colliding galaxies

[telesto]

hi all

heres my question, if the universe is expanding at the same speed in all directions ( they use dots on a balloon and slowly inflate it to demonstrate the expansion) how can two galaxies collide because everything is moving away from each other ????????

look forward to the answers

clive:confused:

[ronin]

The universe is expanding at rate X. X being dependant on the distance separating the galaxies.

The 2 galaxies however are attracted to each other by gravity at rate Y.

If Y > X then the are moving towards each other faster then the universe between them is expanding. So then eventually collide.

[acey]

Expansion operates at cosmological distances but not within gravitationally bound systems: the Earth and Sun are not being separated by cosmological expansion. Galaxy clusters (such as our own Local Group which contains the Milky Way, M31 etc) are gravitationally bound, and not expanding. So galaxies within clusters can collide: the Milky Way and M31 will possibly do so in future.

[telesto]

thanks to ronin and acey for your answers i now understand and you explained it so well

regards

clive

[Karlos]

Simply because they are within each others' proximity, where gravity can overcome the expansion of space.