Void Abundance at High Redshift: The Formation Epoch of the Largest Underdensities

The largest voids are late arrivals. This paper follows the computed abundance of voids at two radii, 30 and 60 megaparsecs per h, from the present day back to redshift two, and defines the formation epoch of the largest underdensities as the redshift at which their abundance falls to a tenth of its present value. At the present day the 30 megaparsec abundance is 1.85e+04 per cubic gigaparsec per h and the 60 megaparsec abundance is 2.1e+03; by redshift one the 60 megaparsec abundance has fallen to 224, and by redshift two the 30 megaparsec abundance stands at 364, a fraction 0.0197 of its present value. The formation epoch so defined is redshift 1.04. The implication is direct: surveys that find voids at redshift above one are observing a population still in formation, whose profiles and shapes should differ systematically from the relaxed low-redshift population, and analyses that apply low-redshift calibrations to them without correction import a bias of the size computed here. All values are computed from stated assumptions, not measured.