Investigating Cosmic Discordance (2003.04935v2)

Abstract: We show that a combined analysis of CMB anisotropy power spectra obtained by the Planck satellite and luminosity distance data simultaneously excludes a flat universe and a cosmological constant at $99 \%$ CL. These results hold separately when combining Planck with three different datasets: the two determinations of the Hubble constant from Riess et al. 2019 and Freedman et al. 2020, and the Pantheon catalog of high redshift supernovae type-Ia. We conclude that either LCDM needs to be replaced by a different model, or else there are significant but still undetected systematics. Our result calls for new observations and stimulates the investigation of alternative theoretical models and solutions.

• The paper demonstrates that combining Planck and luminosity distance data excludes a flat universe with a cosmological constant at 99% confidence level, questioning the standard LCDM model.
• Analysis shows Planck data prefers a closed universe and a dark energy equation of state w<-1 when combined with extended parameters, contradicting LCDM predictions.
• Tensions between Planck+BAO data and luminosity distance data highlight discrepancies in cosmological observations, suggesting LCDM may not be an ideal model for interpreting all phenomena.

Investigating Cosmic Discordance: An Overview

The paper, "Investigating Cosmic Discordance" by Eleonora Di Valentino, Alessandro Melchiorri, and Joseph Silk, presents a meticulous analysis of current cosmological datasets focusing on tensions that challenge the Lambda Cold Dark Matter (LCDM) model, which is predominantly recognized as the cosmological standard model. The primary aim of the paper is to assess the implications of combining Cosmic Microwave Background (CMB) data from the Planck satellite with various luminosity distance measurements including those from supernovae type-Ia and the Hubble constant determinations. The authors examine whether the existing data support a closed universe and a departure from the cosmological constant, often associated with dark energy in the LCDM framework.

The core of the paper lies in its methodological approach where the authors extend the typical parameter space of the LCDM model. This is accomplished by allowing variations not only in the dark energy equation of state but also in cosmic curvature, neutrino mass, and the running of the spectral index of primordial fluctuations. Importantly, they introduce the lensing amplitude parameter, $A_{lens}$, to model potential systematics in Planck data.

Key Findings

1. Exclusion of Flat Universe and Cosmological Constant: A prominent result is that a combined analysis of Planck data with luminosity distance measurements excludes the scenario of a flat universe with a cosmological constant at 99% confidence level (CL). This significant outcome leads to questioning the adequacy of the LCDM model.
2. Curvature and Dark Energy Equation of State: The paper reveals that the Planck data alone prefer a closed universe model at more than 95% CL when combined with an extended parameter set. The preference extends to a dark energy equation of state, $w$, less than -1, suggesting a phantom-like behavior, which starkly contrasts the LCDM prediction of $w=-1$ associated with a cosmological constant.
3. Tensions Between Datasets: The authors identify that the combined Planck and Baryon Acoustic Oscillation (BAO) data present a narrative compatible with LCDM but do not align well with luminosity distance data, further highlighting the discrepancies in the current cosmological observations. Such tensions question the reliability of the LCDM as an ideal model for interpreting cosmological phenomena.
4. Systematic Effects: Allowing for adjustments in the lensing amplitude parameter $A_{lens}$ can reconcile these tensions to an extent, albeit it suggests the presence of significant systematic errors in the Planck dataset.

Implications and Future Directions

This paper significantly contributes to the ongoing conversation about the validity of the LCDM model in light of contemporary data. The pronounced tensions between cosmic datasets underscore potential systematic issues or, more provocatively, indicate that the theoretical framework underpinning cosmology might need reconsideration.

The paper implies that further investigations are critical in resolving these tensions, particularly improving the accuracy and calibration of cosmological datasets. Additionally, the results suggest revisiting theoretical models to explore alternatives to LCDM, potentially incorporating elements such as dynamic dark energy models and non-zero curvature scenarios.

The discrepancies highlighted may also impact the derived parameters critical for testing our understanding of fundamental physics, such as neutrino masses. The paper advocates for additional cosmological observations and encourages exploring diverse theoretical models. As future datasets from CMB and large-scale structure surveys become available, they may help clarify these discordances, potentially paving the way for precision cosmology that ventures beyond the confines of the current standard model.