BullFrog: Multi-step perturbation theory as a time integrator for cosmological simulations

Author(s)

Cornelius Rampf, Florian List, Oliver Hahn

Abstract

Modelling the cosmic large-scale structure can be done through numerical

N-body simulations or by using perturbation theory. Here, we present an

N-body approach that effectively implements a multi-step forward model

based on Lagrangian Perturbation Theory (LPT) in a $\Lambda$CDM

Universe. This is achieved by introducing the second-order accurate

BullFrog integrator, which automatically performs 2LPT time steps to

second order without requiring the explicit computation of 2LPT

displacements. Importantly, we show that BullFrog trajectories rapidly

converge to the exact solution as the number of time steps increases, at

any moment in time, even though 2LPT becomes invalid after

shell-crossing. As a validation test, we compare BullFrog against other

N-body integrators and high-order LPT, both for a realistic $\Lambda$CDM

cosmology and for simulations with a sharp UV cutoff in the initial

conditions. The latter scenario enables controlled experiments against

LPT and, in practice, is particularly relevant for modelling

coarse-grained fluids arising in the context of effective field theory.

We demonstrate that BullFrog significantly improves upon other

LPT-inspired integrators, such as FastPM and COLA, without incurring any

computational overhead compared to standard N-body integrators.

Implementing BullFrog in any existing N-body code is straightforward,

particularly if FastPM is already integrated.

Organisation(s)

Department of Mathematics, Department of Astrophysics

Journal

Journal of Cosmology and Astroparticle Physics (JCAP)

ISSN

1475-7516

Publication date

09-2024

Peer reviewed

Yes

Austrian Fields of Science 2012

103003 Astronomy, 103004 Astrophysics

Keywords

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