Why is the Andromeda Galaxy moving towards us if the universe is expanding? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.
The universe is expanding.
The Local Group of galaxies is not. It is a gravitationally bound structure.
Let me explain.
The early universe was filled pretty much uniformly with gas that was expanding everywhere. However, it had small “density perturbations”: In some places, it was a little denser, in some places it was a little less dense.
Wherever it was denser, it had more gravity. Gravity that was pulling it together. If the density was high enough, gravity was sufficient to overcome the expansion: those bits of matter stopped flying apart and instead, ended up in gravitationally bound structures.
These structures are the largest gravitationally bound structures today, clusters of galaxies such as the Local Group, to which Andromeda and the Milky Way both belong.
So clusters of galaxies do not individually expand, but clusters of galaxies do still fly apart from each other everywhere.
Having said that, there is also a degree of randomness involved. Take the Virgo cluster, for instance. It is a cluster of galaxies some 50–60 million light years from here, moving away from us at nearly a thousand kilometers per second. But within that cluster, galaxies move about rather rapidly, in chaotic orbits in their mutual gravitational field. So there is M86, for instance, a member of the Virgo cluster that happens to be moving toward the Milky Way at the moment. It is still gravitationally bound within Virgo, and a few billion years from now, it will be receding from us just as rapidly as it is in a different segment of its trajectory within Virgo.
To make sense of it, imagine that you are looking at a child on a merry-go-round. Say you are slowly backing away from the merry-go-round, so the distance between you and the merry-go-round steadily increases. But if the merry-go-round spins fast enough, for every half revolution the child may actually be approaching you. Yet the child is still bound to the merry-go-round and, averaged over several revolutions, the distance between you and the child will continue to increase.
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