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Andromeda galaxy

acurno

We've always been told that the expansion of the universe is like a balloon inflating. Each galaxy rushing away from each other, observed through redshift on the spectrum.
Why then do galaxies collide? Why is Andromeda on a collision course with our own?

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muskyj

I think the simple answer is gravity.
The universe is expanding everywhere but not fast enough to counteract the localised effects of gravity. if it did then then stars and galaxies etc. would never have formed in the first place.

Cú Giobach

acurno wrote: »

We've always been told that the expansion of the universe is like a balloon inflating. Each galaxy rushing away from each other, observed through redshift on the spectrum.
Why then do galaxies collide? Why is Andromeda on a collision course with our own?

Think of it like two ice skaters standing on an ice rink holding hands, with the rink "expanding" beneath them. They wont be pulled apart because the friction between the skates and the ice wouldn't be strong enough to overcome the force of them holding hands, and also it wouldn't be hard for the skaters to actually pull themselves together, against the force of the expansion.
Space itself is frictionless so bound objects (eg, objects bound by gravity, or atoms with the nuclear force) wont be pulled apart.