<p>The Universe is remarkably homogeneous, i.e. the same everywhere, and isotropic, i.e. the same in every direction (NB homogeneity does not imply isotropy and there are toy cosmological models which are homogeneous but not isotropic) and this is the underlying physical assumption of big bang cosmology.</p>

<p>When modelling the Universe the simplification of it being 100% homogeneous and isotropic is most often used as because on the very largest scale (i.e. the scale of the observable Universe) this is very close to being true and any deviations from this are small enough that they do not effect most of the properties being studied. In 100% homogeneous and isotropic Universe, expansion takes place at all places and at all scales.</p>

<p>However we know that the Universe is not 100% homogeneous and isotropic, especially on a smaller scale; for example the centre of a star is a very different place from intergalactic space. It is a fair question then to ask in view of this how does expansion manifest itself on smaller scales.</p>

<p>We know that in gravitationally bound systems such as our local group (which includes Andromeda) matter has the tendency to be drawn closer by gravity rather than to move away as objects do in an expanding Universe, so it seems our local group is not expanding. Still, it is very tempting to see expansion on this scale as being like a small repulsive force which acts against gravity, but is not strong enough to overcome gravity. So in this sense even the local group would be expanding</p>

<p>However the view of expansion acting like a small repulsive force on a small scale is not necessarily the right view. A counterexample would be the Einstein-Strauss Swiss-cheese model where expansion only takes place in deep space and not in the space around stars (modeled as vacuoles). The Einstein-Strauss model though cannot be taken as the last word on the subject as the way it models stars in space is not that realistic. </p>

<p>Overall then how expansion manifests itself on a smaller scale is still very much an open question and we just don't know what tiny corrections if any it would make on the dynamics of a very small system such as the solar system.</p>

CC BY-SA 4.0
2018-10-30T20:19:39.463Z
how does the ESSC model differ from a small repulsive force?  The small repulsive force leveled by gravity 'seems' descriptive, since the primary differentiator between inter-galactic space and intra-galactic space is precisely that - matter/gravity-field density
theRiley
CC BY-SA 3.0
2015-03-14T08:06:22.637Z
2015-03-14T08:06:22.637Z
John Davis
<p>Actually, space is expanding everywhere. It is more noticeable between galaxies since space between galaxies is much larger than space between stars and space between the planets in the solar system.
A 1% expansion of space only represents 1.5 million kilometers between the Sun and the Earth.  But the same 1% expansion between two galaxies that are four billion light-years away means that they are now 40 light-years farther away.
And 40 light-years is ten times the distance from the Sun to the nearest star Proxima Centaury.</p>

CC BY-SA 3.0
2015-03-14T00:19:50.210Z
Yet the closest Galaxy to us, Andromeda, is moving towards us and not away from us ?
Peter U
CC BY-SA 3.0
2015-03-14T07:58:51.867Z
@PeterU Andromeda is moving through space faster than space expands between it and us.
LocalFluff
CC BY-SA 3.0
2015-03-14T10:21:23.127Z
Actually, it is not true that space is expanding everywhere. Space in NOT expanding on such small scales as the Solar system, and not even on galactic scales. If it did, it would mean that when Earth was formed it would have been much closer to the Sun than it is today, and for example molecular clouds would have a much higher density in the early Universe, both of which we know is not the case. I think John Davis' answer it good. Space and matter is "bound" to each other, matter curves space in such a way as to prevent expansion on small scales.
pela
CC BY-SA 3.0
2015-03-14T14:22:42.507Z
What is the cause of this expansion, is it still from the original "Big Bang" ?
Peter U
CC BY-SA 3.0
2015-03-14T14:26:03.990Z
@LocalFluff What is pulling Andromeda towards the Milky Way Galaxy ? Is it the gravitational force of the Milky Way ? The Milky Way must be bigger than Andromeda because otherwise it would be pulling us towards it instead , is this correct ?
Peter U
CC BY-SA 3.0
2015-03-14T14:36:42.057Z
The original cause of expansion was Big Bang, or rather inflation (we think). But that expansion slowly decreased in magnitude due to the mutual gravitational attraction of matter (mostly dark matter). Recently, however, another effect took over, namely dark energy, which causes the Universe to accelerate its expansion (we think). Wrt. how space is "attached" to matter, see also [my previous post here](http://physics.stackexchange.com/questions/164866/expansion-of-the-universe-in-everyday-live/164872#164872). Concerning Andromeda, both attract each other, it's just a matter of reference frame.
pela
CC BY-SA 3.0
2015-03-14T14:58:22.730Z
@PeterU It is the bending of space that makes Andromeda and the Milky Way fall toward each other. They don't "pull", that was the Newtonian idea scrapped a century ago. They "fall" down curved spacetime. Space is bent by the mass of matter nearby. Mostly by unknown dark matter. But then there's also unknown dark energy which accelerates the expansion of space. I don't know, I "learn" mostly by watching lectures held by persons who obviously never really left their student lifestyle. It's a most intriguing reality TV show anyway. The one who predicts how it ends wins the Nobel Prize.
LocalFluff
CC BY-SA 3.0
2015-03-14T15:28:15.647Z
Gravitational bounding negates expansion
Mithoron
CC BY-SA 3.0
2015-03-13T22:41:41.627Z
2015-03-13T22:41:41.627Z
Carlos
<p>If Space is expanding between the Galaxies then why isn't it also expanding between the Stars within the Galaxies as well ? In fact why isn't Space expanding within our Solar System ?</p>

CC BY-SA 3.0
2016-10-28T15:56:56.253Z
Similar: http://astronomy.stackexchange.com/questions/18609/in-km-h-what-actually-is-the-speed-of-andromeda-away-from-us-cosmologically
Fattie
CC BY-SA 4.0
2018-10-30T20:16:18.073Z
i came here to ask that very question!
theRiley
CC BY-SA 3.0
2015-03-14T15:35:16.757Z
related http://astronomy.stackexchange.com/questions/1530/contradict-the-expansion-of-the-universe?rq=1
Mithoron
<space><expansion>
Expansion of Space
Peter U
# Expansion of Space

(Asked by: username_0 on 2015-03-13)

If Space is expanding between the Galaxies then why isn't it also expanding between the Stars within the Galaxies as well ? In fact why isn't Space expanding within our Solar System ?

**Tags:** space, expansion

> **Comments:**
> **username_3**: related http://astronomy.stackexchange.com/questions/1530/contradict-the-expansion-of-the-universe?rq=1  
> **username_1**: Similar: http://astronomy.stackexchange.com/questions/18609/in-km-h-what-actually-is-the-speed-of-andromeda-away-from-us-cosmologically  
> **username_2**: i came here to ask that very question!  

---

## 2 answers

### Answer by username_4 on 2015-03-14. Score: 4

The Universe is remarkably homogeneous, i.e. the same everywhere, and isotropic, i.e. the same in every direction (NB homogeneity does not imply isotropy and there are toy cosmological models which are homogeneous but not isotropic) and this is the underlying physical assumption of big bang cosmology.

When modelling the Universe the simplification of it being 100% homogeneous and isotropic is most often used as because on the very largest scale (i.e. the scale of the observable Universe) this is very close to being true and any deviations from this are small enough that they do not effect most of the properties being studied. In 100% homogeneous and isotropic Universe, expansion takes place at all places and at all scales.

However we know that the Universe is not 100% homogeneous and isotropic, especially on a smaller scale; for example the centre of a star is a very different place from intergalactic space. It is a fair question then to ask in view of this how does expansion manifest itself on smaller scales.

We know that in gravitationally bound systems such as our local group (which includes Andromeda) matter has the tendency to be drawn closer by gravity rather than to move away as objects do in an expanding Universe, so it seems our local group is not expanding. Still, it is very tempting to see expansion on this scale as being like a small repulsive force which acts against gravity, but is not strong enough to overcome gravity. So in this sense even the local group would be expanding

However the view of expansion acting like a small repulsive force on a small scale is not necessarily the right view. A counterexample would be the Einstein-Strauss Swiss-cheese model where expansion only takes place in deep space and not in the space around stars (modeled as vacuoles). The Einstein-Strauss model though cannot be taken as the last word on the subject as the way it models stars in space is not that realistic.

Overall then how expansion manifests itself on a smaller scale is still very much an open question and we just don't know what tiny corrections if any it would make on the dynamics of a very small system such as the solar system.

> **Comments:**
> **username_2**: how does the ESSC model differ from a small repulsive force?  The small repulsive force leveled by gravity 'seems' descriptive, since the primary differentiator between inter-galactic space and intra-galactic space is precisely that - matter/gravity-field density  

---

### Answer by username_5 on 2015-03-13. Score: 0

Actually, space is expanding everywhere. It is more noticeable between galaxies since space between galaxies is much larger than space between stars and space between the planets in the solar system.
A 1% expansion of space only represents 1.5 million kilometers between the Sun and the Earth. But the same 1% expansion between two galaxies that are four billion light-years away means that they are now 40 light-years farther away.
And 40 light-years is ten times the distance from the Sun to the nearest star Proxima Centaury.

> **Comments:**
> **username_0**: Yet the closest Galaxy to us, Andromeda, is moving towards us and not away from us ?  
> **username_6**: @PeterU Andromeda is moving through space faster than space expands between it and us.  
> **username_7**: Actually, it is not true that space is expanding everywhere. Space in NOT expanding on such small scales as the Solar system, and not even on galactic scales. If it did, it would mean that when Earth was formed it would have been much closer to the Sun than it is today, and for example molecular clouds would have a much higher density in the early Universe, both of which we know is not the case. I think username_4' answer it good. Space and matter is "bound" to each other, matter curves space in such a way as to prevent expansion on small scales.  
> **username_0**: What is the cause of this expansion, is it still from the original "Big Bang" ?  
> **username_0**: @username_6 What is pulling Andromeda towards the Milky Way Galaxy ? Is it the gravitational force of the Milky Way ? The Milky Way must be bigger than Andromeda because otherwise it would be pulling us towards it instead , is this correct ?  
> **username_7**: The original cause of expansion was Big Bang, or rather inflation (we think). But that expansion slowly decreased in magnitude due to the mutual gravitational attraction of matter (mostly dark matter). Recently, however, another effect took over, namely dark energy, which causes the Universe to accelerate its expansion (we think). Wrt. how space is "attached" to matter, see also [my previous post here](http://physics.stackexchange.com/questions/164866/expansion-of-the-universe-in-everyday-live/164872#164872). Concerning Andromeda, both attract each other, it's just a matter of reference frame.  
> **username_6**: @PeterU It is the bending of space that makes Andromeda and the Milky Way fall toward each other. They don't "pull", that was the Newtonian idea scrapped a century ago. They "fall" down curved spacetime. Space is bent by the mass of matter nearby. Mostly by unknown dark matter. But then there's also unknown dark energy which accelerates the expansion of space. I don't know, I "learn" mostly by watching lectures held by persons who obviously never really left their student lifestyle. It's a most intriguing reality TV show anyway. The one who predicts how it ends wins the Nobel Prize.  
> **username_3**: Gravitational bounding negates expansion  

---

