<h3>Expansion depends on the amount of mass</h3>

<p>Yes, space expands more in regions with less matter — in fact this has been proposed as an alternative explanation to dark energy as the cause of the observed accelerated expansion: If by chance we happen to be located near the center of a "<a href="https://en.wikipedia.org/wiki/Void_(astronomy)" rel="noreferrer">cosmic void</a>", then nearby space expands faster than distant space, and since nearby space corresponds to recent times, this would be interpreted as recent accelerated expansion. This idea was popular some ten years ago (e.g. <a href="https://ui.adsabs.harvard.edu/abs/2007ApJ...664L..13C" rel="noreferrer">Conley et al. 2007</a>; <a href="https://ui.adsabs.harvard.edu/abs/2008PhRvD..78h4032W" rel="noreferrer">Wiltshire 2008</a>), but isn't to my knowledge so hot anymore.</p>

<p>Similarly, in regions with <em>more</em> matter, space expands less. In fact, on scales as small as galaxies, and even galaxy groups, space doesn't expand at all. Gravity prevents galaxies from expanding, and keeps galaxies near each other from receding. On even smaller scales, electromagnetic (and nuclear) forces keeps objects like stars, planets, and bicycles from expanding.</p>

<h3>Expansion is physical</h3>

<p>For this reason, space between two stars doesn't expand. So, to address your question, let's consider instead two <em>galaxies</em>, separated by millions of light-years. However, fundamentally we cannot place a ruler between them and watch it expand. Electromagnetic forces would <em>try</em> to hold it together, fail, and the ruler would break.</p>

<p>Why is this? Imagine a rigid ruler, one megaparsec long (i.e. 3.26 million light-years). Since the Hubble constant is <span class="math-container">$H_0 \simeq 70\,\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{Mpc}^{-1}$</span>, the region at the other end of the ruler recedes at <span class="math-container">$70\,\mathrm{km}\,\mathrm{s}^{-1}$</span>. That is, space "drags" the two ends of the ruler away from each other at <span class="math-container">$70\,\mathrm{km}\,\mathrm{s}^{-1}$</span>. If the ruler doesn't break, a local observer will see the end of the ruler whizzing by at high speeds.</p>

<p>Perhaps you might be able to construct a rod able to withstand this stress. If I were a solid state physicist I might be able to calculate the maximum possible length of a rod, given an optimal material, but I'm not. But the point is that, no matter what material you come up with it is not even practically impossible to make it arbitrarily long, but "theoretically" impossible. There is a fundamental limit to the length of a physical ruler, namely the length corresponding to the distance at which two regions of space recedes at the speed of light, i.e. <span class="math-container">$d = c/H_0 = 4400\,\mathrm{Mpc}$</span>, or <span class="math-container">$14.4\,\mathrm{Glyr}$</span>. If the ruler were longer than this and didn't break, then a local observer would see the end move by faster than the speed of light, which is not possible.</p>

<p>Luckily, there are other ways to measure distances, e.g. by "<a href="https://en.wikipedia.org/wiki/Cosmic_distance_ladder#Standard_candles" rel="noreferrer">standard candles</a>". And yes, if you measure the physical distance between the two galaxies at two different times, it will have increased in the meantime, physically.</p>

<p>(You could also imagine "freezing" space by magic, laying out 1-meter rods. In that case, second time you do it you will need more rods.)</p>

CC BY-SA 4.0
2019-12-27T10:54:11.097Z
Why do electromagnetic forces keep small objects from expanding but fail to do so for the intergalactic large ruler?
Hartmut Braun
CC BY-SA 4.0
2019-12-27T12:46:44.563Z
@HartmutBraun See my update :)
pela
CC BY-SA 4.0
2019-12-27T16:35:56.647Z
So you are saying: objects up to a size of, say, galaxies are dense enough to avoid expansion of spacetime *locally* around them. But on larger scales density is not high enough to counteract the global expansion of the universe (leaving dark energy aside it may still slow down expansion). In addition, a thin rod does not add enough mass (because it’s essentially too thin) and will be ripped apart. Right?
Hartmut Braun
CC BY-SA 4.0
2019-12-28T00:54:23.567Z
@HartmutBraun Yes, exactly! Except not only galaxies, but also somewhat larger scales avoid expansion, up to groups and clusters of galaxies. And yes, I'm neglecting the mass of the rod. If the rod has a large enough mass to affect expansion, it would probably start collapsing under its own weight, initiating nuclear fusion, essentially becoming a star.
pela
CC BY-SA 4.0
2019-12-28T08:30:53.627Z
Pela do you agree that there are several confusing things? That a galaxy does not expand might be apparent as for it being gravitationally bound but in a nevertheless expanding space. This is how I get things. Obviously as objects that are bound are also relatively close, the expansion of that region wouldn't be much. But this is another story. My point is: does the space containing, say, a galaxy does not expand or it does but gravity keeps things at the same distance? It becomes abstract but I am for the second picture.
Alchimista
CC BY-SA 4.0
2019-12-28T12:55:17.183Z
@Alchimista I thought the same way as you do but I never felt completely comfortable with that idea. That‘s exactly why I find pela‘s answer intriguing.
Hartmut Braun
CC BY-SA 4.0
2019-12-28T18:13:16.587Z
@Alchimista Okay so first a disclaimer: This is not my research area, but my interpretation is the following: Space and matter is intimately connected, as expressed by John Wheeler in his famous quote "_space tells matter how to move, and matter tells space how to curve_". If space expands, matter tends to follow, while if you insist on matter _not_ moving (by applying a force), space tends to stay. In regions where gravity (or other forces) are strong enough, space does _not_ expand [cont'd below]
pela
CC BY-SA 4.0
2019-12-28T18:13:41.287Z
Space doesn't expand inside a galaxy, and the distance between Earth and Sun doesn't increase, not even ever-so-slightly (well, it _does_ because Sun loses mass, but that's another story). But the distance at which we orbit _is_ ever-so-slightly larger than predicted by Kepler/Newton/GR alone; the equilibrium distance is slightly modified by the dynamics of space, so in a expanding space our orbit around Sun, and the size of a galaxy, and the distance between nearby galaxies, is slightly larger than it would be if space didn't expand.
pela
CC BY-SA 4.0
2019-12-28T18:13:54.900Z
(also @HartmutBraun)
pela
CC BY-SA 4.0
2019-12-29T07:38:10.610Z
@Hartmut Braun, and Pela of course. I came back to write more on my confusion considering ideal sticks and how do they behave and realized that I should have read your previous comments :) This things are really puzzling though I realise there is not much difference between a not expanding bubble containing things and an expanding one containing things that don't move relative to each others, and especially so in a - if it is - infinite universe.
Alchimista
CC BY-SA 4.0
2019-12-29T21:07:09.897Z
@Alchimista Well, there _is_ a difference between 1) "_not expanding bubble containing things_" and 2) "_an expanding one containing things that don't move relative to each other_" in the sense that in case #2, those thing will have a proper motion through space, as measured by a comoving observer. And the speed will have a limit, namely the speed of light.
pela
CC BY-SA 4.0
2019-12-30T08:09:52.530Z
Yes but I think that discerning the two is impossible as for nearby things are bound and far away ones are nevertheless within the Flow. The problem is being inside a bound region if you get my point.. No polemics here of course. :)
Alchimista
CC BY-SA 4.0
2019-12-30T13:55:50.373Z
@Alchimista I think I get your point :)
pela
CC BY-SA 4.0
2019-12-26T21:40:30.083Z
2019-12-27T12:46:32.293Z
2019-12-27T12:46:32.293Z
pela
<p>I'm trying to understand the very basis of the current cosmology.
I searched in the old questions but I found nothing that answers my questions specifically.</p>
<p>As far as I understand, we have to think the &quot;expansion&quot; more like a &quot;stretching&quot; of the space that makes room for &quot;new&quot; space. This is confirmed because the Universe is homogeneous and non-local.</p>
<p>First question: does the gravity counteract this expansion? In other words: does the space expand more in the areas with less gravity (i.e. among galaxies) and less in the nearby of stars and planets?</p>
<p>If we imagine to place a ruler between two stars, say at 10 ly, it would &quot;extend&quot; over time - even if we will still read 10 ly, right? But if now we place another ruler that was kept in our (huge) pocket, it will measure a longer distance. Is this correct?</p>
<p>That is because the first ruler has expanded together the space between the stars, while the second one endured the force of gravity and was kept rolled up so it wasn't able to expand.</p>

<expansion>
Does the gravity oppose to the Universe expansion?
Mark
# Does the gravity oppose to the Universe expansion?

(Asked by: username_0 on 2019-12-26)

I'm trying to understand the very basis of the current cosmology.
I searched in the old questions but I found nothing that answers my questions specifically.

As far as I understand, we have to think the "expansion" more like a "stretching" of the space that makes room for "new" space. This is confirmed because the Universe is homogeneous and non-local.

First question: does the gravity counteract this expansion? In other words: does the space expand more in the areas with less gravity (i.e. among galaxies) and less in the nearby of stars and planets?

If we imagine to place a ruler between two stars, say at 10 ly, it would "extend" over time - even if we will still read 10 ly, right? But if now we place another ruler that was kept in our (huge) pocket, it will measure a longer distance. Is this correct?

That is because the first ruler has expanded together the space between the stars, while the second one endured the force of gravity and was kept rolled up so it wasn't able to expand.

**Tags:** expansion

---

## 1 answers

### Answer by username_1 on 2019-12-26. Score: 8 | Accepted answer

### Expansion depends on the amount of mass

Yes, space expands more in regions with less matter — in fact this has been proposed as an alternative explanation to dark energy as the cause of the observed accelerated expansion: If by chance we happen to be located near the center of a "[cosmic void](https://en.wikipedia.org/wiki/Void_(astronomy))", then nearby space expands faster than distant space, and since nearby space corresponds to recent times, this would be interpreted as recent accelerated expansion. This idea was popular some ten years ago (e.g. [Conley et al. 2007](https://ui.adsabs.harvard.edu/abs/2007ApJ...664L..13C); [Wiltshire 2008](https://ui.adsabs.harvard.edu/abs/2008PhRvD..78h4032W)), but isn't to my knowledge so hot anymore.

Similarly, in regions with *more* matter, space expands less. In fact, on scales as small as galaxies, and even galaxy groups, space doesn't expand at all. Gravity prevents galaxies from expanding, and keeps galaxies near each other from receding. On even smaller scales, electromagnetic (and nuclear) forces keeps objects like stars, planets, and bicycles from expanding.

### Expansion is physical

For this reason, space between two stars doesn't expand. So, to address your question, let's consider instead two *galaxies*, separated by millions of light-years. However, fundamentally we cannot place a ruler between them and watch it expand. Electromagnetic forces would *try* to hold it together, fail, and the ruler would break.

Why is this? Imagine a rigid ruler, one megaparsec long (i.e. 3.26 million light-years). Since the Hubble constant is $H\_0 \simeq 70\,\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{Mpc}^{-1}$, the region at the other end of the ruler recedes at $70\,\mathrm{km}\,\mathrm{s}^{-1}$. That is, space "drags" the two ends of the ruler away from each other at $70\,\mathrm{km}\,\mathrm{s}^{-1}$. If the ruler doesn't break, a local observer will see the end of the ruler whizzing by at high speeds.

Perhaps you might be able to construct a rod able to withstand this stress. If I were a solid state physicist I might be able to calculate the maximum possible length of a rod, given an optimal material, but I'm not. But the point is that, no matter what material you come up with it is not even practically impossible to make it arbitrarily long, but "theoretically" impossible. There is a fundamental limit to the length of a physical ruler, namely the length corresponding to the distance at which two regions of space recedes at the speed of light, i.e. $d = c/H\_0 = 4400\,\mathrm{Mpc}$, or $14.4\,\mathrm{Glyr}$. If the ruler were longer than this and didn't break, then a local observer would see the end move by faster than the speed of light, which is not possible.

Luckily, there are other ways to measure distances, e.g. by "[standard candles](https://en.wikipedia.org/wiki/Cosmic_distance_ladder#Standard_candles)". And yes, if you measure the physical distance between the two galaxies at two different times, it will have increased in the meantime, physically.

(You could also imagine "freezing" space by magic, laying out 1-meter rods. In that case, second time you do it you will need more rods.)

> **Comments:**
> **username_2**: Why do electromagnetic forces keep small objects from expanding but fail to do so for the intergalactic large ruler?  
> **username_1**: @HartmutBraun See my update :)  
> **username_2**: So you are saying: objects up to a size of, say, galaxies are dense enough to avoid expansion of spacetime *locally* around them. But on larger scales density is not high enough to counteract the global expansion of the universe (leaving dark energy aside it may still slow down expansion). In addition, a thin rod does not add enough mass (because it’s essentially too thin) and will be ripped apart. Right?  
> **username_1**: @HartmutBraun Yes, exactly! Except not only galaxies, but also somewhat larger scales avoid expansion, up to groups and clusters of galaxies. And yes, I'm neglecting the mass of the rod. If the rod has a large enough mass to affect expansion, it would probably start collapsing under its own weight, initiating nuclear fusion, essentially becoming a star.  
> **username_3**: Pela do you agree that there are several confusing things? That a galaxy does not expand might be apparent as for it being gravitationally bound but in a nevertheless expanding space. This is how I get things. Obviously as objects that are bound are also relatively close, the expansion of that region wouldn't be much. But this is another story. My point is: does the space containing, say, a galaxy does not expand or it does but gravity keeps things at the same distance? It becomes abstract but I am for the second picture.  
> **username_2**: @Alchimista I thought the same way as you do but I never felt completely comfortable with that idea. That‘s exactly why I find pela‘s answer intriguing.  
> **username_1**: @Alchimista Okay so first a disclaimer: This is not my research area, but my interpretation is the following: Space and matter is intimately connected, as expressed by John Wheeler in his famous quote "_space tells matter how to move, and matter tells space how to curve_". If space expands, matter tends to follow, while if you insist on matter _not_ moving (by applying a force), space tends to stay. In regions where gravity (or other forces) are strong enough, space does _not_ expand [cont'd below]  
> **username_1**: Space doesn't expand inside a galaxy, and the distance between Earth and Sun doesn't increase, not even ever-so-slightly (well, it _does_ because Sun loses mass, but that's another story). But the distance at which we orbit _is_ ever-so-slightly larger than predicted by Kepler/Newton/GR alone; the equilibrium distance is slightly modified by the dynamics of space, so in a expanding space our orbit around Sun, and the size of a galaxy, and the distance between nearby galaxies, is slightly larger than it would be if space didn't expand.  
> **username_1**: (also @HartmutBraun)  
> **username_3**: @Hartmut Braun, and Pela of course. I came back to write more on my confusion considering ideal sticks and how do they behave and realized that I should have read your previous comments :) This things are really puzzling though I realise there is not much difference between a not expanding bubble containing things and an expanding one containing things that don't move relative to each others, and especially so in a - if it is - infinite universe.  
> **username_1**: @Alchimista Well, there _is_ a difference between 1) "_not expanding bubble containing things_" and 2) "_an expanding one containing things that don't move relative to each other_" in the sense that in case #2, those thing will have a proper motion through space, as measured by a comoving observer. And the speed will have a limit, namely the speed of light.  
> **username_3**: Yes but I think that discerning the two is impossible as for nearby things are bound and far away ones are nevertheless within the Flow. The problem is being inside a bound region if you get my point.. No polemics here of course. :)  
> **username_1**: @Alchimista I think I get your point :)  

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