CMB power spectrum parameter degeneracies in the era of precision cosmology

Abstract

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