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#Horndeski in the Cosmic Linear Anisotropy Solving System

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), k-essence and quintesssence. Hi_CLASS relies on a powerfull formulation 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: Horndeski in the Cosmic Linear Anisotropy Solving System M. Zumalacarregui, E. Bellini, I. Sawicki, J. Lesgourgues, P. Ferreira 1605.06102

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

• Nonlinear evolution of the BAO scale in alternative theories of gravity E. Bellini, M. Zumalacarregui PRD92 (2015) 063522
• Constraints on deviations from LCDM within Horndeski gravity E. Bellini, A. Cuesta, R. Jimenez, L. Verde JCAP 1602 (2016)
• Gravity at the horizon: on relativistic effects, CMB-LSS correlations and ultra-large scales in Horndeski's theory J. Renk, M. Zumalacarregui, F. Montanari 1604.03487
• The Observational Future of Cosmological Scalar-Tensor Theories D. Alonso, E. Bellini, P. G. Ferreira, M. Zumalacarregui PRD95 (2017) 063502

The Code

Hi_CLASS is currently being developed by

• Emilio Bellini (main developer)
• Pedro G. Ferreira
• Julien Lesgourgues
• Ignacy Sawicki
• Miguel Zumalacarregui (main developer)

We are very grateful to Thomas Tram for his invaluable advice and Janina Renk for puting Hi_CLASS to work during her Master's thesis and suggesting improvements.

Hi_CLASS has been tested against EFTCAMB and the Galileon code developed by Barreira et al. (also based on CAMB). In the latter case the results agree within 1% for the CMB-TT spectra, 0.1% for matter power spectra and, 0.01% for the background expansion (using default parameters).