Are the galaxies dispersing? What is it that has the Andromeda Galaxy on a collision course with the Milky Way?
In the expanding universe, galaxies tend to get further away from one another. However, this is only the case on average.
I.e., that the density of galaxies decreases as they expand; however, galaxies and clusters of galaxies fly around concerning each of their counterparts. They collide, smash into one another, spin around, and so on. Anything that the attractive gravitational force instructs them to do.
What is the cause?
What's happening is that two different events are taking place, and it comes down to which side takes the lead. It comes down to a question of size.
Space expands, drawing objects away from one another.
However, at the same time, gravity pulls everything into one.
On average, expansion prevails in very, very long intervals and over extremely, very long distances.
Another way to look at it is as follows:
- At huge scales (much larger than the size of clusters of galaxies), You can think of the universe's expansion and leave out the individual galaxies' movements.
- In the galaxy clusters, it is possible to observe galaxies whizzing through (e.g., The Milky Way and Andromeda). For the majority of calculations regarding the dynamic of the cluster, you can disregard the growth of space. Attractive gravity between galaxies overpowers it.
- When you look at the size of a single galaxy, its internal processes within the universe are completely controlled by gravitational pull - it absolutely and UTTERLY overpowers any impact on space expansion. Therefore, at this scale, it's possible to disregard the increase completely. This is why galaxies don't expand with the speed of intergalactic space.
- We are electrically neutral on the level of you and me and the atoms that make up our body. Electrostatic forces thwart the gravitational force and an expansion in space. This is why we don't grow like the universe and do not clump together (sexual attraction aside). We gain weight when we consume too much food and revolve around our sexual preferences.
- In the nucleus of your atoms, The seats are not electrically neutral. But the weak and strong nuclear forces can overwhelm the repelling electromagnetic forces, which is why protons stay together.