This is an extension of the popular 21cmFAST code that interfaces with CLASS to generate initial conditions at recombination that are consistent with the input cosmological model. These initial conditions can be set during the time of recombination, allowing one to compute the 21cm signal (and its spatial fluctuations) throughout the dark ages, as well as in the proceeding cosmic dawn and reionization epochs, just like in the standard 21cmFAST.

21cmFirstCLASS tracks both the CDM density field $\delta_c$ as well as the baryons density field $\delta_b$.

21cmFirstCLASS allows you to consistently compute the brightness temperature field at the dark ages, as well as in the cosmic dawn and reionization epochs (like in 21cmFAST).

In addition, the user interface in 21cmFirstCLASS has been improved and allows one to easily plot the 21cm power spectrum while including noise from the output of 21cmSense.

Comprehensive jupyter notebook tutorials have been prepared for this code, check them out at the following links.

• Notebook #1 for installation instructions and basic usage.
• Notebook #2 for learning about the new physical features (in ΛCDM cosmology) that have been introduced to 21cmFirstCLASS, like running the simulation through the dark ages, evolving the baryon density field, and more.
• Notebook #3 for studying beyond ΛCDM models with 21cmFirstCLASS.
• Notebook #4 for studying the detectability of the 21cm signal with 21cmSense.

21cmFirstCLASS is an open source code and you are encouraged to use it for your studies. If you use this code please cite:

• Jordan Flitter and Ely D. Kovetz, "New tool for 21-cm cosmology. I. Probing ΛCDM and beyond", Phys. Rev. D 109 (2024) 4, 043512 (arXiv: 2309.03942).
• Jordan Flitter and Ely D. Kovetz, "New tool for 21-cm cosmology. II. Investigating the effect of early linear fluctuations", Phys. Rev. D 109 (2024) 4, 043513 (arXiv: 2309.03948).

As 21cmFirstCLASS is based on 21cmFAST, please also cite the associated 21cmFAST papers.

• Andrei Mesinger, Steven Furlanetto and Renyue Cen, "21CMFAST: a fast, seminumerical simulation of the high-redshift 21-cm signal", Mon. Not. Roy. Astron. Soc. 411 (2011) 955 (arXiv: 1003.3878).
• Muñoz, J.B., Qin, Y., Mesinger, A., Murray, S., Greig, B., and Mason, C., "The Impact of the First Galaxies on Cosmic Dawn and Reionization", Mon. Not. Roy. Astron. Soc. 511 (2022) 3, 3657-3681 (arXiv: 2110.13919).

Moreover, besides of incorporating new features in 21cmFAST, 21cmFirstCLASS also integrates various open source codes. Make sure you cite the relevant papers from the following github links if you use 21cmFirstCLASS to...

• Generate consistent initial conditions for the 21cmFAST simulation (or to perform a joint 21cm-CMB analysis) with CLASS.
• Compute precisely the free electron fraction with HYREC.
• Compute the power spectrum of any box with powerbox.
• Compute the noise of 21cm interferometers like HERA with 21cmSense.
• Study fuzzy dark matter with AxiCLASS.
• Study scattering dark matter with dmeff-CLASS.