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2 Standard Cosmological Model > 2.3 Gravitational Instability - Pg. 15

2.3 Gravitational Instability 15 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 c . 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.