Is the cosmic expansion uniform everywhere?

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I seek a little help in understanding why we can see so far into the past to be able to view events where the light from these events should have long ago overtaken wherever the masses ended up.

I get it that there was a considerable time during which the limits of the space expansion exceeded the speed of light, and that it is is reasonable that there are galaxies that ended up so far from us that the time for us to get a view of them in any state of accretion may mean we never see some of them, and those we do see may no longer be there now.

If the universe is expanding, so that the space between all galaxies is increasing, and there is a red shift between a view of any one from any other (occasional blue shift excepted), then are the local stars within the galaxies similarly being set farther apart?

By extension, if the expansion is uniform everywhere, then right down to the length of a football field, are the goal posts getting farther apart?

Is the red shift that we see because of the expansion, or because the those farthest from us are only so because they always had the higher speeds from the beginning, and some were slower (why?), or is it maybe a combination of both?

Please forgive that my questions are so basic. There are many excellent science documentaries, and YouTube educational videos, graphically showing the universe from it's beginnings, but never quite making clear how if the space itself is expanding, whether the distances between things, and their very sizes are expanding with it, everywhere, without exception.
 

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  • #2
DaveC426913
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then are the local stars within the galaxies similarly being set farther apart?
By extension, if the expansion is uniform everywhere, then right down to the length of a football field, are the goal posts getting farther apart?
No and no.

The expansion of the universe on a cosmic scale is an exceedingly small effect. So small that even the weak gravity holding galaxies in clusters overwhelms it. It's really only seen in the vast distance between whole galaxy clusters, where gravity is near zero to several decimal places.

Think about blowing up a balloon. Glue a bunch of pennies to the balloon, then blow it up. Do the pennies get larger? No. Not even a little bit.

The forces that act to keep the pennies as solid units far overwhelm the force of the balloon trying to "tear" them apart.

Though only held together by gravity, one could say that - as far as cosmological expansion is concerned - galaxies are solid objects.
 
  • #3
Orodruin
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I get it that there was a considerable time during which the limits of the space expansion exceeded the speed of light, and that it is is reasonable that there are galaxies that ended up so far from us that the time for us to get a view of them in any state of accretion may mean we never see some of them, and those we do see may no longer be there now.
The expansion is not a speed, it is a rate. However, the separation speed may exceed the speed of light because it is not due to anything moving.

Why do you think that the light should have overtaken us? If you consider a single object that is continuously emitting light in our direction and has been doing so forever, then there is a continuous beam of light betweeen us and that object. While the light emitted now may never reach us, the light that is already closer will, but this does not stop the beam from being continuous. What effect do you imagine would cut the beam?


Is the red shift that we see because of the expansion, or because the those farthest from us are only so because they always had the higher speeds from the beginning, and some were slower (why?), or is it maybe a combination of both?
The "speeds" you are referring to are not really speeds due to objects moving. That being said, you really cannot disentangle these two. The reason is that those interpretations are based on particular set of coordinates and the interpretations do not survive a coordinate change. In other words, there are alternative descriptions of the exact same thing where the interpretations change. I wrote a PF Insight about this, but it was aimed at an A level audience.

There are many excellent science documentaries, and YouTube educational videos, graphically showing the universe from it's beginnings, but never quite making clear how if the space itself is expanding, whether the distances between things, and their very sizes are expanding with it, everywhere, without exception.
What do you mean by "excellent"? It is certainly not the case that you can learn science from them. The only way that popular science can be excellent is in terms of entertainment value and teaching laymen about science and what scientists are currently up to. If you want to understand actual science you need to pick up a textbook.
 
  • #4
PAllen
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Well, among other things you can find on YouTube are the complete lectures of Sidney Coleman for a course on QFT. YouTube contains everything from the most sublime to the most idiotic.
 
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Well, among other things you can find on YouTube are the complete lectures of Sidney Coleman for a course on QFT.
I think we both know that the OP was not watching lectures by Coleman ...
 
  • #6
PAllen
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I think we both know that the OP was not watching lectures by Coleman ...
Ah yes. But lighter than Coleman's QFT lectures, anyone with basic intro to QM or more, would love watching Coleman's "Quantum Mechanics In Your Face" on youtube. Rivals anything by Feynman, IMO.
 
  • #7
phinds
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I seek a little help in understanding ...
I suggest the link in my signature
 
  • #8
I have a somewhat related thought experiment I'd like to offer up. Sorry if this is redundant of another post. I looked and couldn't find anything similar.

If there were a string(of the twine variety not the 11-dimensional variety) that was a billion light-years long that was laid out across open space with no initial net tension would it have tension induced over time due to the expansion of the universe? If so, at what scale would tension become zero and why?

I understand expansion is an extremely small force that doesn't apply to scales such as the space between particles/atoms/people/etc. but I wonder if it truly has zero effect at all or is it that the effect is just so tiny that the it's useless to talk about. The wiki page on expansion regarding small scale objects seems to say there would be an effect on an objects equilibrium state, albeit undetectable. Also, it seems to imply that popular theories suggest an ever-increasing rate of expansion. With an eventual(I'm guessing about a Grahams number worth of years from now) near infinite expansion rate would that have an effect down to the particle level?(assuming there would be any particles still around that haven't decayed)
https://en.wikipedia.org/wiki/Expansion_of_the_universe#Effects_of_expansion_on_small_scales

Sorry for basing some of my thoughts on wiki....newbie-status excuse? ¯\_(ツ)_/¯
 
  • #9
phinds
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I have a somewhat related thought experiment I'd like to offer up. Sorry if this is redundant of another post. I looked and couldn't find anything similar.

If there were a string(of the twine variety not the 11-dimensional variety) that was a billion light-years long that was laid out across open space with no initial net tension ...
You have to specify how the string gets there. Magic ("it just suddenly appears") is not allowed. When you figure out how to put the string there, you'll have your answer. It's likely more complicated than you have considered.
 
  • #10
Oops, sorry. I was envisioning something like two astronauts unspooling the twine over a very long distance for a very long time then cutting it. So long as it doesn't screw up the spirit of the thought I'd prefer to think of a hypothetical void in our universe(not necessarily our observable universe) where gravity, dark flow, or any other variable other than expansion are essentially null.

I think I see what you are getting at and I'm not thinking that the ends of the twine would instantly move away from each other at light speed as though the twine popped into existence 13.8Bly away. I understand it would take infinite energy/time to get something of mass that far away and then slowed down. I'm thinking something more like the unspooling is at 0.01c for 100B years then release.
 
  • #11
Bandersnatch
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This looks very much like the tethered galaxy problem. In short, what happens depends on whether the expansion is accelerating or decelerating. The expansion alone doesn't cause the the astronauts to either approach or recede (so tension in the rope doesn't change). If the universe is accelerating, the astronauts move away from each other. If it's decelerating, they move closer. In the specific case of an empty universe, mentioned above, their initial proper distance doesn't change.

This paper analyses the though experiment:
Solutions to the tethered galaxy problem in an expanding universe and the observation of receding blueshifted objects; Davis, Lineweaver, Webb
See if you can grok it. Even if you skip over the maths, you can get the gist from the wordy bits. You might need to brush on some terminology and concepts, though.
Also, I'm almost certain there were some earlier discussions about this on PF, that might be of some value. Try searching the forum for 'tethered galaxy problem'. Or come back with more questions, if that's not sufficient or over your head.

The wiki page on expansion regarding small scale objects seems to say there would be an effect on an objects equilibrium state, albeit undetectable. Also, it seems to imply that popular theories suggest an ever-increasing rate of expansion. With an eventual(I'm guessing about a Grahams number worth of years from now) near infinite expansion rate would that have an effect down to the particle level?(assuming there would be any particles still around that haven't decayed)
That effect is constant in a universe with constant dark energy, like ours looks to be. It corresponds to astronauts tethered by a short rope having tiny tension in it, but that tension never increases, so the bound system does not become unbound. The Big Rip scenario you seem to allude to - unbinding even particles - is only possible if dark energy grows with time.
Constant dark energy makes expansion accelerate, eventually doing so exponentially, but the rate of expansion (i.e. H) decreases, approaching a constant value. Expansion with a constant rate is exponential. But locally it's just those astronauts with their imperceptibly tensioned ropes.
 
  • #12
This is all perfect! I figured there had to be a common name for this idea just didn't know how to phrase it...the "expanding universe infinite twine" google/PF search wasn't getting me anywhere and PBS spacetime hasn't mentioned it yet :P

I love getting into the weeds on maths and learning some new stuff :) Thanks for the direction and fast, concise response!
 
  • #13
Grinkle
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I seek a little help in understanding why we can see so far into the past to be able to view events where the light from these events should have long ago overtaken wherever the masses ended up.
This opening question in your post suggests to me that you picture the early universe as finite in extent. If the early universe is very dense and infinite in extent, then it seems obvious why the further out we look the older the structures we are seeing. Its not as though masses had to race away from where we are now, and emit light from where the masses ended up, to use your phrasing. There was always stuff way out there - it just gets less dense as time increases.
 
  • #14
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Hmmm.. The cosmological principle and homogeneity of space holds for the universe on a large scale.. But fails when it comes to smaller distances, like the distance between the earth and other planets, the milky way and Andromeda. On a small scale as stated in your question, the goal post does not go farther apart.. So to answer the general question, on a larger scale the expansion is homogeneous (the same everywhere), but on a small scale such expansion is difficult to observe.
 
  • #15
DaveC426913
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... on a small scale such expansion is difficult to observe.
Not merely difficult to observe.

We expect it is not happening at all at small scales. Planets, stars and galaxies are gravitationally bound. They are not being slowly pulled apart by cosmic expansion - any more than the Earth is being slowly pulled apart by the Sun.
 
  • #16
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Not merely difficult to observe.

We expect it is not happening at all at small scales. Planets, stars and galaxies are gravitationally bound. They are not being slowly pulled apart by cosmic expansion - any more than the Earth is being slowly pulled apart by the Sun.
Good point, noted..
 

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