Cosmic Microwave Background Anisotropy numerical solution (CMBAns) I: An introduction to $C_l$ calculation

TL;DR

CMBAns is a new, flexible, and efficient Boltzmann code for calculating the CMB power spectrum and BipoSH coefficients, aiming to improve accuracy and usability for cosmological research.

Contribution

It introduces a novel Boltzmann code, CMBAns, with a C/C++ hybrid design and standalone tools, enhancing accuracy and flexibility over existing packages.

Findings

Provides accurate CMB power spectrum calculations

Includes tools for BipoSH coefficient computation

Designed for ease of use and customization

Abstract

Cosmological Boltzmann codes are often used by researchers for calculating the CMB angular power spectra from different theoretical models, for cosmological parameter estimation, etc. Therefore, the accuracy of a Boltzmann code is of utmost importance. Different Markov Chain Monte Carlo based parameter estimation algorithms typically require $10^3 - 10^4$ iterations of Boltzmann code. This makes the time complexity of such codes another critical factor. In the last two decades, several Boltzmann packages, such as CMBFAST, CAMB, CMBEasy, CLASS etc., have been developed. In this paper, we present a new cosmological Boltzmann code, CMBAns, that can be used for accurate calculation of the CMB power spectrum and BipoSH coefficients. At present, CMBAns is developed for a flat background matrix. It is mostly written in the C language. However, we borrowed the concept of class from C++. This gives researchers the flexibility to develop their own independent package based on CMBAns, without an in-depth understanding of the source code. We also develop multiple stand-alone facilities which can be directly compiled and run on a given parameter set. In this paper, we discuss all the mathematical formulation, approximation schemes, integration methods etc., that are used in CMBAns. The package will be made available through github for public use in the near future.