Voids and the Baryon Acoustic Scale: A Standard-Ruler Cross-Check from Underdense Regions

The baryon acoustic scale is usually measured from the clustering of galaxies, but it is also present, inverted in character, in the clustering around voids, and this paper evaluates that underdense standard ruler on its own terms. The sound horizon at the drag epoch for the adopted cosmology is 147.1 megaparsecs, or 99.1 megaparsecs per h. A void-stack measurement over an effective volume of five cubic gigaparsecs per h is forecast to reach a fractional precision of 1.1 per cent on the scale, using about 2e+04 large voids. A bounced expansion history matched to the distance data shifts the inferred scale by 0.3 per cent, which is 0.27 of the forecast error. The void ruler is therefore a genuine cross-check on the galaxy ruler, valuable because its systematics, void finding, tracer bias and boundary effects, are almost disjoint from those of the galaxy measurement, while it is not, at forecast precision, a discriminator of the early bounce. Agreement between the two rulers at the per cent level would constrain the systematics of both more than it would constrain any single cosmological parameter. All values are computed from stated assumptions, not measured.