hi_class: Background Evolution, Initial Conditions and Approximation
  Schemes
  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 $\Lambda$CDM 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.
