The Cosmic Linear Anisotropy Solving System (CLASS) III: Comparision with CAMB for LambdaCDM

Abstract: By confronting the two independent Boltzmann codes CLASS and CAMB, we establish that for concordance cosmology and for a given recombination history, lensed CMB and matter power spectra can be computed by current codes with an accuracy of 0.01%. We list a few tiny changes in CAMB which are necessary in order to reach such a level. Using the common limit of the two codes as a set of reference spectra, we derive precision settings corresponding to fixed levels of error in the computation of a CMB likelihood. We find that for a given precision level, CLASS is about 2.5 times faster than CAMB for computing the lensed CMB spectra of a LambdaCDM model. The nature of the main improvements in CLASS (which may each contribute to these performances) is discussed in companion papers.