<p>The universe is expanding on a large scale. But locally things are always messy.</p>

<p>Locally, galaxies are not set in stone, they move relative to each other, and the directions are random. If they're moving towards each other fast enough, then they will collide.</p>

<p>Also, there's gravity. Some galaxies are bound to each other by gravity, and that will tend to pull them together.</p>

<p>As to why galaxies move at all, relative to each other - well, things in this universe have kinetic energy, and it's distributed randomly. Being distributed randomly, all kinds of scenarios are possible - things running away from each other, zooming past each other, bumping into each other, etc.</p>

<p>It's a messy and random universe, and the order of expansion becomes apparent only on the largest scale.</p>

CC BY-SA 4.0
2019-05-14T13:23:24.230Z
Is there proof that the energy distribution is indeed random? If this entails a separate Stack question, just let me know.
Mindwin Remember Monica
CC BY-SA 4.0
2019-05-14T14:01:21.320Z
@Mindwin: [CMB](https://en.wikipedia.org/wiki/Cosmic_microwave_background) is random. Mass is random. What makes you think kinetic energy could be non-random?
DevSolar
CC BY-SA 4.0
2019-05-14T15:03:18.013Z
While I have zero formal education in physics of any kind, I'm of the opinion that nothing is truly random. They're simply governed by physics we don't understand yet or we don't have enough information to accurately model their behavior. There's billions of years of history and interactions that led the universe to its current state and I feel like if we simply don't understand enough of that history to know exactly how every object got to where it is now.
aleppke
CC BY-SA 4.0
2019-05-14T16:12:06.963Z
@aleppke - I think you're confusing two different meanings of "random" - that of something behaving in a way that cannot be modelled (i.e. it is truly unpredictable), and that of something that can be modelled, but has multiple states to choose from (e.g. the roll of a dice). In this case, DevSolar is stating that just like CMB and mass, kinetic energy is not evenly distributed. If you imagine kinetic energy as arrows on paper pointing in random directions - the spaces between them may be moving outwards gradually, but some of the arrows are still pointing at each other.
Myles
CC BY-SA 4.0
2019-05-14T17:14:47.777Z
@DevSolar Nothing makes me thing it is not. Just being skeptic. We spent three and a half centuries knowing Fermat's Last Theorem to be probably true without a proof. Same thing here.
Mindwin Remember Monica
CC BY-SA 4.0
2019-05-14T18:23:04.467Z
@aleppke That's actually a very big discussion. True randomness does exist, it's the outcome of certain quantum phenomena. But it is true that trajectories of bodies are quite deterministic. However, many bodies moving under gravity make complex, chaotic systems. The behavior after a long time is essentially impossible to predict except on a statistical level. Don't get stuck on the word "random" in the reply as it was not used in a strict, naive sense. What I meant is - it's a complex distribution that simulates some aspects of true randomness well enough for this discussion.
Florin Andrei
CC BY-SA 4.0
2019-05-14T19:19:14.973Z
@Mindwin: Simply put, it is the most plausible: Considering lots of masses moving independently (i.e., any gravitational influence is secondary), they could be all at rest relatively to each other - but that would give preference to one inertial reference frame (at least regionally); hence they move. Any preferred direction or common rotation would be against the anisotropy we do strongly expect
Hagen von Eitzen
CC BY-SA 4.0
2019-05-15T02:15:00.167Z
Both the direction _and velocities_ are random. The same as stars in our own galaxy. Some small stars are moving at incredible speeds relative to our sun! Much higher than the average speed of nearby stars.
CJ Dennis
CC BY-SA 4.0
2019-05-15T14:52:45.373Z
"If they're moving towards each other fast enough, then they will collide": Isn't "fast enough" any speed greater than zero?  Or is it that if they're moving towards each other with a speed below some threshold, they *won't* collide?  How could that work?
phoog
CC BY-SA 4.0
2019-05-15T19:53:21.050Z
@phoog they could be moving toward each other at what would be a constant rate if the universe were not expanding, but the space between them is expanding, and the rate of expansion is accelerating so that even though they are currently moving toward each other, eventually there will be a net movement away from each other.. Or they could currently be moving toward each other, but a third body moving in a perpendicular direction will pass close enough to one of them to alter its course preventing collision. Or...
Mr.Mindor
CC BY-SA 4.0
2019-05-15T20:35:06.020Z
@Mr.Mindor but the expansion of space is a component of relative motion; that's how we discovered the expansion of space.  So "at what would be a constant rate if the universe were not expanding" isn't particularly useful.  I agree that the acceleration of the expansion could prevent collision of two galaxies currently moving towards each other.  The third body argument seems unhelpful, though, since the "fast enough" statement implies "under current conditions," and the third body isn't a current condition.
phoog
CC BY-SA 4.0
2019-05-14T05:43:22.870Z
2019-05-14T05:43:22.870Z
Florin Andrei
<p>The galaxies don't really get "off track" - it's not impossible, but that kind of thing probably doesn't happen anymore (as space continues to expand). What actually happens is that galaxies form gravitationally bound clusters - within the cluster, the acceleration due to gravity is larger than the equivalent expansion of space between the galaxies, so rather than growing more distant, the galaxies in question actually get closer together over time. Eventually, this results in a collision and a merger.</p>

<p>If the expansion remains roughly constant, there will come a point where we'll no longer be able to see any galaxies outside of our own cluster. But for those close enough, this has little effect - just like the expansion of space doesn't cause atoms, planets, solar systems or galaxies to get bigger.</p>

CC BY-SA 4.0
2019-05-14T13:20:06.250Z
" the galaxies in question actually get closer together over time. Eventually, this results in a collision and a merger". Really? How do they shed their kinetic energy? Are gravitational waves sufficient? What's the supposed time scale?
Peter - Reinstate Monica
CC BY-SA 4.0
2019-05-14T14:42:51.907Z
@PeterA.Schneider gas/dust clouds collide directly, tidal interactions throw streamers of stars away from the combined mass of the 2 galaxies carrying momentum away.  AFAIK gravitational waves a total nonfactor in the galaxies merger (but would be involved in an eventual merger of the central black holes).  The timescale is hundreds of millions to a billion or two years.
Dan Is Fiddling By Firelight
CC BY-SA 4.0
2019-05-14T14:55:08.607Z
@DanNeely I see. The reason being that while intergalactic space is vast, it's not as relatively vast as interstellar space...
Peter - Reinstate Monica
CC BY-SA 4.0
2019-05-14T16:57:27.627Z
"Eventually, this results in...a merger" - just wait. "Dear Sol: as you're aware, the recent merger of the Milky Way and Andromeda galaxies has formed the new MilkyMeda super-galaxy. Unfortunately, due to consolidation and restructuring efforts a limited number of positions have been made redundant including yours. We sincerely regret this, and are further saddened to note that no positions requiring your unique set of skills are available. Please gather your personal effects, including all moons and planets, and report to the nearest black hole for repurposing. Wishing you all the best..."
Bob Jarvis - Слава Україні
CC BY-SA 4.0
2019-05-14T19:28:11.993Z
@PeterA.Schneider Intergalactic space is quite a bit more vast than interstellar space. The former means "space between galaxies", while the latter means "space between stars".
Acccumulation
CC BY-SA 4.0
2019-05-14T21:51:18.330Z
@Acccumulation Yes of course; the keyword was "relative", to the sizes of planetary systems and galaxies, respectively. Galaxies in clusters are much closer to each other relative to their diameters than planetary systems to each other, relative to their diameter. That's why we don't have many tidal perturbations between planetary systems, but more frequently between galaxies.
Peter - Reinstate Monica
CC BY-SA 4.0
2019-05-14T08:16:05.947Z
2019-05-14T08:16:05.947Z
Luaan
<p>I'm not sure that anyone has answered the question asked. The root cause is indeed that gravitationally bound structures with freefall timescales that are much shorter than the age of the universe are not greatly affected by the general expansion of the universe (NB: Structures with freefall timescales longer than this are <em>not</em> going to be the source of many galaxy collisions). That is, locally, the expansion within such structures is negligible. However, that does not necessarily lead to collisions on a a timescale shorter than the age of the universe.</p>

<p>The first reason for galaxy collisions is that galaxy clusters have a very large number density - that is, the spacing between galaxies is not vastly larger than the "size" of a galaxy, where here, "size" means the effective interaction cross-sectional radius. As a result of these high densities, the freefall dynamical timescales in rich clusters (and even smaller groups of galaxies) are of order billions of years and so there is plenty of time for the galaxies to interact. As a contrast, think about how you might construct a <strong>scale</strong> model of stars in the local neighbourhood and compare the sizes of stars with their separations. It would in fact be difficult to make such a scale model with any meaningfully sized stars. On the other hand, you <em>can</em> make a scale model of say the local group of galaxies because their separations are only <span class="math-container">$\sim 10$</span> times their sizes.</p>

<p>The second reason is that many galaxies contain gas and that gas can easily dissipate kinetic energy and also transfer angular momentum. Another factor is that massive clusters of galaxies contain intracluster gas that can also serve to dissipate kinetic energy. In a gravitationally bound system, then objects that are in orbit around each other or around a common centre of mass need ways in which kinetic energy and angular momentum can be lost in order for a collision to take place. Even without gas, the fact that galaxies exist in groups and clusters means that n-body interactions can serve to dissipate energy and angular momentum to make a collision happen.</p>

CC BY-SA 4.0
2019-07-15T09:11:05.610Z
The last paragraph is a bit misleading. Galaxies *do* need to lose kinetic energy and angular momentum in order to merge, but the intracluster gas is way too dilute to have any direct effect on stellar systems (just as the ISM doesn't appreciably remove kinetic energy from stellar motions within a galaxy). What's important in groups and clusters is *dynamical friction*, for which dark matter is the dominant source.
Peter Erwin
CC BY-SA 4.0
2019-05-14T17:46:00.943Z
My very vague understanding suggestion that *"gravitationally bound structures are not affected by the general exapansion of the universe"* is limited to those whose orbital periods are short compared to the inverse Hubble constant. Is that reasonable or am I off track?
dmckee --- ex-moderator kitten
CC BY-SA 4.0
2019-05-14T17:47:37.397Z
Also, is it worth being explicit that clusters are multi-body situation meaning that the simple conic sections of two-body orbital mechanics are not the whole story?
dmckee --- ex-moderator kitten
CC BY-SA 4.0
2019-05-15T05:26:43.127Z
`+1` but I've always been uncomfortable with the oft-repeated phrasing (in various forms) of "gravitationally bound structures are not affected by metric expansion of space" Doesn't metric expansion happen everywhere, but is less *observable* in gravitationally bound systems because its effect is dominated by local motion due to the system being gravitationally bound? It doesn't actually "repel" or block or shut down metric expansion, as much as it simply dominates it observationally? I can ask this as a separate question if that provides a better format.
uhoh
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2019-05-15T14:57:20.843Z
Is "facto" a typo or some sort of specialized usage I've never seen before?
phoog
CC BY-SA 4.0
2019-05-16T18:35:21.040Z
@phoog I propose that we make "facto" a word. Defined as a a piece of information often presented as fact which is defensible under specific circumstances but usually incomplete or misleading to a level that it should not be presented as fact. This definition makes popular science writing the art of lining up factos in a pleasing manner.
dmckee --- ex-moderator kitten
CC BY-SA 4.0
2019-05-14T17:02:04.203Z
2019-05-15T15:51:18.727Z
2019-05-15T15:51:18.727Z
ProfRob
<p>Galaxies don't get "off track" - to see how collisions happen, we need to go right back to galaxy formation early on.</p>

<p>So, Big Bang happens. Space starts to expand - dramatically and to a huge extent. That's space itself expanding, not galaxies moving within space, by the way - distances themselves change. (Which is why it's called a "metric" expansion, metric being a term for distance-measures, and also why cosmologists say the Big Bang happened "everywhere").</p>

<p>At some tiny fraction of a second, the massive expansion winds down. Space continues to expand, but at a much slower rate. The last of the fundamental forces breaks away, and the cosmos is left as an insanely hot dense mix, so hot that even basic particles like protons, neutrons and electrons can't yet exist - although quarks can. </p>

<p>But there are some very subtle things going on. Even though expansion left us with an incredibly uniform, homogenous universe, the density does sliiightly vary between places. As things cool down, and particles start to condense (and annihilate, and other things), the universe is left with what cosmologists call acoustic waves - basically standing waves. And if you've ever seen <a href="https://youtube.com/watch?v=wvJAgrUBF4w" rel="nofollow noreferrer">videos of a tray of sand being vibrated</a>, you'll know that one effect is that it leaves some places with more sand, some with less, due to interference patterns. So our universe ends up, as it expands, with some areas denser, some less dense.</p>

<p>A second effect comes into play. You'll know (or have heard of) dark matter. We don't know what it is made of, but we know it exists (galaxies couldn't form without it, they'd fly apart or take longer than the age of the universe to form), and we know a lot about how it behaves - what forces it responds to, and what forces it doesn't. Interact via gravity - yes, very weakly. Interact via electromagnetic force - no, not at all. That latter bit is crucial. </p>

<p>When "ordinary" matter collapses, it heats up. That's how we get stars, for example. The radiation released during collapse also acts as a kind of pressure, opposing collapse, slowing it down. That's why stars like our sun are stable for so long. Dark matter doesn't interact electromagnetically (as far as we know) so it can't experience or create electromagnetic radiation. So when it collapses, it doesn't get hot, it doesn't release radiation... I think you can see where this is going. <em>There's no radiation released during collapse to resist further collapse, so it can collapse much faster than ordinary matter</em>. As an aside, <strong>because</strong> it can't release radiation, it also can't jettison the energy that must be got rid of to allow dense objects to form. So it ends up quickly collapsing to a hazy diffuse "halo", but then can't collapse much more. And no surprise, it collapses in those places where the universe was fractionally denser. So you get what cosmologists call "filaments" and "halos" of dark matter, a bit like a sponge or a swiss cheese, with comparative "voids" separating them. Ordinary matter is more strongly attracted to these already-existing dark matter filaments and halos. It collapses towards them. The self-gravity of the ordinary matter is enhanced by the gravity due to the concentrations of dark matter there - and ordinary matter <strong>can</strong> lose energy by radiation, so it collapses more than dark matter does, to form the galaxies and their contents which we can see today.</p>

<p>Gravity can do this, because the expansion of the universe has by now slowed down so much from its "heyday", that gravity can pull some of the matter together <strong>within</strong> space faster than the expansion can <strong>add space between them</strong>. Over cosmic distances, gravity is much much weaker, and expansion dominates, so clusters and superclusters still move apart, but within clusters, the galaxies and groups of galaxies are accelerated by gravity enough that they mostly stay in their groups and clusters, and move around or orbit within them.</p>

<p>So we end up with a universe that, on a cosmic scale, we see expansion "winning" as gravity is weak, so we see superclusters moving apart. But within clusters and galaxy groups, we see gravity "winning" because it's stronger over smaller distances, so clusters and gravitationally bound entities like galaxies stay together.</p>

<p>What this in turn means, is that galaxies and galaxy groups are bound by gravity more than they are separated by expansion. So they remain moving within their clusters and groups, despite universal expansion. And, occasionally, because motion of 3 or more separate bodies under gravity is chaotic (and clusters can contain billions or trillions of galaxies),  entire galaxies will get ejected, or collide, or do whatever galaxies do. And that's how it happens.</p>

<p><em>(Although you didn't ask, it's a natural question to wonder what happens next. We believe that the rate of expansion has slowly sped up. That means that in the far future (tens and hundreds of billions of years), that galaxies will have to be even closer together, for gravity to dominate expansion. So clusters that are stable now, might break apart in the far future. If expansion accelerates enough then even smaller bodies could ultimately break up, perhaps galaxies themselves, or even stars and atoms. But that's something nobody knows.)</em></p>

CC BY-SA 4.0
2019-05-14T22:31:22.567Z
2019-05-15T20:54:05.507Z
2019-05-15T20:54:05.507Z
Stilez
<p>I think it's also worth noting that cosmological expansion is measurable only on the very largest of scales. Hubble's law tells us that the farther away an object is, the faster it's being pulled away by the expansion. That rate is approximately <span class="math-container">$70 km.s^{-1}Mpc^{-1}$</span> -- in other words, for every megaparsec (Mpc -- about 3 million light years) that you look farther out in space, an additional <span class="math-container">$70 km.s^{-1}$</span> is tacked onto its recessional speed. For a relatively close galaxy, its actual velocity through space can be much larger than this, and in the specific case of the Andromeda Galaxy, which is only about 2.5 million light years (0.77 MPc) away, it is actually approaching us at around <span class="math-container">$110 km.s^{-1}$</span>. For much, much farther galaxies, billions of light years away, all of those <span class="math-container">$70 km.s^{-1}$</span> add-ons stack up to a much, much higher recessional speed.</p>

CC BY-SA 4.0
2019-05-14T19:34:40.090Z
2019-05-14T19:34:40.090Z
Jim421616
<p>Although the Universe is expanding and, in general, the further away a galaxy is from us the faster it appears to be moving away from us. This does not apply to the galaxies in the Local Group. which is a gravitationally bound structure. The Andromeda galaxy is moving towards the Milky Way at about 400,000 km/h and the Milky Way and Andromeda are expected to collide in about 4 billion years time. When this happens, a large new single galaxy will be formed.  The new galaxy which will be formed by the merger is sometimes given the name Milkomeda
. For more details see <a href="https://thesciencegeek.org/2015/05/08/ultimate-fate-of-the-universe/" rel="nofollow noreferrer">my recent blog post</a> on this topic.</p>

<p>Over billions of years, Milkomeda will gradually absorb the other Local Group members.</p>

<p>In general any gravitationally bound structure such as: stellar systems (e.g. solar system ) our galaxy and groups and clusters of galaxies will not get larger as the Universe expands)</p>

CC BY-SA 4.0
2019-05-15T12:24:11.483Z
2019-05-16T09:14:09.013Z
2019-05-16T09:14:09.013Z
Science Geek
<p>If the universe is expanding outward, what is the processes for one galaxy to get off track enough to collide with another? </p>

<p>Say, the <a href="https://en.wikipedia.org/wiki/Andromeda_Galaxy" rel="noreferrer">Andromeda Galaxy</a> and the Milky Way.</p>

CC BY-SA 4.0
2019-05-14T04:50:03.620Z
This is a *very little* bit like asking why do molecules in an expanding cloud of gas collide.
uhoh
CC BY-SA 4.0
2019-05-14T15:02:08.193Z
Also 'expansion' in space-time is weird.
Strawberry
CC BY-SA 4.0
2019-05-14T16:41:34.717Z
@uhoh: except for all the differences, this is "same question, different scale". :-)
Bob Jarvis - Слава Україні
CC BY-SA 4.0
2019-05-14T17:50:48.137Z
Not to be rude, but I just want to clarify that galaxies can't really get "off track".  There's no set track for them to begin with (unless your question is about a universe simulator or something).  They just are where they are and the laws of physics determine their motions.  Gravity is the primary force that attracts galaxies to each other.
Ben Sandeen
CC BY-SA 4.0
2019-05-15T05:08:21.217Z
@uhoh: Or like "Why do people run into each other on the street if the universe is expanding. ^^
Zaibis
CC BY-SA 4.0
2019-05-15T09:10:02.923Z
As for the initial *if* statement, Hubble measured a red shift, applied the Doppler Effect, and concluded that *distant* galaxies are expanding away from us from an observers dataset of 1. I'm curious as to what the best critical works writing about this conclusion are.
Henrik Erlandsson
CC BY-SA 4.0
2021-04-20T13:45:48.250Z
To summarize what I've read; It isn't the galaxies that are moving away from each other — it's the galaxy clusters. Within any cluster, gravity and kinetic motion are the dominating forces that cause galaxies to merge (or not). On a larger scale, galaxy clusters are far enough apart for gravity to have minimal effect, and thus, expansion dominates (and thus, the galaxy clusters are flying away from each other). Caveat;
Some smart folks don't accept the premise of universal expansion AND/OR galaxy collisions/mergers, and thus, the question may be wrong to begin with. Thanks for the fish.
Freddy Atton
<big-bang-theory>
Why do galaxies collide?
Dave
# Why do galaxies collide?

(Asked by: username_0 on 2019-05-14)

If the universe is expanding outward, what is the processes for one galaxy to get off track enough to collide with another?

Say, the [Andromeda Galaxy](https://en.wikipedia.org/wiki/Andromeda_Galaxy) and the Milky Way.

**Tags:** big-bang-theory

> **Comments:**
> **username_1**: This is a *very little* bit like asking why do molecules in an expanding cloud of gas collide.  
> **username_2**: Also 'expansion' in space-time is weird.  
> **username_3**: @username_1: except for all the differences, this is "same question, different scale". :-)  
> **username_4**: Not to be rude, but I just want to clarify that galaxies can't really get "off track".  There's no set track for them to begin with (unless your question is about a universe simulator or something).  They just are where they are and the laws of physics determine their motions.  Gravity is the primary force that attracts galaxies to each other.  
> **username_5**: @username_1: Or like "Why do people run into each other on the street if the universe is expanding. ^^  
> **username_6**: As for the initial *if* statement, Hubble measured a red shift, applied the Doppler Effect, and concluded that *distant* galaxies are expanding away from us from an observers dataset of 1. I'm curious as to what the best critical works writing about this conclusion are.  
> **username_7**: To summarize what I've read; It isn't the galaxies that are moving away from each other — it's the galaxy clusters. Within any cluster, gravity and kinetic motion are the dominating forces that cause galaxies to merge (or not). On a larger scale, galaxy clusters are far enough apart for gravity to have minimal effect, and thus, expansion dominates (and thus, the galaxy clusters are flying away from each other). Caveat; Some smart folks don't accept the premise of universal expansion AND/OR galaxy collisions/mergers, and thus, the question may be wrong to begin with. Thanks for the fish.  

---

## 6 answers

### Answer by username_8 on 2019-05-14. Score: 38

The universe is expanding on a large scale. But locally things are always messy.

Locally, galaxies are not set in stone, they move relative to each other, and the directions are random. If they're moving towards each other fast enough, then they will collide.

Also, there's gravity. Some galaxies are bound to each other by gravity, and that will tend to pull them together.

As to why galaxies move at all, relative to each other - well, things in this universe have kinetic energy, and it's distributed randomly. Being distributed randomly, all kinds of scenarios are possible - things running away from each other, zooming past each other, bumping into each other, etc.

It's a messy and random universe, and the order of expansion becomes apparent only on the largest scale.

> **Comments:**
> **username_9**: Is there proof that the energy distribution is indeed random? If this entails a separate Stack question, just let me know.  
> **username_10**: @Mindwin: [CMB](https://en.wikipedia.org/wiki/Cosmic_microwave_background) is random. Mass is random. What makes you think kinetic energy could be non-random?  
> **username_11**: While I have zero formal education in physics of any kind, I'm of the opinion that nothing is truly random. They're simply governed by physics we don't understand yet or we don't have enough information to accurately model their behavior. There's billions of years of history and interactions that led the universe to its current state and I feel like if we simply don't understand enough of that history to know exactly how every object got to where it is now.  
> **username_12**: @username_11 - I think you're confusing two different meanings of "random" - that of something behaving in a way that cannot be modelled (i.e. it is truly unpredictable), and that of something that can be modelled, but has multiple states to choose from (e.g. the roll of a dice). In this case, username_10 is stating that just like CMB and mass, kinetic energy is not evenly distributed. If you imagine kinetic energy as arrows on paper pointing in random directions - the spaces between them may be moving outwards gradually, but some of the arrows are still pointing at each other.  
> **username_9**: @username_10 Nothing makes me thing it is not. Just being skeptic. We spent three and a half centuries knowing Fermat's Last Theorem to be probably true without a proof. Same thing here.  
> **username_8**: @username_11 That's actually a very big discussion. True randomness does exist, it's the outcome of certain quantum phenomena. But it is true that trajectories of bodies are quite deterministic. However, many bodies moving under gravity make complex, chaotic systems. The behavior after a long time is essentially impossible to predict except on a statistical level. Don't get stuck on the word "random" in the reply as it was not used in a strict, naive sense. What I meant is - it's a complex distribution that simulates some aspects of true randomness well enough for this discussion.  
> **username_13**: @Mindwin: Simply put, it is the most plausible: Considering lots of masses moving independently (i.e., any gravitational influence is secondary), they could be all at rest relatively to each other - but that would give preference to one inertial reference frame (at least regionally); hence they move. Any preferred direction or common rotation would be against the anisotropy we do strongly expect  
> **username_14**: Both the direction _and velocities_ are random. The same as stars in our own galaxy. Some small stars are moving at incredible speeds relative to our sun! Much higher than the average speed of nearby stars.  
> **username_15**: "If they're moving towards each other fast enough, then they will collide": Isn't "fast enough" any speed greater than zero?  Or is it that if they're moving towards each other with a speed below some threshold, they *won't* collide?  How could that work?  
> **username_16**: @username_15 they could be moving toward each other at what would be a constant rate if the universe were not expanding, but the space between them is expanding, and the rate of expansion is accelerating so that even though they are currently moving toward each other, eventually there will be a net movement away from each other.. Or they could currently be moving toward each other, but a third body moving in a perpendicular direction will pass close enough to one of them to alter its course preventing collision. Or...  
> **username_15**: @username_16 but the expansion of space is a component of relative motion; that's how we discovered the expansion of space.  So "at what would be a constant rate if the universe were not expanding" isn't particularly useful.  I agree that the acceleration of the expansion could prevent collision of two galaxies currently moving towards each other.  The third body argument seems unhelpful, though, since the "fast enough" statement implies "under current conditions," and the third body isn't a current condition.  

---

### Answer by username_17 on 2019-05-14. Score: 9

The galaxies don't really get "off track" - it's not impossible, but that kind of thing probably doesn't happen anymore (as space continues to expand). What actually happens is that galaxies form gravitationally bound clusters - within the cluster, the acceleration due to gravity is larger than the equivalent expansion of space between the galaxies, so rather than growing more distant, the galaxies in question actually get closer together over time. Eventually, this results in a collision and a merger.

If the expansion remains roughly constant, there will come a point where we'll no longer be able to see any galaxies outside of our own cluster. But for those close enough, this has little effect - just like the expansion of space doesn't cause atoms, planets, solar systems or galaxies to get bigger.

> **Comments:**
> **username_18**: " the galaxies in question actually get closer together over time. Eventually, this results in a collision and a merger". Really? How do they shed their kinetic energy? Are gravitational waves sufficient? What's the supposed time scale?  
> **username_19**: @PeterA.Schneider gas/dust clouds collide directly, tidal interactions throw streamers of stars away from the combined mass of the 2 galaxies carrying momentum away.  AFAIK gravitational waves a total nonfactor in the galaxies merger (but would be involved in an eventual merger of the central black holes).  The timescale is hundreds of millions to a billion or two years.  
> **username_18**: @DanNeely I see. The reason being that while intergalactic space is vast, it's not as relatively vast as interstellar space...  
> **username_3**: "Eventually, this results in...a merger" - just wait. "Dear Sol: as you're aware, the recent merger of the Milky Way and Andromeda galaxies has formed the new MilkyMeda super-galaxy. Unfortunately, due to consolidation and restructuring efforts a limited number of positions have been made redundant including yours. We sincerely regret this, and are further saddened to note that no positions requiring your unique set of skills are available. Please gather your personal effects, including all moons and planets, and report to the nearest black hole for repurposing. Wishing you all the best..."  
> **username_20**: @PeterA.Schneider Intergalactic space is quite a bit more vast than interstellar space. The former means "space between galaxies", while the latter means "space between stars".  
> **username_18**: @username_20 Yes of course; the keyword was "relative", to the sizes of planetary systems and galaxies, respectively. Galaxies in clusters are much closer to each other relative to their diameters than planetary systems to each other, relative to their diameter. That's why we don't have many tidal perturbations between planetary systems, but more frequently between galaxies.  

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### Answer by username_21 on 2019-05-14. Score: 9

I'm not sure that anyone has answered the question asked. The root cause is indeed that gravitationally bound structures with freefall timescales that are much shorter than the age of the universe are not greatly affected by the general expansion of the universe (NB: Structures with freefall timescales longer than this are *not* going to be the source of many galaxy collisions). That is, locally, the expansion within such structures is negligible. However, that does not necessarily lead to collisions on a a timescale shorter than the age of the universe.

The first reason for galaxy collisions is that galaxy clusters have a very large number density - that is, the spacing between galaxies is not vastly larger than the "size" of a galaxy, where here, "size" means the effective interaction cross-sectional radius. As a result of these high densities, the freefall dynamical timescales in rich clusters (and even smaller groups of galaxies) are of order billions of years and so there is plenty of time for the galaxies to interact. As a contrast, think about how you might construct a **scale** model of stars in the local neighbourhood and compare the sizes of stars with their separations. It would in fact be difficult to make such a scale model with any meaningfully sized stars. On the other hand, you *can* make a scale model of say the local group of galaxies because their separations are only $\sim 10$ times their sizes.

The second reason is that many galaxies contain gas and that gas can easily dissipate kinetic energy and also transfer angular momentum. Another factor is that massive clusters of galaxies contain intracluster gas that can also serve to dissipate kinetic energy. In a gravitationally bound system, then objects that are in orbit around each other or around a common centre of mass need ways in which kinetic energy and angular momentum can be lost in order for a collision to take place. Even without gas, the fact that galaxies exist in groups and clusters means that n-body interactions can serve to dissipate energy and angular momentum to make a collision happen.

> **Comments:**
> **username_23**: My very vague understanding suggestion that *"gravitationally bound structures are not affected by the general exapansion of the universe"* is limited to those whose orbital periods are short compared to the inverse Hubble constant. Is that reasonable or am I off track?  
> **username_23**: Also, is it worth being explicit that clusters are multi-body situation meaning that the simple conic sections of two-body orbital mechanics are not the whole story?  
> **username_1**: `+1` but I've always been uncomfortable with the oft-repeated phrasing (in various forms) of "gravitationally bound structures are not affected by metric expansion of space" Doesn't metric expansion happen everywhere, but is less *observable* in gravitationally bound systems because its effect is dominated by local motion due to the system being gravitationally bound? It doesn't actually "repel" or block or shut down metric expansion, as much as it simply dominates it observationally? I can ask this as a separate question if that provides a better format.  
> **username_15**: Is "facto" a typo or some sort of specialized usage I've never seen before?  
> **username_23**: @username_15 I propose that we make "facto" a word. Defined as a a piece of information often presented as fact which is defensible under specific circumstances but usually incomplete or misleading to a level that it should not be presented as fact. This definition makes popular science writing the art of lining up factos in a pleasing manner.  
> **username_22**: The last paragraph is a bit misleading. Galaxies *do* need to lose kinetic energy and angular momentum in order to merge, but the intracluster gas is way too dilute to have any direct effect on stellar systems (just as the ISM doesn't appreciably remove kinetic energy from stellar motions within a galaxy). What's important in groups and clusters is *dynamical friction*, for which dark matter is the dominant source.  

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### Answer by username_24 on 2019-05-14. Score: 2

Galaxies don't get "off track" - to see how collisions happen, we need to go right back to galaxy formation early on.

So, Big Bang happens. Space starts to expand - dramatically and to a huge extent. That's space itself expanding, not galaxies moving within space, by the way - distances themselves change. (Which is why it's called a "metric" expansion, metric being a term for distance-measures, and also why cosmologists say the Big Bang happened "everywhere").

At some tiny fraction of a second, the massive expansion winds down. Space continues to expand, but at a much slower rate. The last of the fundamental forces breaks away, and the cosmos is left as an insanely hot dense mix, so hot that even basic particles like protons, neutrons and electrons can't yet exist - although quarks can.

But there are some very subtle things going on. Even though expansion left us with an incredibly uniform, homogenous universe, the density does sliiightly vary between places. As things cool down, and particles start to condense (and annihilate, and other things), the universe is left with what cosmologists call acoustic waves - basically standing waves. And if you've ever seen [videos of a tray of sand being vibrated](https://youtube.com/watch?v=wvJAgrUBF4w), you'll know that one effect is that it leaves some places with more sand, some with less, due to interference patterns. So our universe ends up, as it expands, with some areas denser, some less dense.

A second effect comes into play. You'll know (or have heard of) dark matter. We don't know what it is made of, but we know it exists (galaxies couldn't form without it, they'd fly apart or take longer than the age of the universe to form), and we know a lot about how it behaves - what forces it responds to, and what forces it doesn't. Interact via gravity - yes, very weakly. Interact via electromagnetic force - no, not at all. That latter bit is crucial.

When "ordinary" matter collapses, it heats up. That's how we get stars, for example. The radiation released during collapse also acts as a kind of pressure, opposing collapse, slowing it down. That's why stars like our sun are stable for so long. Dark matter doesn't interact electromagnetically (as far as we know) so it can't experience or create electromagnetic radiation. So when it collapses, it doesn't get hot, it doesn't release radiation... I think you can see where this is going. *There's no radiation released during collapse to resist further collapse, so it can collapse much faster than ordinary matter*. As an aside, **because** it can't release radiation, it also can't jettison the energy that must be got rid of to allow dense objects to form. So it ends up quickly collapsing to a hazy diffuse "halo", but then can't collapse much more. And no surprise, it collapses in those places where the universe was fractionally denser. So you get what cosmologists call "filaments" and "halos" of dark matter, a bit like a sponge or a swiss cheese, with comparative "voids" separating them. Ordinary matter is more strongly attracted to these already-existing dark matter filaments and halos. It collapses towards them. The self-gravity of the ordinary matter is enhanced by the gravity due to the concentrations of dark matter there - and ordinary matter **can** lose energy by radiation, so it collapses more than dark matter does, to form the galaxies and their contents which we can see today.

Gravity can do this, because the expansion of the universe has by now slowed down so much from its "heyday", that gravity can pull some of the matter together **within** space faster than the expansion can **add space between them**. Over cosmic distances, gravity is much much weaker, and expansion dominates, so clusters and superclusters still move apart, but within clusters, the galaxies and groups of galaxies are accelerated by gravity enough that they mostly stay in their groups and clusters, and move around or orbit within them.

So we end up with a universe that, on a cosmic scale, we see expansion "winning" as gravity is weak, so we see superclusters moving apart. But within clusters and galaxy groups, we see gravity "winning" because it's stronger over smaller distances, so clusters and gravitationally bound entities like galaxies stay together.

What this in turn means, is that galaxies and galaxy groups are bound by gravity more than they are separated by expansion. So they remain moving within their clusters and groups, despite universal expansion. And, occasionally, because motion of 3 or more separate bodies under gravity is chaotic (and clusters can contain billions or trillions of galaxies), entire galaxies will get ejected, or collide, or do whatever galaxies do. And that's how it happens.

*(Although you didn't ask, it's a natural question to wonder what happens next. We believe that the rate of expansion has slowly sped up. That means that in the far future (tens and hundreds of billions of years), that galaxies will have to be even closer together, for gravity to dominate expansion. So clusters that are stable now, might break apart in the far future. If expansion accelerates enough then even smaller bodies could ultimately break up, perhaps galaxies themselves, or even stars and atoms. But that's something nobody knows.)*


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### Answer by username_25 on 2019-05-14. Score: 1

I think it's also worth noting that cosmological expansion is measurable only on the very largest of scales. Hubble's law tells us that the farther away an object is, the faster it's being pulled away by the expansion. That rate is approximately $70 km.s^{-1}Mpc^{-1}$ -- in other words, for every megaparsec (Mpc -- about 3 million light years) that you look farther out in space, an additional $70 km.s^{-1}$ is tacked onto its recessional speed. For a relatively close galaxy, its actual velocity through space can be much larger than this, and in the specific case of the Andromeda Galaxy, which is only about 2.5 million light years (0.77 MPc) away, it is actually approaching us at around $110 km.s^{-1}$. For much, much farther galaxies, billions of light years away, all of those $70 km.s^{-1}$ add-ons stack up to a much, much higher recessional speed.


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### Answer by username_26 on 2019-05-15. Score: 0

Although the Universe is expanding and, in general, the further away a galaxy is from us the faster it appears to be moving away from us. This does not apply to the galaxies in the Local Group. which is a gravitationally bound structure. The Andromeda galaxy is moving towards the Milky Way at about 400,000 km/h and the Milky Way and Andromeda are expected to collide in about 4 billion years time. When this happens, a large new single galaxy will be formed. The new galaxy which will be formed by the merger is sometimes given the name Milkomeda
. For more details see [my recent blog post](https://thesciencegeek.org/2015/05/08/ultimate-fate-of-the-universe/) on this topic.

Over billions of years, Milkomeda will gradually absorb the other Local Group members.

In general any gravitationally bound structure such as: stellar systems (e.g. solar system ) our galaxy and groups and clusters of galaxies will not get larger as the Universe expands)


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