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 104 to 105 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 ΛCDM 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.
Journal of Cosmology and Astroparticle Physics
- Pub Date:
- July 2011
- Astrophysics - Cosmology and Nongalactic Astrophysics
- 35 pages, 10 figures, 5 tables. Ultra-relativistic fluid approximation better justified in a new Appendix B. Version accepted in JCAP. Code available at http://class-code.net