An emulator for the Lyman-α forest in beyond-ΛCDM cosmologies

Abstract

Interpreting observations of the Lyman-α forest flux power spectrumrequires interpolation between a small number of expensive simulations.We present a Gaussian process emulator modelling the 1D flux power spectrum asa function of the amplitude and slope of the small-scale linear matter powerspectrum, and the state of the intergalactic medium at the epoch of interest(2 < z < 4).This parameterisation enables the prediction of the flux power spectrum inextended cosmological models that are not explicitly included in the training set,eliminating the need to construct bespoke emulators for a number of extensionsto ΛCDM.Our emulator is appropriate for cosmologies in which the linear matter powerspectrum is described to percent level accuracy by just an amplitude andslope across the epoch of interest, and in the regime probed by eBOSS/DESI data.We demonstrate this for massive neutrino cosmologies, where the emulator is ableto predict the flux power spectrum in a Σ mν=0.3 eV neutrino cosmologyto sub-percent accuracy, without including massive neutrinos in the training simulations.Further parameters would be required to describe models with sharp features in thelinear power, such as warm or light axion dark matter.This work will facilitate the combination of upcoming DESI datawith observations of thecosmic microwave background, to obtain constraints on neutrino mass andother extensions to ΛCDM cosmology.