Planck evidence for a closed Universe and a possible crisis for cosmology (1911.02087v1)

Abstract: The recent Planck Legacy 2018 release has confirmed the presence of an enhanced lensing amplitude in CMB power spectra compared to that predicted in the standard $\Lambda$CDM model. A closed universe can provide a physical explanation for this effect, with the Planck CMB spectra now preferring a positive curvature at more than $99 \%$ C.L. Here we further investigate the evidence for a closed universe from Planck, showing that positive curvature naturally explains the anomalous lensing amplitude and demonstrating that it also removes a well-known tension within the Planck data set concerning the values of cosmological parameters derived at different angular scales. We show that since the Planck power spectra prefer a closed universe, discordances higher than generally estimated arise for most of the local cosmological observables, including BAO. The assumption of a flat universe could, therefore, mask a cosmological crisis where disparate observed properties of the Universe appear to be mutually inconsistent. Future measurements are needed to clarify whether the observed discordances are due to undetected systematics, or to new physics, or simply are a statistical fluctuation.

• The paper’s main finding is evidence for a closed universe with a 3.4σ deviation in the curvature parameter ΩK based on enhanced CMB lensing signals.
• The analysis integrates Planck CMB power spectra with BAO, H0, and cosmic shear data, uncovering significant systematic tensions exceeding 3σ.
• The results call for a reassessment of standard ΛCDM and inflation models, suggesting that closed inflation scenarios might resolve key cosmological discrepancies.

Planck Evidence for a Closed Universe and Cosmological Tensions

The paper by Di Valentino et al. presents an analysis of the data from the Planck Legacy 2018 (PL18) release, which has led to results that challenge the widely accepted standard cosmological model—$\Lambda$CDM. The key finding is an indication of a closed universe, suggested by the enhanced lensing amplitude observed in the Cosmic Microwave Background (CMB) power spectra. This observation contradicts the standard $\Lambda$CDM model that predominantly assumes a flat universe, as it suggests a positive curvature with more than 99% confidence level (C.L.).

The authors focus their investigation on the curvature parameter $\Omega_K$, reporting a significant deviation at 3.4 standard deviations in favor of a closed universe. This claim is bold given the foundational premise of inflationary theory, which naturally predicts a flat universe. Nonetheless, they argue that closed inflation models, albeit requiring precision fine-tuning, might reconcile the observed discrepancies without invoking complex new physics or extraordinary systematic errors.

In substantiating their claim, the paper underscores the discord between the PL18 data and several key cosmological observables. Most notably, the paper highlights discrepancies when comparing PL18 results with Baryon Acoustic Oscillations (BAO), the Hubble constant ($H_0$) measurements, and cosmic shear data. The tensions are substantial—the PL18's preferred closed model contrasts sharply with the $H_0$ measurements by Riess et al., showing a discrepancy at the level of 5.2 standard deviations, thus exacerbating the known $H_0$ tension. Similarly, the disagreement with BAO results, supported by SDSS and BOSS data, signifies a 3σ conflict, showcasing a systematic deviation when a closed universe is assumed.

The authors further enrich their analysis by considering the implications of curvature on internal data consistency. They identify a model-dependent shift that reconciles cosmological parameters derived at different angular scales of the CMB, specifically without resorting to the otherwise employed $A_{lens}$ parameter alterations. Intriguingly, the curvature also addresses the observed lensing amplitude's discordance within the PL18 dataset alone.

However, a closed universe model fails to cohesively align with other local cosmological measurements. The cosmic shear results, particularly from the KiDS-450 survey, aggravate the tension, resulting in a 3.8σ discrepancy under a non-flat universe assumption. The inconsistencies present a robust argument for either the potential systematic error within PL18, previously undetected anomalies or a hint towards novel physics beyond $\Lambda$CDM.

The paper concludes with a call for further precision measurements and scrutiny to ascertain the veracity of these discordances and the preference for positive curvature. The authors acknowledge the significance of these findings, suggesting they necessitate critical reassessment within cosmological paradigms, potentially instigating shifts in the accepted concordance model of the universe. This insight is invaluable for advancing the field, prompting rigorous examination of both Planck data and the cosmological models it challenges.