Numerical discreteness errors in multispecies cosmological N-body simulations

Journal Article · 03 May 2023 · Monthly Notices of the Royal Astronomical Society

DOI: https://doi.org/10.1093/mnras/stad1176 · OSTI ID:1972751

Liu, Xin ^[1]; Emberson, J. D. ^[2]; Buehlmann, Michael ^[2]; Frontiere, Nicholas ^[2]; Habib, Salman ^[2]

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1. Argonne National Laboratory (ANL), Argonne, IL (United States); Univ. of Chicago, IL (United States)
2. Argonne National Laboratory (ANL), Argonne, IL (United States)

We present a detailed analysis of numerical discreteness errors in two-species, gravity-only, cosmological simulations using the density power spectrum as a diagnostic probe. In a simple set-up where both species are initialized with the same total matter transfer function, biased growth of power forms on small scales when the solver force resolution is finer than the mean interparticle separation. The artificial bias is more severe when individual density and velocity transfer functions are applied. In particular, significant large-scale offsets in power are measured between simulations with conventional offset grid initial conditions when compared against converged high-resolution results where the force resolution scale is matched to the interparticle separation. These offsets persist even when the cosmology is chosen so that the two particle species have the same mass, indicating that the error is sourced from discreteness in the total matter field as opposed to unequal particle mass. We further investigate two mitigation strategies to address discreteness errors: the frozen potential method and softened interspecies short-range forces. The former evolves particles under the approximately ‘frozen’ total matter potential in linear theory at early times, while the latter filters cross-species gravitational interactions on small scales in low-density regions. By modelling closer to the continuum limit, both mitigation strategies demonstrate considerable reductions in large-scale power spectrum offsets.

• Research Organization: Argonne National Laboratory (ANL), Argonne, IL (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF); USDOE National Nuclear Security Administration (NNSA)
• Grant/Contract Number: AC02-06CH11357; AC02-05CH11231; 17-SC-20-SC
• OSTI ID: 1972751
• Alternate ID(s): OSTI ID: 2375484
• Journal Information: Monthly Notices of the Royal Astronomical Society, Vol. 522, Issue 3; ISSN 0035-8711
• Publisher: Oxford University PressCopyright Statement
• Country of Publication: United States
• Language: English