Evidence for a ~300 Mpc Scale Under-density in the Local Galaxy
  Distribution
  Galaxy counts and recent measurements of the luminosity density in the
near-infrared (NIR) 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. In this effort, we combine photometry from UKIDSS and
2MASS with redshifts from the SDSS, 2DFGRS, 6DFGRS, 2MR, and GAMA surveys. We
find that the overall shape of the z=0 rest-frame K-band luminosity function
(M* = -22.15 +/- 0.04 and alpha = -1.02 +/- 0.03) appears to be relatively
constant as a function of environment and redshift out to z ~0.2. We find a
local (z < 0.07) luminosity density that is in good agreement with previous
studies. Beyond z ~ 0.07 we detect a rising luminosity density that reaches a
value ~1.5 times higher than that measured locally at z>0.1. This suggests that
the stellar mass density as a function of redshift 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 value on a scale and amplitude sufficient to introduce significant
biases into the determination of basic cosmological observables, such as the
expansion rate. An under-density of roughly this scale and amplitude would be
sufficient to resolve the apparent tension between direct measurements of the
Hubble constant and those inferred by Planck.
