Where the world stands still: turnaround as a strong test of ACDM cosmology

Our intuitive understanding of cosmic structure formation works best in scales small enough so that isolated, hound, 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 ACDM cosmology concerns the scale that separates these limits: the turnaround radius, which is the non-expanding shell furthest away from tile center of a bound structure. We show that the maximum possible value of the turnaround radius within the framework of the ACDM model is, for a given mass equal to (3CM/Lambda c(2))(1/3), with CI 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 ACDM cosmology. Current data appear to favor ACDM over alternatives with local inhomogeneities and no A. However there exist several local-universe structures that have, within errors, reached their limiting size. With improved determination's of their turnaround radii and the enclosed mass, these objects may challenge the limit and ACDM cosmology.