An emulator for the Lyman-$\alpha$ forest in beyond-$\Lambda$CDM cosmologies

TL;DR

This paper introduces a Gaussian process emulator for the Lyman-alpha forest flux power spectrum that efficiently predicts results for extended cosmological models, including massive neutrinos, without needing new simulations.

Contribution

The novel emulator models the flux power spectrum as a function of small-scale matter power spectrum parameters, enabling predictions for extended cosmologies beyond the standard model.

Findings

Accurately predicts flux power spectrum in massive neutrino cosmologies.

Eliminates need for bespoke simulations for various extensions to ΛCDM.

Achieves sub-percent accuracy in relevant regimes.

Abstract

Interpreting observations of the Lyman-$\alpha$ forest flux power spectrum requires interpolation between a small number of expensive simulations. We present a Gaussian process emulator modelling the 1D flux power spectrum as a function of the amplitude and slope of the small-scale linear matter power spectrum, 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 in extended cosmological models that are not explicitly included in the training set, eliminating the need to construct bespoke emulators for a number of extensions to $\Lambda$CDM. Our emulator is appropriate for cosmologies in which the linear matter power spectrum is described to percent level accuracy by just an amplitude and slope 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 able to predict the flux power spectrum in a $\Sigma m_\nu=0.3$ eV neutrino cosmology to sub-percent accuracy, without including massive neutrinos in the training simulations. Further parameters would be required to describe models with sharp features in the linear power, such as warm or light axion dark matter. This work will facilitate the combination of upcoming DESI data with observations of the cosmic microwave background, to obtain constraints on neutrino mass and other extensions to $\Lambda$CDM cosmology.