Horndeski in the Cosmic Linear
Anisotropy Solving System

M. Zumalacarregui, E. Bellini,
I. Sawicki, J. Lesgourgues, P. Ferreira

The Science

hi_class implements Horndeski's theory in the modern Cosmic Linear Anisotropy Solving System. It can be used to compute any linear observable in seconds, including cosmological distances, CMB, matter power and number count spectra. hi_class can be readily interfaced with Monte Python to test Gravity and Dark Energy models.

Horndeski is the most general scalar-tensor theory described by second-order equations of motion, and contains many well known models, including (but by no means limited to) covariant Galileons, Brans-Dicke, f(R), chameleons, k-essence and quintesssence. hi_class relies on a reformulation of the Effective Field Theory for Dark Energy developed by E. Bellini and I. Sawicki (see JCAP 1407 (2014) 050).

The publicly available version (hi_class teaser) is presented and described in:

hi_class has been used to obtain results in the following publications:

The Code

hi_class computes the cosmological predictions of alternative theories of gravity. The code solves the linear equations starting deep in the radiation era, and can compute any cosmological observable, including (but not limited to) cosmological distances, the matter power spectrum, Cosmic Microwave Background temperature and polarization, as well as their correlation with the matter distribution. The publicly available version incorporates parameterized models based on the Effective Field Theory of Dark Energy.

hi_class has been tested for a range of models against several codes in a dedicated paper A comparison of Einstein-Boltzmann solvers for testing General Relativity . The codes validated include EFTCAMB , COOP and the Galileon code developed by Barreira et al. (based on CAMB). The results agree within 0.1% for CMB and matter power spectra, as good as for base CLASS/CAMB using default precision parameters (at low multipoles the agreement is within 0.5%, well within cosmic variance). The achieved precision is sufficient for tests of gravity with next-generation surveys.


hi_class is freely available to the scientific community. If you use it in a publication/preprint please cite at least the original CLASS paper and

The code can be cloned from the GitHub repository or downloaded as a compressed file. To get started and find detailed information on the available models and code functionality please read the hi_class.ini file.


The Team

hi_class is currently being developed by

We are very grateful to Thomas Tram for his invaluable advice, Janina Renk for puting hi_class to work during her Master's thesis and Carlos Garcia-Garcia for his continuous work with the development version.


If you are interesing in using a beta version or for other inquiries about hi_class please contact emilio - bellini -- physics.ox.ac.uk or miguelzuma -- berkeley.edu