Evidence for a sign change of the ISW effect in the very recent universe: hot voids and cold overdensities at $z<0.03$

TL;DR

The paper tests a potential sign change of the ISW effect at very low redshift by measuring CMB temperatures around local voids ($z<0.03$) and contrasting them with simulations. Using Planck maps (PR3/PR4) and two void finders (Sparkling and Revolver) with wavelet filtering, the authors find voids to be significantly warmer than predicted while nearby filaments are cooler, implying a negative ISW effect in the recent Universe. The signal strengthens with void size and LOS underdensity and remains robust across data releases and map choices, reaching up to ~4$\sigma$ in voids and ~6.5$\sigma$ for the void–galaxy temperature contrast relative to simulations. These results are discussed in the context of dynamical dark energy and modified gravity, with DESI results and other puzzles giving additional motivation for non-standard evolution of gravitational potentials at $z\lesssim0.1$. The work emphasizes the need for further theoretical modeling and cross-checks to confirm an evolving-potential ISW mechanism.

Abstract

We find a significant CMB temperature excess in the direction of local underdensities within $z<0.03$. By contrast, less than $0.2\%$ of simulated CMB maps show a similarly significant temperature excess in nearby voids. Combined with earlier findings showing a $>5σ$ cooling of CMB photons in galactic filaments in the same redshift range, we now have possible evidence for a negative Integrated Sachs-Wolfe (ISW) effect in the very recent Universe. In addition to having opposite sign, the observed amplitude is an order of magnitude larger than the predicted Rees-Sciama and ISW effects for the nearby Universe and similar to the expectation of some dark energy and modified gravity models predicting altered growth of gravitational potentials at very low redshift. We discuss the results in light of the latest Data Release 2 results of the Dark Energy Spectroscopic Instrument (DESI) showing evidence for dynamical dark energy. Removing the quadrupole, we find the CMB temperatures measured in nearby voids to a large degree uncorrelated with the temperature measured around nearby galaxies and the observed mean difference between these temperatures is almost $6.5σ$ larger than found in simulations.

Figures (17)

• Figure 1: Void weight maps. Left panels: based on 200 void maps created from Sparkling combining 100 realizations for the volume limited sample of galaxies and 100 realizations from the sample using all galaxies. Right panels: based on Revolver voids identified in the sample using all tracers as well as in the volume limited sample. Each pixel represents a weight proportial to how many times that pixel was flagged as belonging to a void among those 200 void maps (Sparkling ) or among overlapping voids (Revolver ). In these maps, a pixel was defined as belonging to a void when it was found within $r<0.5R_\mathrm{void}$ for voids identified in the redshift range $z=[0,0.03]$ based on the 2MRS redshift catalogue. The upper panels show the result when all open voids were included, the middle panels for all voids larger than the median size of a given void set and the lower panels for the largest quartile of voids. Similar void maps (not shown) were created using only pixels within $r<0.25R_\mathrm{void}$ and $r<0.75R_\mathrm{void}$.
• Figure 2: Definition of $\Delta_\mathrm{LOS}$ considering the density within the solid angle subtended by the void between $1$ and $3R_\mathrm{void}$
• Figure 3: The combined weight maps created from the mean between the Revolver void maps and the Sparkling void maps in Figure \ref{['fig:voidmaps']}. The larger number of times a pixel has been flagged as belonging to a void in Sparkling and Revolver , the higher the pixel weight. A weight larger than 0.5 indicates that the pixel has been flagged as a void several times in both methods. From upper to lower panel, void map based on: all open voids, the largest half of the voids and the larger quartile of voids.
• Figure 4: The Planck SMICA CMB temperature fluctuation map, masked such that only the areas around nearby galaxies (upper panel) and voids (lower panel) are clearly visible. The mask consists of holes of $1$ Mpc radius around all large nearby spiral galaxies in the most massive galactic filaments within $z<0.02$ (see H2025 for details). The void holes includes one realization of Sparkling voids, including all voids with $\Delta_{23}<0$ out to $z<0.03$ and extend to 1/2 void radius. The full CMB temperature map can be seen in the background. It can be clearly seen that the CMB temperature is mostly negative around galaxies and mostly positive in voids.
• Figure 5: The mean CMB temperature within $0.5R_\textrm{void}$ of each individual void for voids within the larges quartile. Voids are taken from the combined Sparkling sample where only voids appearing in $>50\%$ of the random realizations are included, corresponding to the voids having the largest weights in the void weight maps of Figure \ref{['fig:voidmaps']}. The fact that the void position and radius differ slightly for different realizations is reflected in the varying temperature within one single void for some of the voids.