CAS AS 100 Chapter Notes - Chapter 23: Cosmic Microwave Background, Cosmological Principle, Big Crunch
19 May 2016
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~dark matter"s gravitational pull must be the primary force holding galaxies and clusters together and might"ve pulled them together in the first place. After the big bang, the universe expanded everywhere. The stronger gravity in regions of enhanced density pulled in matter until these regions stopped expanding and became clouds even as the rest of the universe continued to expand. Early on, all galaxies that will eventually end up in a cluster are flying apart with the expansion but the gravity of the dark matter associated with the cluster reverses the trajectories. The galaxies fall back inward and start orbiting each other with random orientations. Many other large clusters of galaxies appear to be drawing in new members, judging is from the velocities of galaxies near the outskirts of those clusters (rate of separation slowing) Clusters also seem to be tugging on one another, hinting that they are part of even bigger gravitationally bound systems called superclusters.
