Question: If I understand the various articles I’ve read and programs I’ve seen on TV and the internet. It has been determined that the Andromeda and Milky Way galaxies are on a collision course and are destined to “collide” in about 4 billion years. My question is this. If the collision will happen, how can it? Given the observations thus far that all of the galaxies are receding from each other due to the expansion of space-time itself. – Guy
Answer: You are right in that on the large scale all galaxies are expanding away from each other. On smaller scales, though, there are what are called “peculiar motions” where galaxies can have motions relative to each other that are due to local gravitational attraction. The Andromeda and Milky Way galaxies have peculiar motions whereby they are moving toward each other, which results in the prediction that they will eventually collide.