Evidence for a ~300 Mpc Scale Under-density in the Local Galaxy Distribution

Abstract: 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.

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

The study conducted by Keenan et al. presents a thorough investigation into the luminosity density of the local universe, particularly focusing on a potential large-scale under-density on the order of hundreds of Megaparsecs. This paper employs a robust dataset primarily sourced from the UKIDSS Large Area Survey, reinforced by spectroscopic data from several comprehensive redshift surveys including SDSS and GAMA. The sample comprises approximately 35,000 galaxies, constituting a nearly complete spectroscopic catalog that permits the examination of the K-band luminosity function (LF) across a diverse redshift span ranging from 0.01 to 0.2.

The methodology centers on the construction of a K-band selected galaxy catalog, meticulously cross-matched with redshift data to facilitate accurate distance measurements. The team meticulously addresses potential biases such as Petrosian aperture clipping in UKIDSS data, which could skew results by underestimating fluxes of extended galaxies, particularly at lower redshifts. These biases are systematically corrected through model-based extrapolations, enhancing the reliability of the luminosity density measurements.

A compelling result of the analysis is the identification of a rising luminosity density at z > 0.07, achieving levels approximately 1.5 times greater than those detected locally at z < 0.07. This observation suggests a significant under-density within the local universe, implying that the stellar mass density increases with distance beyond a proximate scale of 300 Mpc. The authors posit that, assuming a consistent relationship between luminous and dark matter, this under-density could introduce notable biases in cosmological measurements—specifically, those related to dark energy and the rate of universal expansion.

By exploring the consistency of the LF shape with redshift, the paper draws remarkable conclusions regarding the structure of the universe, proposing potential resolutions to current cosmological tensions, such as discrepancies in the Hubble constant derived from local versus cosmic microwave background (CMB) observations. The under-density hypothesis necessitates a reconsideration of observational data under the assumption that local environments differ substantially in density from average cosmic values.

Additionally, the research supports ongoing efforts by utilizing mock catalogs to simulate expected variances under the Lambda-Cold-Dark-Matter (ΛCDM) model, underscoring that observed deviations might ambitiously align with such simulated predictions but require further cosmic observations to establish definitive correlations.

The implications of this work extend beyond mere theoretical musings, emphasizing the importance of accounting for local inhomogeneities in cosmological models. This underlines the necessity for a revision of cosmological parameters inferred from observations within local cosmic structures, which may not be fully representative of the universe at large. This paper supports the continuous push toward observing larger volumes to attain a homogenized cosmic view, free from the biases introduced by local structure variations.

In sum, this research adds a substantive layer to the understanding of cosmic structure, offering a lens through which the subtleties of local density variations might be seen as influential factors in the determination of fundamental cosmological constants. Future investigations could build upon this foundation, employing more expansive datasets and refined techniques to unravel the intricacies of the universe's luminous and dark constituents.