Structure formation in \Lambda CDM cosmology is a cosmic battle between expansion momentum, gravity, and vacuum energy. Our intuitive understanding of the process works best in scales small enough so that bound, relaxed gravitating systems are no longer adjusting their radius; and large enough so that space and matter follow the average expansion of the Universe. Yet one of the most robust predictions of \Lambda CDM cosmology concerns the scale that separates these limits: the turnaround radius, which is the non-expanding shell furthest away from the center of a bound structure. The maximum possible value of the turnaround radius within the framework of the \Lambda CDM model is, for a given mass M, equal to (3GM/\Lambda c^2)^{1/3}, with G Newton’s constant and c the speed of light, independently of cosmic epoch, detailed assumptions, exact nature of dark matter, or baryonic effects. We discuss the possible use of this prediction as an observational test for \Lambda CDM cosmology.

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Date added: Wed, 9 Oct 13

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