If the universe is becoming larger, why would galaxies collide?

[Realitycheck]

It certainly looks like the larger the universe gets, the more space you get between galaxies. Which is why I have trouble picturing why we would collide with Andromeda.

[Martin]

It certainly looks like the larger the universe gets, the more space you get between galaxies. Which is why I have trouble picturing why we would collide with Andromeda.

 

Excellent! You are asking a question about something you realize you don't understand and it's coming across very clear. Thanks!

 

Andromeda and Milky are part of a group of 10 or so galaxies which are gravitationally bound together.

 

it is called the Local Group.

 

In GR the distances between gravitationally bound objects do not expand.

 

You should try not to be confused by WORDS like "expanding universe" and "expanding space". These are merely VERBAL concepts that imperfectly reflect the mathematical reality of a dynamic metric with changing distances (in our case on average mostly expanding).

 

I gather from a recent article by Larry Krauss, one of the world's top cosmologists, that we can expect in the very long term to have all the galaxies in the Local Group coalesce into one big galaxy. It won't be as pretty probably. When spiral galaxies merge they lose the spiral structure and typically form socalled elliptical galaxies.

 

You know that the distance between atoms in metal ruler is not expanding (because atomic and crystal bonds hold at fixed distance)

And you know that solar system distances are not expanding (because analogous gravitational structure).

So there is no reason to suppose that distances between members of the Local Group should be expanding either!

 

It is also a bound structure, like a rock or a ruler or the solar system.

 

So if Andromeda is falling towards Milky (as i believe it is) then the general expansion of largescale distances has nothing to do with this and can not be expected to interfere.

 

Expansion of distances refers to LARGESCALE, between things too far apart to be bound in a coherent system. And it is only observed at such large scales (much larger than distances within Local Group)

[Edtharan]

The simple answer is Gravity.

 

The further away an object is from another, the lower the strength that the gravity is between them. So if the acceleration due to this gravitational attraction is less than the expansion rate between the objects, then they will move apart.

 

However, if the acceleration due to gravity is greater than the rate of expansion between the objects, then they will move together.

 

Here is an experiemnt:

1) Get together a lot of blocks of around the same size (lego blocks wou8ld be good for this).

2) Start with 10 blocks in a row (like this ABCDEFGHIJ)

3) Place 3 blocks (1, 2 and 3), the first on top of A, the Second on G and the last on J.

4) You will need to repeat these next steps in sequence to perform the experiment

4a) Place a block between every two blocks: eg if you have ABC then you will place blocks (Y and Z) like this: AYBZC). DO this for all blocks

4b) Move the block "1" 4 blocks along the line towards block "J"

4c) Move the block "2" 4 blocks along the line towards block "J"

 

Notice that even though both blocks "1" and "2" are moving the same speed, only block "2" will ever reach block "J". Block "1" will be moving along, but the distance between it and block "j" will actually get bigger.

 

Adding in the blocks is like the expansion of space (it is not the same, it is just an simple analogy - don't take it too far). The movemnt of the blocks "1" and "2" is like the movments of the galaxies. If a galaxy is near enough and moving fast enough then it will eventually reach us, if it is too far away or moving too slowly, then it will never reach us.

 

As an extra exploration: What occurs if you start a Block on "F" and only move it 4 palces towards "J". Will it ever reach "J", will it get further away from "J" or will it do something else?

[Daecon]

Aren't clusters of galaxies also just parts of a larger supercluster?

[antimatter]

It certainly looks like the larger the universe gets, the more space you get between galaxies. Which is why I have trouble picturing why we would collide with Andromeda.

 

It's just gravity. The Universe is expanding due to Dark Energy,

but it's a local group of galaxies and gravity is pulling them together.

The actual collison shouldn't affect us all that much, it's the gravitational pull that will throw off our Solar System.

[Klaynos]

The expansion is getting faster because of dark energy, not expanding because of it... Although what we know about dark energy is rather limited...

[antimatter]

Sorry, my mistake, I'm a little tired today.

[lakmilis]

Alright I am a little outdated on certain things with these respects... I remember though something I was sceptical too during my younger years... the cosmological principle is it?.. where all celestial objects were meant to be generally expanding or moving away from each other....

 

 

Now... I always have wondered when it comes to black holes... if they are actual 'holes' ... then they should not move away but seem rather static in their relative position eg. they would function as 'local' centres.... if they are dense .. as do indeed have the mass and it is not swallowed into some singularity we still are boggled about... then they will behave as other objects... can anyone update me a bit on this?

 

cheers

[Daecon]

About black holes moving... I'm not sure if it's an accurate analogy or not, but imagine digging a hole in a football field or something, then lifting the entire field up and moving it somewhere. The hole and it's surrounding area of space still move relative to the rest of the world.

 

Can that be extrapolated to a black hole and the surrounding space? Say if it's "football field" is the size of as far as it's gravitational influence extends?

[Klaynos]

A blackhole moves in the same way as a star of identical mass...