Evidence against a supervoid causing the CMB Cold Spot (1704.03814v1)

Abstract: We report the results of the 2dF-VST ATLAS Cold Spot galaxy redshift survey (2CSz) based on imaging from VST ATLAS and spectroscopy from 2dF AAOmega over the core of the CMB Cold Spot. We sparsely surveyed the inner 5$^{\circ}$ radius of the Cold Spot to a limit of $i_{AB} \le 19.2$, sampling $\sim7000$ galaxies at $z<0.4$. We have found voids at $z=$ 0.14, 0.26 and 0.30 but they are interspersed with small over-densities and the scale of these voids is insufficient to explain the Cold Spot through the $\Lambda$CDM ISW effect. Combining with previous data out to $z\sim1$, we conclude that the CMB Cold Spot could not have been imprinted by a void confined to the inner core of the Cold Spot. Additionally we find that our 'control' field GAMA G23 shows a similarity in its galaxy redshift distribution to the Cold Spot. Since the GAMA G23 line-of-sight shows no evidence of a CMB temperature decrement we conclude that the Cold Spot may have a primordial origin rather than being due to line-of-sight effects.

• The paper used the 2dF-VST ATLAS Cold Spot galaxy redshift survey (2CSz) to identify voids near the Cold Spot, finding three voids but noting they were too small and sparse to cause the anomaly via the Integrated Sachs-Wolfe effect.
• Findings suggest the CMB Cold Spot is unlikely caused by a foreground supervoid, even when considering combined voids and over-densities, challenging explanations based on standard gravity and the Integrated Sachs-Wolfe effect alone.
• The results strengthen the case for a primordial origin of the Cold Spot, potentially linked to non-Gaussian fluctuations in the early universe, and highlight the need for deeper galaxy surveys to further investigate such large-scale anomalies.

Overview of "Evidence against a supervoid causing the CMB Cold Spot"

The paper "Evidence against a supervoid causing the CMB Cold Spot" by R. Mackenzie et al. provides a meticulous examination of the existence of a supervoid correlated with the Cosmic Microwave Background (CMB) Cold Spot, a prominent feature observed in the CMB anisotropy maps. The CMB Cold Spot is a perplexing anomaly diverging from the expected Gaussian fluctuations predicted by the standard cosmological model, ΛCDM, raising the possibility of non-standard physics in the early universe.

Research Context

The paper is anchored within the continuing discourse on the origin of the CMB Cold Spot. Previous hypotheses suggested that a supervoid, a large region with a low density of galaxies, could account for this anomaly through the Integrated Sachs-Wolfe (ISW) effect, a phenomenon where gravitational potential wells decay with time in a universe with a cosmological constant, leaving an imprint on the CMB. The ISW effect has been statistically confirmed yet remains subject to contention regarding its capability to explain the Cold Spot entirely.

Methodology and Findings

The authors conducted the 2dF-VST ATLAS Cold Spot galaxy redshift survey (2CSz) to investigate this hypothesis. By surveying around 7000 galaxies within a 5-degree radius of the Cold Spot up to a redshift of 0.4, they identified three voids at redshifts 0.14, 0.26, and 0.30. However, these voids, intercalated with over-dense regions, were deemed insufficient in scale and density to evoke the Cold Spot via the ISW effect. The voids, even when combined, imparted a negligible ISW temperature decrement against the observed properties of the Cold Spot.

A comparative analysis with the GAMA G23 field, serving as a control with similar galaxy redshift features but no corresponding CMB anomaly, further substantiates the conclusion that the observed voids are not the source of the Cold Spot. Through rigorous statistical analysis, including Akaike Information Criterion (AIC) assessments, the authors surmise that any discernible ISW-like effect is improbable, aligning with previous contentions against such causal attribution.

Implications

The implications extend into several realms of cosmological inquiry:

• Primordial Non-Gaussianities: The persistence of the CMB Cold Spot, after excluding a supervoid explanation, lends credence to theories positing primordial non-Gaussian fluctuations, potentially connected to topological defects or variations in the inflationary epoch.
• Modified Gravity: The results challenge models advocating for enhanced ISW effects through non-standard cosmologies or modified gravity scenarios, indicating a need for novel approaches to reconcile observed anomalies without standard ISW contributions.
• Future Survey Requirements: By delineating the limits of current observational approaches, the paper underscores the necessity for more comprehensive surveys probing deeper into earlier redshift ranges to further constrain potential explanations of large-scale CMB features.

Conclusion

Through a discriminative approach, this paper significantly constrains the plausibility of a supervoid causing the CMB Cold Spot, reinforcing the conjecture for a primordial origin. As such, it paves the way for further investigations into the early universe's initial conditions and challenges cosmologists to refine models anticipating non-Gaussianity or modifications beyond ΛCDM. In future developments, enhanced spectroscopic surveys and complementary modes of cosmological observation will be pivotal in elucidating unresolved CMB phenomena.