21cmFirstCLASS I. Cosmological tool for $\Lambda$CDM and beyond

In this work we present 21cmFirstCLASS, a modified version of 21cmFAST, the most popular code in the literature for computing the anisotropies of the 21-cm signal. Our code uses the public cosmic microwave background (CMB) Boltzmann code CLASS, to establish consistent initial conditions at recombination for any set of cosmological parameters and evolves them throughout the dark ages, cosmic dawn, the epoch of heating and reionization. We account for inhomogeneity in the temperature and ionization fields throughout the evolution, crucial for a robust calculation of both the global 21-cm signal and its fluctuations. We demonstrate how future measurements of the CMB and the 21-cm signal can be combined and analyzed with 21cmFirstCLASS to obtain constraints on both cosmological and astrophysical parameters and examine degeneracies between them. As an example application, we show how 21cmFirstCLASS can be used to study cosmological models that exhibit non-linearities already at the dark ages, such as scattering dark matter (SDM). For the first time, we present self-consistent calculations of the 21-cm power spectrum in the presence of SDM during the non-linear epoch of cosmic dawn. The code is publicly available at https://github.com/jordanflitter/21cmFirstCLASS.