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Chapter 2 - Standard Cosmological Model > 2.3 Gravitational Instability

2.3 Gravitational Instability

Now we are at a position to understand how objects, like the Milky Way galaxy, have formed out of small density inhomogeneities that get amplified by gravity

Let us consider for simplicity the background of a marginally bound (flat) Universe which is dominated by matter. In such a background, only a slight enhancement in density is required to exceed the critical density ρc. Because of Birkhoff’s theorem, a spherical region that is denser than the mean will behave as if it is part of a closed Universe and increase its density contrast with time, while an underdense spherical region will behave as if it is part of an open Universe and appear more vacant with time relative to the background, as illustrated in figure 2.1. Starting with slight density enhancements that bring them above the critical value ρc, the overdense regions will initially expand, reach a maximum radius, and then collapse upon themselves (like the trajectory of a rocket launched straight up, away from the center of the Earth). An initially slightly inhomogeneous Universe will end up clumpy, with collapsed objects forming out of overdense regions. The material to make the objects is drained out of the intervening underdense regions, which end up as voids.


  

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