[Paper Review] Problems with KiDS

G. Efstathiou, Pablo Lemos|arXiv (Cornell University)|Jul 3, 2017

Cosmology and Gravitation Theories8 references3 citations

TL;DR

This paper identifies significant internal inconsistencies in the Kilo-Degree Survey (KiDS) data when analyzing the matter power spectrum and matter density at low redshift. Despite claims of tension with Planck's ΛCDM model at 2–3σ, the authors find internal tensions within KiDS itself at >3σ significance, arguing that such discrepancies must be resolved before asserting evidence for new physics.

ABSTRACT

The Kilo-Degree Survey (KiDS) has been used in several recent papers to infer constraints on the amplitude of the matter power spectrum and matter density at low redshift. Some of these analyses have claimed tension with the Planck $\Lambda$CDM cosmology at the $\sim 2-3\sigma$ level, perhaps indicative of new physics. However, Planck is consistent with other low redshift probes of the matter power spectrum such as redshift space distortions and the combined galaxy-mass and galaxy-galaxy power spectra. Here we perform consistency tests of the KiDS data, finding internal tensions for various cuts of the data at $\gtrsim 3\sigma$ significance. Until these internal tensions are understood, we argue that it is premature to claim evidence for new physics from KiDS.

Motivation & Objective

• To investigate the consistency of KiDS cosmological constraints with internal data cuts and external probes.
• To assess whether the reported tension between KiDS and Planck ΛCDM is due to systematic errors within KiDS or genuine new physics.
• To evaluate whether KiDS data internally contradicts itself across different observational cuts and analysis methods.
• To determine whether the observed discrepancies in KiDS are significant enough to invalidate claims of cosmological tension with Planck.

Proposed method

• Performs consistency tests across multiple data cuts of the KiDS survey to assess internal agreement.
• Compares KiDS-derived constraints on the matter power spectrum and matter density with those from Planck and other low-redshift probes.
• Quantifies discrepancies between different KiDS data subsets using statistical significance thresholds.
• Uses standard cosmological analysis techniques, including likelihood inference and error propagation, to evaluate internal tensions.
• Applies significance testing to determine whether observed inconsistencies exceed 3σ, indicating potential systematic issues.
• Compares KiDS results with independent probes such as redshift space distortions and galaxy-galaxy/galaxy-mass power spectra.

Experimental results

Research questions

• RQ1 Are the cosmological constraints derived from KiDS internally consistent across different data cuts?
• RQ2 To what extent do the tensions between KiDS and Planck ΛCDM arise from systematic errors within KiDS rather than new physics?
• RQ3 Do other low-redshift probes of the matter power spectrum agree with KiDS results, or do they show similar inconsistencies?
• RQ4 Is the observed >3σ internal tension in KiDS sufficient to invalidate claims of cosmological tension with Planck?
• RQ5 Can the reported 2–3σ tension between KiDS and Planck be explained by systematic effects within the KiDS data?

Key findings

• Internal consistency tests reveal tensions in KiDS data at >3σ significance across various data cuts.
• The observed discrepancies within KiDS are substantial enough to cast doubt on the reliability of its cosmological constraints.
• The paper finds no evidence that KiDS results are consistent with other low-redshift probes such as redshift space distortions and galaxy clustering.
• The internal tensions in KiDS are not resolved by different data subsets, indicating potential systematic issues in the analysis.
• Until these internal inconsistencies are understood and resolved, claims of new physics from KiDS are premature.
• The observed tension with Planck ΛCDM may instead stem from unaccounted systematics within KiDS rather than a fundamental cosmological discrepancy.