Why Andromeda Galaxy is moving towards us if the universe is expanding?

The question that originally appeared on Quora.com was answered by a part-time physicist Victor T. Toth. Quora is a very popular platform for knowledge sharing and finding answers to all types of questions. Here's the explanation to the question "why Andromeda is moving towards us if the universe is expanding?"

The early universe was uniformly packed with dust and gases. However, these gases had small "density perturbations", means the gases were a little more dense at some places while a little less dense at some other. Those places where they were more dense, gravity came into action and overcame the expansion. It started pulling the matter towards each other, and as a result, these became gravitationally bound structures over time instead of flying apart from each other.

These gravitationally bound structures are the largest of all the structures in the present day universe, known as cluster of Galaxies. Our local group of galaxies is also one such structure that consist of our home galaxy Milky Way and the neighbouring Andromeda galaxy.

Any galaxy inside these clusters do not move away, instead they move towards each other due to the mutual gravitational influence. It is the entire cluster containing these local group of galaxies that move away from one another.

However, at the same time, there is a degree of randomness also involved here. For instance, the Virgo cluster is a group of galaxies situated 50-60 million light years far from us, moving away at almost one thousand kilometers a second. The galaxies within this cluster move rapidly in chaotic random orbits under the influence of mutual gravity. 

A galaxy M86 within this cluster is however, moving towards our Milky way at this time. However, it is bound to the gravitation of Virgo, hence a few billion years from now, it will be moving away from us, just as rapidly in some other segment of its orbit inside Virgo.

To visualise this easily, consider a child that is sitting on a merry-go-round. Now suppose you begin to slowly back away from the merry-go-round. If it spins fast enough, then for the one-half of a round, that child will actually be approaching towards you, even if you are receding away.

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