CMB power spectrum parameter degeneracies in the era of precision cosmology

TL;DR

This paper investigates the degeneracies in CMB power spectrum parameters, emphasizing the importance of physical effects like lensing and the integrated Sachs-Wolfe effect in breaking these degeneracies with high-precision data.

Contribution

It provides a detailed analysis of physical versus numerical degeneracy-breaking effects in CMB data, and assesses the numerical accuracy of CAMB and CosmoMC in parameter estimation.

Findings

Numerical calculations are sufficiently accurate to focus on physical effects for degeneracy breaking.

CMB lensing provides significant degeneracy-breaking information even without non-Gaussian data.

Recent updates to CAMB improve the accuracy of massive neutrino perturbation modeling.

Abstract

Cosmological parameter constraints from the CMB power spectra alone suffer several well-known degeneracies. These degeneracies can be broken by numerical artefacts and also a variety of physical effects that become quantitatively important with high-accuracy data e.g. from the Planck satellite. We study degeneracies in models with flat and non-flat spatial sections, non-trivial dark energy and massive neutrinos, and investigate the importance of various physical degeneracy-breaking effects. We test the CAMB power spectrum code for numerical accuracy, and demonstrate that the numerical calculations are accurate enough for degeneracies to be broken mainly by true physical effects (the integrated Sachs-Wolfe effect, CMB lensing and geometrical and other effects through recombination) rather than numerical artefacts. We quantify the impact of CMB lensing on the power spectra, which inevitably provides degeneracy-breaking information even without using information in the non-Gaussianity. Finally we check the numerical accuracy of sample-based parameter constraints using CAMB and CosmoMC. In an appendix we document recent changes to CAMB's numerical treatment of massive neutrino perturbations, which are tested along with other recent improvements by our degeneracy exploration results.