How can galaxies collide if the Universe is expanding?
Galaxies collide for the same reason that is responsible for their existence, which is gravitational forces. To better understand, we will have to look further back into time and our Universe.
Expansionary powers at play
For a long time, even scientists were confused by the state of the Universe – they were juggling different theories. When Einstein wanted a static Universe, Friedmann said that there could be two sets of solutions to how the universe truly was – a contracting and an expanding Universe. It became even more clear when Hubble, Slipher learned to differentiate distances of distant objects through the shift and redshift in their wavelengths. Thus, arriving at a consensus that the Universe is indeed expanding, at frantic speeds in distances further away from us.
It is now believed that dark energy is the force behind the expansion of the universe. As in, it provides a constant outwards push that enables the universe to expand. However, there is exists a gravitational pull between the objects, matter, and what-not in the universe.
At the beginning of the universe, before expansion, it was tightly packed with gravity having the upper side. Stars were formed by gas clouds, it was then pulled into galaxies, galaxies came together to form clusters. But as they began to form groups and clusters, this process also began to put distance between them allowing expansion to take over.
So, we can safely say that for the last 6 billion years, Universe is in an ever-expanding mode. And yet, the question remains, how do they still collide?
Gravitational Forces at Play
The biggest competition that takes place in space would the one between expansion and gravitational forces. The universe is a non-uniform fabric. There are solitary galaxies; as well as Local Groups and bigger groups such as clusters. At a clustering stage, it is unlikely and very rare for collisions to take place. Here expansion wins as they move at frantic speeds due to distances between them even when gravity is so strong.
But trouble comes in the form of Local Groups, where objects move at moderate speed. Here gravity is the stronger force. It is here where histories of galaxies are written. This is because, in close-bound galaxies or objects, the speed of expansion is relatively smaller. While at large distances, it easily overwhelms any other forces at play.
Another point to note is that just because two galaxies are under the spell of gravity, doesn’t necessarily mean they will collide. We need to remember that the universe we are talking about is so vast that galaxies are minor targets. In the Local Groups, as mentioned, the likelihood of galaxies bumping into each other is much greater. But then again it is not necessary for collisions to take place. In most cases, galaxies usually merge with each other. And even for that to happen, it may take billions of years.
For example, the coveted collision of the Andromeda Galaxy and the Milky Way, which have been in each other’s gravitational pull for years is predicted to take another 4 billion years to collide.
Therefore, our Universe is in a never-ending tango between gravitational forces and expansion. On the largest scale, an expansion might have the upside. But if a force is at play, especially in close quarters, the expansion will have very little influence and gravity will be the hero.