The epoch of reionisation marks a major shift from a cold neutral Universe to a warm ionised one, a transition which was thought to be powered by UV radiation emitted from young massive stars in the first galaxies.

Our understanding of this epoch is still limited : observationally we glimpse a handful of the most luminous galaxies existing at the end of the epoch, but with the advent of the James Webb Telescope and other upcoming instruments we will soon start getting better information about the sources powering reionisation. Theoretically, the best way to gain an understanding is with cosmological simulations. However, those are very expensive, so either people simulate large patches of the reionising Universe without resolving the galactic sources, or go for tiny patches where the stellar sources inside one or a few galaxies are resolved, but the large-scale process is lost. I will present a new suite of simulations, called the Sphinx project, where we have developed new methods that allow us to perform radiation-hydrodynamical simulations of cosmic reionisation, resolving the emission and escape of radiation in the inter-stellar medium of hundreds of galaxies all evolving together in the same simulation, hence capturing the interplay of small- and large-scale processes.

I will describe the simulations and the key developments that made them possible, and show results from a pilot Sphinx study looking into the surprising impact on reionisation of spectral energy distribution (SED) models containing binary stellar populations, compared to more classical SEDs.