The DES view of the Eridanus supervoid and the CMB Cold Spot (2112.07699v1)

Abstract: The Cold Spot is a puzzling large-scale feature in the Cosmic Microwave Background temperature maps and its origin has been subject to active debate. As an important foreground structure at low redshift, the Eridanus supervoid was recently detected, but it was subsequently determined that, assuming the standard $\Lambda$CDM model, only about 10-20$\%$ of the observed temperature depression can be accounted for via its Integrated Sachs-Wolfe imprint. However, $R\gtrsim100~h^{-1}\mathrm{Mpc}$ supervoids elsewhere in the sky have shown ISW imprints $A_{\mathrm{ISW}}\approx5.2\pm1.6$ times stronger than expected from $\Lambda$CDM ($A_{\mathrm{ISW}}=1$), which warrants further inspection. Using the Year-3 redMaGiC catalogue of luminous red galaxies from the Dark Energy Survey, here we confirm the detection of the Eridanus supervoid as a significant under-density in the Cold Spot's direction at $z<0.2$. We also show, with $\mathrm{S/N}\gtrsim5$ significance, that the Eridanus supervoid appears as the most prominent large-scale under-density in the dark matter mass maps that we reconstructed from DES Year-3 gravitational lensing data. While we report no significant anomalies, an interesting aspect is that the amplitude of the lensing signal from the Eridanus supervoid at the Cold Spot centre is about $30\%$ lower than expected from similar peaks found in N-body simulations based on the standard $\Lambda$CDM model with parameters $\Omega_{\rm m} = 0.279$ and $\sigma_8 = 0.82$. Overall, our results confirm the causal relation between these individually rare structures in the cosmic web and in the CMB, motivating more detailed future surveys in the Cold Spot region.

• The paper confirms the detection of the Eridanus supervoid at z<0.2 using DES Year-3 galaxy counts and lensing data, characterizing it with δ₀ ≈ -0.2 and R ≈ 200 h⁻¹ Mpc.
• It employs three gravitational lensing methods that achieve S/N ≥ 5, revealing a signal approximately 30% weaker than ΛCDM predictions.
• The study indicates that the supervoid may only partially explain the CMB Cold Spot anomaly, highlighting the need for further research into cosmic voids and dark energy.

The DES View of the Eridanus Supervoid and the CMB Cold Spot

This paper investigates the enigmatic Cosmic Microwave Background (CMB) Cold Spot (CS) by exploring its potential association with the Eridanus supervoid, an extensive under-density in the universe. Utilizing data from the Dark Energy Survey (DES) Year-3, the authors employ both galaxy distribution and gravitational lensing data to assess the properties of the supervoid and its possible impact on the CMB.

Overview and Methodology

The Cold Spot, located in the Southern Galactic Hemisphere, has confounded researchers due to its unusually low temperature when compared to standard CMB fluctuations. Various hypotheses have been posited, including the possibility that this anomaly is connected to a supervoid, which might impact the CMB via the Integrated Sachs-Wolfe (ISW) effect. Prior analyses suggested that the Eridanus supervoid, situated at redshift $z < 0.2$, might only account for a minor portion (10-20%) of the observed temperature depression assuming a $\Lambda$CDM cosmology.

In the current work, the authors use the DES redMaGiC catalogue, which provides accurate photometric redshifts, to map the line-of-sight galaxy density and test for under-densities. They supplement this data with weak gravitational lensing observations derived from DES mass maps to paper the projected mass distribution in the Cold Spot's vicinity at multiple scales.

Key Findings

• Detection of the Eridanus Supervoid: The DES Year-3 data confirms the under-density at $z<0.2$ in the direction of the Cold Spot. The void, characterized by $\delta_\mathrm{0} \approx -0.2$ and an approximate radius $R \approx 200 ~h^{-1}$Mpc, is evident both in galaxy number counts and gravitational lensing signals, making it the most significant under-density in the DES coverage area.
• Gravitational Lensing Results: Using maximum a posteriori methods, three versions of gravitational lensing maps (Kaiser-Squires, Wiener filter, and Null B-mode) consistently indicate an under-density. The team reports a $\mathrm{S/N} \gtrsim 5$ detection of the supervoid, corroborating previous simulations that predicted detectability for such large structures.
• Comparison with Simulations: Notably, the lensing signal from the observed supervoid appears less pronounced (by approximately 30%) than the typical signal from simulation-based $\Lambda$CDM models. This discrepancy extends to the innermost regions of the structure, marking a $\sim 2\sigma$ tension with predictions but possibly attributable to large-scale reconstruction challenges.

Discussion and Implications

The paper underlines that while the Eridanus supervoid contributes to the CS phenomenon, the full temperature anomaly remains unexplained under conventional cosmology. The lensing analysis indirectly suggests that large supervoids could have stronger impacts on the ISW effect than previously recognized, which if true, would challenge current $\Lambda$CDM expectations about cosmic evolution and the growth of large-scale structures.

Looking forward, deeper probes into the growth of fluctuations and void modeling are crucial, potentially informing broader issues in cosmology, such as the $S_{8}$ tension and the nature of dark energy. Upcoming data from DES and other surveys, alongside refined CMB observations (including polarization studies), will be pivotal in providing more detailed insights into these cosmic anomalies and their theoretical implications.