Cosmic-Enu: An emulator for the non-linear neutrino power spectrum

Abstract

Cosmology is poised to measure the neutrino mass sum $M_{\nu }$ and has identified several smaller-scale observables sensitive to neutrinos, necessitating accurate predictions of neutrino clustering over a wide range of length scales. The FlowsForTheMasses non-linear perturbation theory for the massive neutrino power spectrum, ${\Delta }_{\nu }^2(k)$, agrees with its companion N-body simulation at the $10\%-15\%$ level for $k\le 1h/$Mpc. Building upon the Mira-Titan IV emulator for the cold matter, we use FlowsForTheMasses to construct an emulator for ${\Delta }_{\nu }^2(k)$ covering a large range of cosmological parameters and neutrino fractions ${\Omega }_{\nu ,0}{h}^2\le 0.01$, which corresponds to $M_{\nu }\le 0.93$~eV. Consistent with FlowsForTheMasses at the $3.5\%$ level, it returns a power spectrum in milliseconds. Ranking the neutrinos by initial momenta, we also emulate the power spectra of momentum deciles, providing information about their perturbed distribution function. Comparing a $M_{\nu }=0.15$~eV model to a wide range of N-body simulation methods, we find agreement to $3\%$ for $k\le 3k_{\mathrm{FS}}=0.17h/$Mpc and to $19\%$ for $k\le 0.4h/$Mpc. We find that the enhancement factor, the ratio of ${\Delta }_{\nu }^2(k)$ to its linear-response equivalent, is most strongly correlated with ${\Omega }_{\nu ,0}{h}^2$, and also with the clustering amplitude ${\sigma }_8$. Furthermore, non-linearities enhance the free-streaming-limit scaling $\partial \log ({\Delta }_{\nu }^2/{\Delta }_{\mathrm{m}}^2)/\partial \log (M_{\nu })$ beyond its linear value of 4, increasing the $M_{\nu }$-sensitivity of the small-scale neutrino density.

17 pages, 14 figures, 3 tables. Emulator code available at: https://github.com/upadhye/Cosmic-Enu