The Cosmic Linear Anisotropy Solving System (CLASS) II: Approximation schemes

TL;DR

This paper introduces new approximation schemes in the CLASS Boltzmann code that significantly improve computational efficiency and accuracy for cosmological parameter estimation in LambdaCDM models.

Contribution

It presents three novel approximation schemes in CLASS, including a baryon-photon tight-coupling, an ultra-relativistic fluid, and a radiation streaming approximation.

Findings

Enhanced speed and precision in cosmological computations

Implementation of an ultra-relativistic fluid approximation

Effective handling of reionisation effects

Abstract

Boltzmann codes are used extensively by several groups for constraining cosmological parameters with Cosmic Microwave Background and Large Scale Structure data. This activity is computationally expensive, since a typical project requires from 10'000 to 100'000 Boltzmann code executions. The newly released code CLASS (Cosmic Linear Anisotropy Solving System) incorporates improved approximation schemes leading to a simultaneous gain in speed and precision. We describe here the three approximations used by CLASS for basic LambdaCDM models, namely: a baryon-photon tight-coupling approximation which can be set to first order, second order or to a compromise between the two; an ultra-relativistic fluid approximation which had not been implemented in public distributions before; and finally a radiation streaming approximation taking reionisation into account.