Evidence for a ∼300 Megaparsec Scale Under-Density in the Local Galaxy Distribution

Abstract

Galaxy counts and recent measurements of the luminosity density in the near-infrared have indicated the possibility that the local universe may be under-dense on scales of several hundred megaparsecs. The presence of a large-scale under-density in the local universe could introduce significant biases into the interpretation of cosmological observables, and, in particular, into the inferred effects of dark energy on the expansion rate. Here we measure the K−band luminosity density as a function of redshift to test for such a local under-density. For our primary sample in this study, we select galaxies from the UKIDSS Large Area Survey and use spectroscopy from the SDSS, 2DFGRS, GAMA, and other redshift surveys to generate a K−selected catalog of ∼ 35, 000 galaxies that is ∼ 95% spectroscopically complete at K AB < 16.3 (K AB < 17 in the GAMA fields). To complement this sample at low redshifts, we also analyze a K−selected sample from the 2M++ catalog, which combines 2MASS photometry with redshifts from the 2MASS redshift survey, the 6DFGRS, and the SDSS. The combination of these samples allows for a detailed measurement of the K−band luminosity density as a function of distance over the redshift range 0.01 < z < 0.2 (radial distances D ∼ 50 − 800 h −1 70 Mpc). We find that the overall shape of the z = 0 rest-frame K−band luminosity function (M * = −22.15 ± 0.04 and α = −1.02 ± 0.03) appears to be relatively constant as a function of environment and distance from us. We find a local (z < 0.07, D < 300 h −1 70 Mpc) luminosity density that is in good agreement with previous studies. Beyond z ∼ 0.07, we detect a rising luminosity density that reaches a value of roughly ∼ 1.5 times higher than that measured locally at z > 0.1. This suggests that the stellar mass density as a function of distance follows a similar trend. Assuming that luminous matter traces the underlying dark matter distribution, this implies that the local mass density of the universe may be lower than the global mass density on a scale and amplitude sufficient to introduce significant biases into the determination of basic cosmological observables. An under-density of roughly this scale and amplitude could resolve the apparent tension between direct measurements of the Hubble constant and those inferred by Planck.

Asymmetry and dark energy evolution

“…Altogether, the apparent detection of a local under density can be considered as a possible cause of the original tension between low and high redshift observational constraints discussed in [10,11,12,13,15], which as a solution also agree with the analysis of [19] where they proposed the existence of a local under-density to alleviate the tension in the determination of H 0 .…”

“…Although simple extensions of the ΛCDM model were studied, not all of them alleviated the tension. In [19] the authors showed that the assumption of a large local under-density on radial scales of a few Mpc it is enough to alleviate such a tension.…”

Cluster–void Degeneracy Breaking: Dark Energy, Planck, and the Largest Cluster and Void

“…Measurements of the local galaxy luminosity density indicate that the local universe coincides with an extreme void. Based on Keenan et al (2013Keenan et al ( , 2014 . The values are very close to those of the CS Void.…”

The VLT Survey Telescope ATLAS

“…Previous observations from 2MASS have suggested that there exists a galaxy underdensity in the Southern hemisphere (Keenan, Barger & Cowie 2013;Whitbourn & Shanks 2014), and we wish to test for the existence of this 'Local Hole' at optical wavelengths. Since VST ATLAS will also cover the Great Attractor region, this means that it can use the form of bright galaxy counts to search a large area in this direction for clusters and superclusters even behind Shapley 8.…”