Where the world stands still: turnaround as a strong test of \Lambda CDM cosmology

Our intuitive understanding of cosmic structure formation 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, exact nature of dark matter, or baryonic effects. We discuss the possible use of this prediction as an observational test for $\Lambda$CDM cosmology.