Cosmological perturbations for two cold fluids in ΛCDM

The cosmic large-scale structure of our Universe is comprised of baryons and cold dark matter (CDM). Yet it is customary to treat these two components as a combined single-matter fluid with vanishing pressure, which is justified only for sufficiently large scales and late times. Here, we go beyond the single-fluid approximation and develop the perturbation theory for two gravitationally coupled fluids while still assuming vanishing pressure. We mostly focus on perturbative expansions in powers of D (or D+), the linear structure growth of matter in a Lambda CDM Universe with cosmological constant Lambda. We derive in particular (1) explicit recursion relations for the two fluid densities, (2) complementary all-order results in the Lagrangian-coordinates approach, as well as (3) the associated component wavefunctions in a semiclassical approach to cosmic large-scale structure. In our companion paper, we apply these new theoretical results to generate novel higher order initial conditions for cosmological hydrodynamical simulations.

Automated summary

The passage discusses the perturbation theory for two gravitationally coupled fluids in a Lambda CDM Universe, deriving explicit recursion relations for fluid densities, all-order results in Lagrangian-coordinates approach, and component wavefunctions in a semiclassical approach to cosmic large-scale structure. These theoretical results are then applied to generate novel higher order initial conditions for cosmological hydrodynamical simulations in a companion paper.