Hubble's law and Andromeda-Milky way collision? I'm confused

[cicatrix]

For many years I've been taking the Hubble's law for granted - linear distances to other galaxies constantly increase due to space expansion. I've also known the fact that our Milky way will eventually collide with Andromeda galaxy. Now I happen to stumble upon these two facts simultaneously. Now I'm a bit confused. If all galaxies run away from us, according to Hubble's law, why Andromeda still comes towards us? Is there a contradiction or there is some kind of explanation?

[Z-Man]

Hubble's law is only an average law and kicks in at much larger distances; local movements can override it. Hills and mountains don't contradict the fact that overall, the Earth is pretty round.

[Starman4308]

For many years I've been taking the Hubble's law for granted - linear distances to other galaxies constantly increase due to space expansion. I've also known the fact that our Milky way will eventually collide with Andromeda galaxy. Now I happen to stumble upon these two facts simultaneously. Now I'm a bit confused. If all galaxies run away from us, according to Hubble's law, why Andromeda still comes towards us? Is there a contradiction or there is some kind of explanation?

Hubble's law doesn't say "all galaxies are getting farther away from us". It says "on average, they are getting farther away from us". It does not exclude "and this specific one is coming closer out of sheer random chance".

[magnemoe]

Hubble's law doesn't say "all galaxies are getting farther away from us". It says "on average, they are getting farther away from us". It does not exclude "and this specific one is coming closer out of sheer random chance".

Both are part of an cluster of galaxies who stays together, its again part of another larger cluster who might stay together.

At longer distance it start to show up more and more.

[Kryten]

For many years I've been taking the Hubble's law for granted - linear distances to other galaxies constantly increase due to space expansion. I've also known the fact that our Milky way will eventually collide with Andromeda galaxy. Now I happen to stumble upon these two facts simultaneously. Now I'm a bit confused. If all galaxies run away from us, according to Hubble's law, why Andromeda still comes towards us? Is there a contradiction or there is some kind of explanation?

Andromeda is simply close enough that it's gravitational attraction towards us overwhelms the effects of expansion. There are a few (dwarf) galaxies even closer that are actively orbiting ours, for the same reason.

[cicatrix]

Both are part of an cluster of galaxies who stays together, its again part of another larger cluster who might stay together.

At longer distance it start to show up more and more.

And that contradicts another statement of the Big Bang theory that the expansion has no center, i.e. like three dots on a rubber band that is being expanded - there's no 'center' and no difference where to measure - the expansion is everywhere at once.

[Kryten]

Expansion is applied to the space between the galaxies, it's not a force pulling the galaxies apart. The more distance there is, the more expansion between them there will be-this doesn't require a centre.

[cicatrix]

Expansion is applied to the space between the galaxies, it's not a force pulling the galaxies apart. The more distance there is, the more expansion between them there will be-this doesn't require a centre.

Expansion cannot be applied selectively. It has to be applied everywhere. If you have 3 objects A, B, C then the distances ratio AB/BC = const for any given point, no?

[Camacha]

Andromeda is simply close enough that it's gravitational attraction towards us overwhelms the effects of expansion. There are a few (dwarf) galaxies even closer that are actively orbiting ours, for the same reason.

There seems to be evidence that these dwarf galaxies already have 'collided' a couple of times. Very interesting stuff.

[Kryten]

I'm not sure what you're trying to say. Expansion is speed per unit distance; any two points will experience expansion between them, but at lower distances the speed is low enough to be negligible and/or completely overwhelmed by whatever attractive forces there may be.

[PakledHostage]

Expansion cannot be applied selectively. It has to be applied everywhere. If you have 3 objects A, B, C then the distances ratio AB/BC = const for any given point, no?

Sure, but if AB/BC is 1/1000 then it will still be 1/1000 if they remain constant. In other words, if the distance between A and C along your rubber band from your earlier example was 1001 mm and you stretched it by 100%, the distance between A and B will only stretch by 1 mm (1mm to 2mm) while the distance from B to C stretches by 1000 mm (1000 mm to 2000 mm).

As has already been stated, Andromeda is close enough that it and the Milky Way are gravitationally interacting with each other (not to mention the rest of the galaxies in our local group).

Edited November 19, 2014 by PakledHostage
Added a sentence to clarify my point

[Leszek]

Hubble's law can be overridden at local levels by gravity. As has been said the local group is a cluster that is close enough to each other to overcome expanding space. Hubble's law also affects you and I, but we are close enough to the earth that the gravity swamps this effect. Same thing.

This doesn't undermine anything from the big bang at all. All around the universe there are groups of galaxies that are stuck together by gravity, each group is moving away from it neighbors. It might be more illuminating if not technically correct to say that Hubble's law applies to galactic groups and not individual galaxies. If you happened to be in another galactic cluster, you would see the exact same thing hence no center. If you play the tape backwards, the groups get closer together until they all end up in the same place at about the same time. Hence the big bang.

I have seen this kind of oversimplified cosmology before from creationist websites. That topic is not for discussion here, I would just say that if on the off chance that is where this information came from, you should consider getting a second opinion from a science forum somewhere.

Edited November 19, 2014 by Leszek

[cicatrix]

Nah, I deduced about the gravity myself, and it WAS a second opinion. It's just when I thought about it I wanted an independent confirmation. Still, it's a pity they're flying away from us. It would be much more exciting if they were getting closer.

[Scotius]

Exciting? Yes, for a time. And then, as things would get closer and closer to each other eventual witnesses would get less and less comfortable. For example: increasing amount of light arriving from all direction would first eradicate night, and then literally fry everything with the power of starlight

[magnemoe]

Exciting? Yes, for a time. And then, as things would get closer and closer to each other eventual witnesses would get less and less comfortable. For example: increasing amount of light arriving from all direction would first eradicate night, and then literally fry everything with the power of starlight

Yes, however now it looks a bit like the expansion force is increasing over time who is a bit weird, you should expect the expansion to slow down over time because of gravity. Even if gravity was not enough to stop the expansion it should slow it down.

[cicatrix]

If space was rubber I could think of irrecoverable accelerating deformation before its rupture. If you stretch rubber strongly enough, just before it will be torn apart, its expansion rate will be increasing faster and faster. Just a thought to consider LOL.

[Frozen_Heart]

If space was rubber I could think of irrecoverable accelerating deformation before its rupture. If you stretch rubber strongly enough, just before it will be torn apart, its expansion rate will be increasing faster and faster. Just a thought to consider LOL.

Appreciate that thought... :/

[-Velocity-]

Yes, however now it looks a bit like the expansion force is increasing over time who is a bit weird, you should expect the expansion to slow down over time because of gravity. Even if gravity was not enough to stop the expansion it should slow it down.

The expansion "force" is not increasing over time, the expansion rate is accelerating. (Not quite sure if "force" is the right word here, since we're talking about the expansion of space rather than movement through space.) A constant expansion force- the classic, fixed "cosmological constant"- would lead to an acceleration in the expansion rate too. Last I checked, our observations are as yet not precise enough to know whether the cosmological constant is truly constant, or is time-varying.

Edited November 20, 2014 by |Velocity|

[YNM]

Very simple : the more space, the more expansion.

If we can say that "dark energy" is actually a thing that's internal to space itself, then if you have two object with masses, gravity would try to take them closer. But the space itself tries to put them further. If there's enough space between the two, the space would win - the two object would be separated further and further and further. If there's not enough space, then gravity wins - the two object draw closer until it eventually collides. You should also consider what is called as peculiar velocity - a velocity that's independent from the movement of space. Andromeda galaxy is the only most prominent object to approach us, draws closer and closer. There might be enough space actually - but just that the peculiar velocity is there, and over time the space loses to gravity.

Kind of that - peculiar velocity is also a problem when deducting redshift of galaxies, and H0 (from nearby galaxies -further ones experienced a smaller value of it).

[GregA]

Hubble's law can be overridden at local levels by gravity. As has been said the local group is a cluster that is close enough to each other to overcome expanding space. Hubble's law also affects you and I, but we are close enough to the earth that the gravity swamps this effect. Same thing.

This doesn't undermine anything from the big bang at all. All around the universe there are groups of galaxies that are stuck together by gravity, each group is moving away from it neighbors. It might be more illuminating if not technically correct to say that Hubble's law applies to galactic groups and not individual galaxies. If you happened to be in another galactic cluster, you would see the exact same thing hence no center. If you play the tape backwards, the groups get closer together until they all end up in the same place at about the same time. Hence the big bang.

I have seen this kind of oversimplified cosmology before from creationist websites. That topic is not for discussion here, I would just say that if on the off chance that is where this information came from, you should consider getting a second opinion from a science forum somewhere.

It is possible to be critical of Big Bang cosmology without invoking creationism.

For example, many scientists are now openly talking about eternal inflation without fear of it destroying their career. Mainly because of Dark Energy observations, and the complete failure of Big Bang cosmology to predict it.

[Kryten]

'Big bang cosmology' is the observation that the universe is expanding at all, and that such expansion must logically have a beginning. Dark energy doesn't contradict that, just add to it.

[Kerbart]

Consider a river. On average water is flowing in downstream direction. But local currents and eddies still can have water that's flowing in the opposite direction; in the very least compare to the water around it.

[Leszek]

It is possible to be critical of Big Bang cosmology without invoking creationism.

For example, many scientists are now openly talking about eternal inflation without fear of it destroying their career. Mainly because of Dark Energy observations, and the complete failure of Big Bang cosmology to predict it.

Absolutely and I never said otherwise. In fact you can be critical of evolution without invoking creationism. That is part of what it means when something is a scientific theory. I also didn't say the OP was a creationist. I was very careful to be neutral and use unassuming language.

The way the conversation flowed on the first page of the thread was almost identical to some of the stuff I have seen creationists use, I know full well that this doesn't prove anything and is a unreliable indicator. That is why I was careful to make no allegations on the matter.

This is getting off topic though and I will say no more on this issue.

[Justin Hew]

On 11/19/2014 at 11:03 PM, cicatrix said:

Expansion cannot be applied selectively. It has to be applied everywhere. If you have 3 objects A, B, C then the distances ratio AB/BC = const for any given point, no?

There seems to be of a misconception here, immense in fact. Try running a QD Simulation on expansion, critical density would avail if expansion occurs everywhere at given speed and time