads on this forum where exactly the points you raise have been debunked.
 
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  • #7
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@PeterDonis oh my bad , I totally got your comment about constant positive acceleration wrong, I guess you just wanted to say that the acceleration of expansion is constant instead of exponential where it would pick up "speed" as it went ahead, thanks @timmdeeg for pointing this blatantly obvious mistake out I totally got carried away in midst of my other thoughts.

@phinds yup you are correct, simply due to a lack of time I hadn't had the chance to dive into each scientific subject as much as I would like so although I like cosmology I have mostly only read popular science in this subject with some additional info from places like PF. That being said I do check the sites I read and I usually tend to read university sites and NASA related etc links but still this is complicated enough.
 
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Now this would probably need it's own thread but just to get a glimpse of whether I have got the idea even remotely precise I will dare to ask here.
I think that one of the misconceptions that I have (seemingly others as well) is thinking of space as this sphere and all the galaxies and matter within the space as some objects located within the sphere as opposed to thinking of the Lambda CDM Big bang model as the space-time being a sphere and everything within the universe as being on the surface of the sphere spread out almost evenly across the surface , and looking from a time domain picture the expansion of space is just a history of this sphere being smaller and in each next frame getting larger and that means the distances between everything on the surface of the sphere increase , am I going in the right direction here of thinking of space in this 4D sphere model?

If this is correct then that would explain to me tings that are often mentioned like that there is no special place in the universe that could be labeled "center" because everything is everywhere spaced out evenly across a expanding surface. right or wrong?


Again looking from the time frame space would be like a sphere within a sphere with each next frame having a larger spherical surface so they progress like the Russian famous "matroska" dolls one within the other with no limit to how big or far one can go?


Am I right here or off the mark?
 
  • #9
phinds
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Now this would probably need it's own thread but just to get a glimpse of whether I have got the idea even remotely precise I will dare to ask here.
I think that one of the misconceptions that I have (seemingly others as well) is thinking of space as this sphere and all the galaxies and matter within the space as some objects located within the sphere as opposed to thinking of the Lambda CDM Big bang model as the space-time being a sphere and everything within the universe as being on the surface of the sphere spread out almost evenly across the surface , and looking from a time domain picture the expansion of space is just a history of this sphere being smaller and in each next frame getting larger and that means the distances between everything on the surface of the sphere increase , am I going in the right direction here of thinking of space in this 4D sphere model?

If this is correct then that would explain to me tings that are often mentioned like that there is no special place in the universe that could be labeled "center" because everything is everywhere spaced out evenly across a expanding surface. right or wrong?


Again looking from the time frame space would be like a sphere within a sphere with each next frame having a larger spherical surface so they progress like the Russian famous "matroska" dolls one within the other with no limit to how big or far one can go?


Am I right here or off the mark?
Completely off the mark although I think you're getting towards the right idea. I suggest the link in my signature. It should clear up the whole thing.
 
  • #10
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was the main mistake in using the sphere which I now see amounts to a closed spatial geometry universe although I guess the accepted geometry within the lambda CDM model is that of flat Euclidean space? So by this I mistakenly applied CDM model parameters to a non CDM model geometry?

I am looking at your link yes, although too early to speak of any results
 
  • #11
PeterDonis
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I guess you just wanted to say that the acceleration of expansion is constant instead of exponential where it would pick up "speed" as it went ahead
This is still wrong, or at least garbled. Constant acceleration means exponential expansion. Or, using precise math instead of vague ordinary language: the "acceleration" being referred to is ##\ddot{a} / a##, i.e., the second derivative of the scale factor with respect to time, divided by the scale factor. If this is constant, i.e., ##\ddot{a} / a = K## for some constant ##K##, then the scale factor itself increases exponentially with time, ##a = a_0 \exp(\sqrt{K} t)##. (Direct computation of ##\ddot{a} / a## from the expression for ##a## I just gave will confirm what I've said.)
 
  • #12
PeterDonis
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was the main mistake in using the sphere which I now see amounts to a closed spatial geometry universe
No, it doesn't, because you are thinking of "sphere" as a 2-sphere, with some boundary very far away (many billions of light years) from us; but the closed universe is a 3-sphere, with no boundary but a finite total spatial volume.
 
  • #13
PeterDonis
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the Lambda CDM Big bang model as the space-time being a sphere
No, this is not correct. Spacetime is not a 4-sphere in the Lambda CDM model.
 
  • #14
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yes @PeterDonis I just realized my mistake with regard to this and corrected it in the post that was made at the same time as yours

by the way is there a graph that represents the expansion parameter that you just spoke of?
 
  • #15
PeterDonis
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is there a graph that represents the expansion parameter that you just spoke of?
Do you mean the scale factor? Just graph an exponential.
 
  • #16
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I think I found the graph in wikipedia here
https://en.wikipedia.org/wiki/Hubble's_law#/media/File:Friedmann_universes.svg

So originally Hubble only saw redshifts and this served as major evidence for the Big bang model to extrapolate back to past and model the universe as expanding but only later we found out that further away stars have larger redshifts than closer ones so then the scale factor turned from linear to exponential?

And to account for this accelerated expansion , dark matter was proposed?
 
  • #17
PeroK
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And to account for this accelerated expansion , dark matter was proposed?
Dark energy!
 
  • #18
PeterDonis
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think I found the graph in wikipedia here
That graph shows how various different models of the universe extrapolate what we observe today back into the past and out into the future. It does not show anything that would allow you to tell which model matches the actual data we have better.

later we found out that further away stars have larger redshifts than closer ones so then the scale factor turned from linear to exponential?
No. Further away stars will have larger redshifts than closer ones on any model of expansion. The redshift is basically a measure of how much the universe has expanded since the light was emitted. A star further away will have emitted its light earlier so the universe will have expanded more, so its redshift will be larger.

What later measurements (in the 1990s) found was that the precise relationship between redshift and other observable parameters (such as brightness and angular size) indicated that the expansion of the universe stopped decelerating and started accelerating a few billion years ago.
 
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