# Colloque national Action Dark Energy 2020 - 4ème édition

Oct 13 – 15, 2020
Europe/Paris timezone

## Carpooling to solve the cosmological simulation bottleneck

Oct 13, 2020, 12:40 PM
20m
en visio

### en visio

lien zoom envoyé par mail

### Speaker

Nicolas Chartier (LPENS)

### Description

To exploit the power of next-generation large-scale structure surveys, ensembles of numerical simulations are necessary to give accurate theoretical predictions of the statistics of observables. High-fidelity simulations come at a towering computational cost. Therefore, approximate but fast simulations, surrogates, are widely used to gain speed at the price of introducing model error. We propose a general method that exploits the correlation between simulations and surrogates to compute fast, reduced-variance statistics of large-scale structure observables without model error at the cost of only a few simulations. We call this approach Convergence Acceleration by Regression and Pooling (CARPool). In numerical experiments with intentionally minimal tuning, we apply CARPool to a handful of GADGET-III $N$-body simulations paired with surrogates computed using COmoving Lagrangian Acceleration (COLA). We find $\sim 100$-fold variance reduction even in the non-linear regime, up to $k_\mathrm{max} \approx 1.2$ $h {\rm Mpc^{-1}}$ for the matter power spectrum. CARPool realises similar improvements for the matter bispectrum. In the nearly linear regime CARPool attains far larger sample variance reductions. By comparing to the 15,000 simulations from the Quijote suite, we verify that the CARPool estimates are unbiased, as guaranteed by construction, even though the surrogate misses the simulation truth by up to $60\%$ at high $k$. Furthermore, even with a fully configuration-space statistic like the non-linear matter density probability density function, CARPool achieves unbiased variance reduction factors of up to $\sim 10$, without any further tuning. Conversely, CARPool can be used to remove model error from ensembles of fast surrogates by combining them with a few high-accuracy simulations.

### Primary authors

Nicolas Chartier (LPENS) Benjamin Wandelt (IAP) Dr Yashar Akrami (Département de Physique, École Normale Supérieure (ENS)) Mr Francisco Villaescusa-Navarro (Princeton University)

### Presentation materials

 ActionDarkEnergy_13102020.odp ActionDarkEnergy_13102020.pdf