Lensing the darkness: The matter density profile in cosmic voids from UNIONS

TL;DR

This paper presents a significant detection of weak lensing signals from cosmic voids using UNIONS data, revealing differences in matter density profiles between large and small voids and validating a new covariance computation method.

Contribution

It introduces a novel method for covariance calculation in void lensing studies and provides the most significant detection of void lensing from spectroscopic surveys to date.

Findings

Detected void lensing at 6.2σ significance

Large and small voids have different density profiles

Galaxy bias factor measured as 2.45±0.36

Abstract

We measure the distribution of matter contained within the emptiest regions of the Universe: cosmic voids. We use the large overlap between the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS) and voids identified in the LOWZ and CMASS catalogues of the Baryon Oscillation Spectroscopic Survey (BOSS) to constrain the excess surface mass density of voids using weak lensing. We present and validate a novel method for computing the Gaussian component of the conventional weak lensing covariance, adapted for use with void studies. We detect the stacked weak lensing void density profile at the $6.2\sigma$ level, the most significant detection of void lensing from spectroscopically-identified voids to date. We find that large and small voids have different matter density profiles, as expected from numerical studies of void profiles. This difference is significant at the $2.3\sigma$ level. Comparing the void profile to a measurement of the void-galaxy cross-correlation to test the linearity of the relationship between mass and light, we find good visual agreement between the two, and a galaxy bias factor of $2.45\pm0.36$, consistent with other works. This work represents a promising detection of the lensing effect from underdensities, with the goal of promoting its development into a competitive cosmological probe.