hi_class background evolution, initial conditions and approximation schemes

Abstract

Cosmological datasets have great potential to elucidate the nature of dark energy and test gravity on the largest scales available to observation. Theoretical predictions can be computed with hi_class (www.hiclass-code.net), an accurate, fast and flexible code for linear cosmology, incorporating a wide range of dark energy theories and modifications to general relativity. We introduce three new functionalities into hi_class: (1) Support for models based on covariant Lagrangians, including a constraint-preserving integration scheme for the background evolution and a series of worked-out examples: Galileon, nKGB, quintessence (monomial, tracker) and Brans-Dicke. (2) Consistent initial conditions for the scalar-field perturbations in the deep radiation era, identifying the conditions under which modified-gravity isocurvature perturbations may grow faster than adiabatic modes leading to a loss of predictivity. (3) An automated quasistatic approximation scheme allowing order-of-magnitude improvement in computing performance without sacrificing accuracy for wide classes of models. These enhancements bring the treatment of dark energy and modified gravity models to the level of detail comparable to software tools restricted to standard ΛCDM cosmologies. The hi_class code is publicly available (https://github.com/miguelzuma/hi_class_public), ready to explore current data and prepare for next-generation experiments.

Early modified gravity in light of the H0 tension and LSS data

“…We perform a Markov-chain Monte Carlo (MCMC) analysis using a modified version of the HICLASS code [87][88][89][90] interfaced to the publicly available sampling code MONTEPYTHON-v3 5 [91,92] and to the PYBIRD code for the calculation of the full shape of the galaxy power spectrum in the effective field theory of large scale structure [86]. We obtain plots using the GetDist package 6 [93].…”

Larger value for H0 by an evolving gravitational constant

“…We run a Markov-chain Monte Carlo (MCMC) using the publicly available code MontePython-v3 5 [44,45] wrapped either with CLASSig [35], a modified version of the CLASS 6 [46,47] for scalar-tensor theory of gravity, or with a modified version of hiCLASS [48,49] which allows to study consistently oscillating scalar fields. The agreement of CLASSig and hiCLASS for the precision of current and future experiments has been demonstrated in [50].…”

Improvements in cosmological constraints from breaking growth degeneracy

“…The cosmological evolution of the models was computed using the Boltzmann code 1 CLASS (Blas et al 2011), and its modified version 2 hi_class (Zumalacarregui et al 2017;Bellini et al 2020). The cosmological data -hereafter referred to as the "baseline" -contain the SDSS-II/SNLS3 Joint Light-curve Analysis (JLA) sample of type Ia supernova (SNIa; Betoule et al 2014), the Baryon Oscillation Spectroscopic Survey (BOSS) baryon acoustic oscillation (BAO) measurements (Beutler et al 2011;Anderson et al 2014;Ross et al 2015), and the low-and high-multipole temperature and polarisation of Planck 2018 cosmic microwave background (CMB) data (Planck Collaboration 1 www.class-code.net 2 www.hiclass-code.net…”