A Possible Cold Imprint of Voids on the Microwave Background Radiation

TL;DR

This study measures the average temperature change in the CMB caused by voids in the galaxy distribution, finding a signal slightly higher than predicted by standard cosmological models, with implications for understanding large-scale structure effects.

Contribution

It provides the first measurement of the CMB temperature imprint of voids across a range of sizes and redshifts, highlighting the complex effects of voids and their environments on the ISW signal.

Findings

Detected a 2.6-2.9 μK temperature decrement associated with voids.

Found that small voids can produce hot spots due to their environment.

Observed a higher-than-expected ISW signal compared to ΛCDM predictions.

Abstract

We measure the average temperature decrement on the cosmic microwave background (CMB) produced by voids selected in the SDSS DR7 spectroscopic redshift galaxy catalog, spanning redshifts $0<z<0.44$. We find an imprint of amplitude between 2.6 and 2.9$\mu K$ as viewed through a compensated top-hat filter scaled to the radius of each void; we assess the statistical significance of the imprint at ~2$\sigma$. We make crucial use of $N$-body simulations to calibrate our analysis. As expected, we find that large voids produce cold spots on the CMB through the Integrated Sachs-Wolfe (ISW) effect. However, we also find that small voids in the halo density field produce hot spots, because they reside in contracting, larger-scale overdense regions. This is an important effect to consider when stacking CMB imprints from voids of different radius. We have found that the same filter radius that gives the largest ISW signal in simulations also yields close to the largest detected signal in the observations. However, although it is low in significance, our measured signal is much higher-amplitude than expected from ISW in the concordance $\Lambda$CDM universe. The discrepancy is also at the ~2$\sigma$ level. We have demonstrated that our result is robust against the varying of thresholds over a wide range.