Red-shift and Galactic collisions

Being somewhat addicted to learning Physics, Biology, Chemistry and Astronomy, I often find myself confused, even baffled. While I try working most of these issues on my own, there are some where I end up drawing a blank. This is one of them.

If I understand correctly Mr. Hubble discovered and documented the astronomical equivalent of the Doppler effect a.k.a. the"red shift". From his measurements he deducted that the further away the object was the more red-shifted it's emitted light was, hence the expanding universe model was born.

My problem with this is, if all the stars or galaxies are racing away from each other due to universal expansion, how is it that the Milky way is currently colliding with other galaxies and is bound to collide with Andromeda in about 4 billion years?

Doesn't that prove Hubble was wrong? I mean if all galaxies are supposed to be racing away from each other than none of them should ever collide with any other... or am I completely wrong in my thinking?


Kind regards, M.

reply to post by Manawydan


Wow. i've been sitting here thinking about this for a few minutes now and I know that when the Big Bang occurred the explosion was "uneven" which increased as the matter cooled and fell into each other. As these materials spiraled into the center creating galaxies perhaps the spin created some forward momentum in a direction different from the outward movement of the expansion. Times this by many galaxies I guess you get things bumping into each other.

reply to post by Manawydan


Galaxies collide because if they are relatively near each other (relative here is like 2 million lightyears) the gravitational attraction brings the galaxies closer, eventually colliding. But galactic collisions are over such a large area that normally no stars collide with each other, our galaxy and andromeda will collide around the year 3,000,000,009 AD.

Thank you both for your replies. It would appear that I was missing an important part of the equation, which is: The M-31 (Andromeda) Galaxy's red shift is in fact blue-shift. That pretty much makes my original confusion obsolete.

Kind regards, M.