Bodo Schwabe and Mateja Gosenca tell us about AxioNyx, which is a new public code for simulating both Ultralight (or “Fuzzy”) dark matter (FDM) and Cold dark matter (CDM) simultaneously. The code simulates the FDM using adaptive mesh refinement and the CDM using N-body particles. As far as I’m aware it is the first publicly available code that can do both without needing adaptation out of the tin.

The code passes a bunch of sanity/consistency checks, matching linear theory when it should match and deviating when it should deviate. The paper discussed mainly just introduces AxioNyx; the new physics will come in future papers. Things Bodo, Mateja and collaborators will be tackling are: simulations with full cosmological initial conditions for the combination of FDM+CDM, adding baryons (long-term project), gravitational heating of stars in FDM halos, and re-assessment of earlier constraints on FDM with FDM now only a sub-fraction of the total dark matter content (e.g. the Lyman Alpha constraints). Stay tuned, and/or get in touch with them if you’re keen to help make any of that happen :-).

One neat result from this paper was the confirmation of the “Schrodinger-Vlasov” correspondence. This essentially says that FDM and CDM will behave equivalently on large enough scales. On smaller scales the fuzziness of the FDM causes it to deviate (essentially, it is so light that its deBroglie wavelength is astrophysically relevant). This correspondence has been shown statistically, and as a limiting result, in earlier papers but this is (as far as I’m aware) the first paper where the FDM and CDM are in the same gravitational potentials in the same simulation and one can see them do the same stuff on large scales. It wasn’t surprising, but it’s still good to check and see it happen.

The most interesting things happen when the proportions of FDM and CDM are similar (i.e. when one doesn’t just dominate the other entirely), which might be an interesting thing for considering in future papers too.

Paper: https://arxiv.org/abs/2007.08256
AxioNyx: https://github.com/axionyx

Bodo: https://bodoschwabe.github.io/
Mateja: https://cosmology.auckland.ac.nz/2018/08/20/new-postdoc/