Why are Andromeda and the Milky Way approaching each other?

[Endervhar]

I was recently asked a few questions about these galaxies. As I was by no means sure of my responses (apart from the last ) I am posting the questions and answers in the hope that someone will either confirm or better them.

In an expanding Universe, why are Andromeda and the Milky Way moving closer together?

The expansion of the Universe is causing the galaxy groups to separate. These two galaxies are in the same group, so are not influenced by the force of expansion.

Considering the relativity of motion; is it possible to say if either galaxy is moving towards the other?

My suspicion would be that the easiest way to tackle this question would be to regard the Local Group as a static frame of reference and decide which galaxy is moving relative to the LG. I don’t know which that would be.

Is there any other, larger, thing in the Universe that could be taken as a static F of R?

The only thing I can think of is the Cosmic Microwave Background, which seems to approach every point in the Universe, at the same speed, from every angle. Presumably movement relative to the CMB is as near as we could get to “absolute movement”.

Do we know why Andromeda and the Milky Way are moving closer?

I’ve no idea.

 

[whocouldshebe]

Yeah, it's like, once they slam together and pass each other then could we say that they are moving away from each other, nope, they would sling back around and slam together again. It seems to me that everything moves to a constant state of cancellation or combination if you will. To say things are moving towards or away from each other is a contradiction in itself. Instead we should say they are moving towards a state of equilibrium.

 

[Drakkith]

You are correct in that the expansion of the universe is not enough on the scale of our local galaxy group to overcome the attraction of gravity. It is also correct to say that either galaxy is moving toward the other, as it only depends on the frame you take. You could take a frame where you are stationary with the center of mass of the local group, in which case both galaxies are moving towards each other.

Is there any other, larger, thing in the Universe that could be taken as a static F of R?

I suppose you could use a supercluster or something, but the only real universal reference I can think of would be the cosmic microwave background also.

Do we know why Andromeda and the Milky Way are moving closer?

Why? Because gravity? I'm not sure what answer you are looking for here.

 

[Nik_2213]

I suppose the simplest answer is that they're close enough for attraction due to their mutual gravity to beat the universe's general expansion ...

 

[ideasrule]

The Friedmann equations are the solutions to Einstein's field equations assuming a homogeneous and isotropic universe. It's meaningless to talk about the universe's general expansion on a small scale because on an intergalactic scale, the universe isn't even close to being either homogeneous or isotropic, so you can't apply the same equations that you apply to the universe as a whole and expect them to be valid.

 

[Chronos]

As already noted, gravity is much stronger locally than the force of expansion. The force of exansion [hubble constant] is about 70 kilometers per second per megaparsec. The collective velocity [virgocentric infall rate] of the local group is estimated at around 270 km/sec towards the virgo cluster. The peculiar motions of galaxies in the local group [wrt the cmb] averages around 600 km/sec. The local group is only about 1 mpc in size, so, it is apparent the effect of gravity is much stronger than the expansion force acting upon the local group.

 

[Shenstar]

Whats is the explanation for the reason we have local groups of galaxy clusters? How did these groups form if the expansion force is too weak to counter gravity at that level? If these groups or clumps were closer together at some point wouldn't it have been more difficult then to make them separate?

 

[Chronos]

The local group formed from enormous primordial gas clouds whose gravitational attractive force exceeded the Hubble constant. Every star in every galaxy formed from an overdense knot in their local clump of gas.

 

[marcus]

Whats is the explanation for the reason we have local groups of galaxy clusters? How did these groups form if the expansion force is too weak to counter gravity at that level? If these groups or clumps were closer together at some point wouldn't it have been more difficult then to make them separate?

The local group formed from enormous primordial gas clouds whose gravitational attractive force exceeded the Hubble constant. Every star in every galaxy formed from an overdense knot in their local clump of gas.

Chronos answered, I'll add a footnote. Google "Smoot TED". Structure formation is fascinating and is studied a lot using computer simulations of the universe. These can be used to produce short animations, movies, of structure forming.

There is a club called TED that invites people to give 20 minute talks on interesting subjects. Nobelist George Smoot gave a talk on structure formation. He showed a few of these short computer animation movies.

You can set up a geometry with expanding distances, in the computer, where the rate of expansion varies according to the best-fit cosmic model. Then you can put in a certain amount of dark matter and ordinary matter. And you can watch it evolve.

The matter will start to fall towards itself, where there happens to be a slight overdensity (more than average density.) How much structure forms depends on the balance between the factors favoring condensation (like how much matter you put in with how much random fluctuation in the density) and the expansion rate.
There has to be a balance.

If there is no expansion, or not enough, the matter could collapse the universe, or make mostly black holes. If there is too much expansion it could defeat the formation of structure and wouldn't a lot of stars and galaxies.

The formation of structure is fun to watch. Actually very pretty. Enormous cobweb patterns form, with broad irregular fuzzy strands. Clusters of galaxies tend to form where these large strands cross. It is Jackson Pollocky and random at first but then gradually the mess gets kind of "self-organizing".

If you google "Smoot TED" and don't get the 20 minute talk video with the movies, let us know and one of us will get a direct link.

 

[Shenstar]

Thanks sounds interesting, I'll give it a try. I've always wanted to play with one of those computer simulation things using different variables, perhaps evenvchanging some of the constants (like c) to see how it would make different universes.

 

[WhoWee]

Thank you Marcus - the George Smoot video was very interesting.

 

[Endervhar]

I apologise for posting originally in the wrong place. I usually blame blunders like this on my advanced age. I shall aim for better things in the future.

 

[Endervhar]

The collective velocity [virgocentric infall rate] of the local group is estimated at around 270 km/sec towards the virgo cluster.

I assume this means that as a general rule, all the galaxies in the LG are "falling" towards the Virgo cluster at about 270 km/sec.

The peculiar motions of galaxies in the local group [wrt the cmb] averages around 600 km/sec.

Now you've lost me. How do you square these two figures (270 & 600 km/sec)?

 

[marcus]

I assume this means that as a general rule, all the galaxies in the LG are "falling" towards the Virgo cluster at about 270 km/sec.
Now you've lost me. How do you square these two figures (270 & 600 km/sec)?

Endervhar, we need an online source. I didn't see one. Maybe Chronos has one he can give us the link to.
I can't remember all these details.

The figure of 600 km/s is approximate and applies to both the Local Group and to the Virgo Cluster. Both are heading roughly in the same direction at around 600, but their paths are not exactly parallel. They could also be diverging or converging.

Is there any problem with this?

Two cars can both be heading approx. South on the M6 at approx 100 km/h and yet they can also be converging (getting closer together) because one is drifting sideways slowly towards the other, or perhaps diverging (sidling away).

I'm actually not sure about the 270 which is why I would want an online source. Some journal article that lists various speeds and directions in our local cosmos surroundings. But in principle it shouldn't matter much. They can both be going South about the same speed and still one car is sidling away or towards the other, perhaps it is changing lanes.

 

[Chronos]

See: http://ned.ipac.caltech.edu/level5/Sparke/Sparke1_3.html

 

[marcus]

I see. It is more complicated. Distance to Virgo Cluster is 16 Mpc. So the recession speed (disregarding individual motions) would be 71x16=1136 km/s.

You give an online source:
http://ned.ipac.caltech.edu/level5/Sparke/Sparke1_1.html
a section from a book by Sparke that Ned Wright has put up at his UCLA cosmo website.

What it says there if I understand right is that the ACTUAL recession speed is only around
1136 - 270 = 866 km/s.

When you look at Fig. 7.8
http://ned.ipac.caltech.edu/level5/Sparke/Figures/figure7_8.jpeg
You see that 866 as RAW VELOCITY on the left panel, for a big square marker that stands for Virgo Cluster. And then on the right panel you see the same big square marker PUSHED UP by 270---that is "corrected for the infall" so then it sits on the sloping line where it is supposed to sit. Corrected for infall it is more like 1136.

I think that is what that section of the Sparke book is trying to teach us.
http://ned.ipac.caltech.edu/level5/Sparke/Sparke1_3.html

The book talks about "subtracting out -J", that is subtracting out the infall -270. this is the same as adding 270, which you see is how they got from the left panel to the right panel.

If you switch over to thinking it from the point of view of observers at rest relative to the CMB then both Local Group and Virgo Cluster are moving roughly 600 towards Hydra Centaurus in the S. Hemisphere, and converging at the rate 270 (because of infall their paths are not parallel, only roughly in the same direction).
But because of expansion the distance between LG and VC is actually increasing, despite the infall effect of massive VC's gravity.

 

[Endervhar]

Thanks, folks, for a great response. Lots of stuff to try to get my head round.