In a Universe where space is expanding, how do separate galaxies manage to collide? (Strange Universe Series, 2011)

Dark energy, which is the reason for the accelerated expansion, is counteracted by gravitation.
The closer two objects are together, the weaker dark energy gets in comparison to gravitation between the objects (there is less space between them, thus less dark energy, and gravitation is stronger than at greater distance). Now, I am not an astrophysicist, but I can imagine that Milky Way and Andromeda were so close to each other, that the gravitational attraction between them was stronger than the dark energy trying to push them apart.

Also, are al lgalaxies expanding from the same central point? Because, if they are not, this would explain how they could collide with each other despite expanding out from a random point…

If two galaxies are separating due to cosmic expansion they will still collide if the gravitational force between them is strong enough. Eventually collisions will stop as all galaxies will either have merged or be receding from each other beyond the power of gravity to pull them back.

@yankeetooter this is not how the universe expands. See metric expansion of space.

There are objects out there in the universe heavy enough to attract galaxies across long distances. One example being The Great Attractor.

Galaxies themselves did not form perfectly homogenously throughout space in the early universe, but tended to cluster, presumably nucleating in locations that ended up slightly denser in hydrogen and helium. And there are clusters of clusters (and clusters of clusters of clusters? and so on?) and corresponding great voids.

@ragingloli Thanks for a simple explanation.

@yankeetooter From here, everything that is at great distance appears to be moving directly away from us, and the further something is from us, the more rapidly it is receding, according to observed redshifts. That might lead on to conclude that the Big Bang occurred where Earth is today. But spacetime is more complex than that. If you could pack a giant telescope in a super-fact spaceship and travel thousands of light years, or millions of light years from Earth, you would see the same pattern suggesting that this new location was the center of the big bang. This is because spacetime itself is expanding everywhere except in areas of clusters with high gravitational attraction.

@hiphiphopflipflapflop Great links. Thanks so much for those.

Gravity in galaxy clusters beats dark energy. The distant future will become very dark for our descendants. They might think the Virgo cluster is everything out there.

Well, that’s good….wouldn’t want anything to happen to the Virgos, lol!