[neilep]

Dearest Galaxyologists,

As a sheepy, I of course luff galaxies. They are my all time favourite things that contain billions of stars and have nice pretty shapes.

look, here's some



Nice eh ?..Being delivered and turned into a hanging mobile next Tuesday.

I'm confewesed though !..yes..yes I really am !...ewe see , my neighbour told me that as the Universe expands everything we see, all the stars and galaxies expand with it. He got his balloon out and demonstrated it to me.

Well, ewe can imagine my shock and horror when I heard that in fact some galaxies collide !!!...... and that what really worries me even more so is that The Androgynous Galaxy and our own Mouldy Way are also on a collision course !!..Noooooooooooo !!....how can this be ? *le sigh* Why do some galaxies collide when they are racing away from each other ! ?

I have an appointment to have a Prince Albert inserted next Wednesday...is it worth me keeping it ?



Hugs et les shmishes


mwah mwah mwah !!


neil
Galaxy Worry-wart
xxxxxxxxxxxxxxxxx

[lyner]

If you let a gas expand, the molecules are still bashing into each other- just a bit less often. Same sort of argument applies here, I think.

[ukmicky]

The galaxies which collide, like those in our local group and are gravitationally bound to each. The gravitational force between our local group of galaxies is much much stronger than the expansion force which is actually a very weak force and theirfore they move towards each other rather than away.

[neilep]

If you let a gas expand, the molecules are still bashing into each other- just a bit less often. Same sort of argument applies here, I think.

Thank Ewe Sophiecentaur. ...As a firm believer in empirical study I have eaten a lot of beans and drunk lots of fizzy pop........and am ready with mirror and magnifying glass in hand to see gas molecules bashing into each other.
YAYYYY !!

[neilep]

The galaxies which collide, like those in our local group and are gravitationally bound to each. The gravitational force between our local group of galaxies is much much stronger than the expansion force which is actually a very weak force and theirfore they move towards each other rather than away.

Wooo !!..Thanks Michael !...so...even though the nearest galaxy is like...well far away...even gravity still affects it !!..Gosh...gravity is like...well big and all over the place !!

So let me try and get this...even though we are on a collision course yes ?....and that we are moving towrads each other locally yes ?...the space that our local space inhabits is still flying apart eh ?

Are we like ripples moving one way atop a current moving in a different direction?

[lyner]

There is a 'critical density' of materials which governs whether they will keep going apart or coalesce.
On the largest scale, it is used in order to determine whether the Universe will collapse, in the end (closed), keep on expanding more and more (open) or keep expanding but slower and slower (flat).

As material moves away from a centre of a distributed mass (say a group of galaxies or a cloud of dust OR the Universe itself), it has a certain amount of Kinetic Energy (KE), by virtue of its movement. It also has a certain amount of (negative) gravitational potential energy (GPE) by virtue of its being attracted to the centre of gravity of the  mass. If there is more KE than GPE, then the mass will carry on expanding because there will be KE to spare . If it is less, then the mass will collapse because there is not enough KE.
The average density of the region of interest determines how much GPE it has which, in turn, tells you whether it will expand or collapse eventually.