Ann wrote:
Think of it like this. The Andromeda Galaxy is only about two million light-years away from us. The Milky Way and Andromeda are both rather big and massive galaxies, and they are close enough that their mutual gravity holds them in a steady grip. Right now they are dancing around each other, and they - we - will probably collide a couple of billion years from now.
NO to the "mutual gravity holds them in a steady grip" and
NO to the "dancing around each other."
The Andromeda Galaxy is currently careening towards the Milky Way at a whopping 300 km/s (0.1% c).
This pure kinetic velocity is
much faster than either the Hubble expansion rate (55 km/s)
or the Andromeda/Milky Way escape velocity (~65 km/s) and it dominates both.
Since the Andromeda Galaxy seems to be odd man out so far as local velocities are concerned
I can only surmise that Andromeda is an interloper that was accidentally hurdled
this way by dynamical interactions with other more distant galaxies.
It's more like the Koozebanian
galley-oh-hoop-hoop mating finale than
all that Koozebanian "dancing around each other" that goes on prior to that.
Click to play embedded YouTube video.