Overview of hi_class: A Cosmological Boltzmann Code for Modified Gravity
The paper presents a comprehensive account of the hi_class software, a public extension of the Cosmic Linear Anisotropy Solving System (CLASS) designed to investigate a broad range of modifications to general relativity, specifically within the framework of Horndeski theories. This software is tailored for researchers aiming to explore alternative theories of gravity and dark energy models beyond the canonical Lambda Cold Dark Matter ((\Lambda)CDM) paradigm.
Horndeski theories, as the most general scalar-tensor theories with second-order equations of motion, encompass a wide variety of models including quintessence, Brans-Dicke, ( f(R) ) gravity, covariant Galileons, and more. hi_class facilitates the computation of linear structure formation and cosmological parameter extraction within these extended frameworks by modifying the gravitational sector in the Boltzmann code setting.
Implementation and Features
hi_class introduces several modifications to the CLASS code to accommodate the extra degrees of freedom presented by Horndeski theories. Essential features include:
Unified Parameterization: The code employs a parameterization approach using four key functions: the dimensionless cosmological strength of gravity (M_{*}2) and three (\alpha) functions ((\alpha_{\text{B}}), (\alpha_{\text{M}}), (\alpha_{\text{T}})). These functions collectively describe departures from General Relativity in the scalar and tensor sectors, enabling comprehensive analysis of potential gravitational modifications.
Numerical Stability: The code evaluates stability conditions to ensure the robustness of calculated cosmological predictions. These include ensuring positive kinetic terms and sound speeds for both scalar and tensor degrees of freedom. The stability checks are vital to avoid exponential growth associated with ghost and gradient instabilities.
Scalar Field Dynamics: The code utilizes efficient algorithms to solve the perturbation equations for scalar modes introduced in Horndeski frameworks, allowing for use in conjunction with Monte Carlo Markov Chain (MCMC) methodologies for cosmological parameter estimation.
Public Accessibility and Development: hi_class is shared openly with the scientific community to facilitate widespread use and collaborative improvements. The software is intended for continual development, with updates planned to address approximation schemes, extend model variety, and enhance end-user experience.
Practical and Theoretical Implications
The implementation of hi_class represents a valuable tool for investigating the potential impacts of modified gravity theories on cosmological observables, including the Cosmic Microwave Background (CMB) and Large Scale Structure (LSS). By allowing researchers to simulate and constrain models such as quintessence and ( f(R) ) gravity, this software paves the way for more nuanced exploration into the nature of dark energy.
This tool aligns with the current theoretical pursuit to uncover the most accurate model of our universe by contrasting predictions from traditional (\Lambda)CDM and more complex scenarios. As large-scale observational projects continue to provide high-precision cosmological data, hi_class offers a framework to assess how alternative gravitational theories might manifest observationally, potentially guiding future theoretical advancements or refinements.
In summary, hi_class enhances the capability to probe modifications to Einstein's gravity on cosmological scales, offering a crucial step toward a deeper understanding of the universe's fundamental forces and components. Its continuous evolution and integration with cutting-edge data will ensure its relevance in ongoing and future cosmological research efforts.