The Cosmic Linear Anisotropy Solving System (CLASS) I: Overview
Julien Lesgourgues
TL;DR
CLASS targets the need for a user-friendly, flexible, accurate, and fast Boltzmann code for cosmology. It introduces a modular C-based architecture with a centralized precision-control structure, dynamic indexing, and automatic discretization to simplify extensions and maintain performance. The paper details the code organization, module interactions, and error management, arguing that these design choices facilitate experimentation with non-standard cosmologies and new observables. The work aims to enable robust parameter inference and interoperability with external tools like CosmoMC and MultiNest, supporting high-precision cosmology analyses.
Abstract
The Cosmic Linear Anisotropy Solving System (CLASS) is a new accurate Boltzmann code, designed to offer a more user-friendly and flexible coding environment to cosmologists. CLASS is very structured, easy to modify, and offers a rigorous way to control the accuracy of output quantities. It is also incidentally a bit faster than other codes. In this overview, we present the general principles of CLASS and its basic structure. We insist on the friendliness and flexibility aspects, while accuracy, physical approximations and performances are discussed in a series of companion papers.