Evolving ultralight scalars into non-linearity with Lagrangian perturbation theory

Many models of high energy physics suggest that the cosmological dark sector consists of not just one, but a spectrum of ultralight scalar particles with logarithmically distributed masses. To study the potential signatures of low concentrations of ultralight axion (also known as fuzzy) dark matter, we modify Lagrangian perturbation theory (LPT) by distinguishing between trajectories of different dark matter species. We further adapt LPT to include the effects of a quantum potential, which is necessary to generate correct initial conditions for ultralight axion simulations. Based on LPT, our modified scheme is extremely efficient on large scales and it can be extended to an arbitrary number of particle species at very little computational cost. This allows for computation of self-consistent initial conditions in mixed dark matter models. Additionally, we find that shell-crossing is delayed for ultralight particles and that the deformation tensor extracted from LPT can be used to identify the range of redshifts and scales for which the Madelung formalism of fuzzy dark matter can lead to divergences.

Automated summary

Many models in high energy physics suggest that the cosmological dark sector contains a spectrum of ultralight scalar particles. By modifying Lagrangian perturbation theory (LPT) and including the effects of a quantum potential, researchers were able to efficiently compute self-consistent initial conditions for mixed dark matter models involving ultralight axions. The delay of shell-crossing for ultralight particles and the identification of potential divergences using the deformation tensor from LPT were also highlighted in the study.