Cosmological perturbations: non-cold relics without the Boltzmann
  hierarchy
  We present a formulation of cosmological perturbation theory where the
Boltzmann hierarchies that evolve the neutrino phase-space distributions are
replaced by integrals that can be evaluated easily with fast Fourier
transforms. The simultaneous evaluation of these integrals combined with the
differential equations for the rest of the system (dark matter, photons,
baryons) are then solved with an iterative scheme that converges quickly. The
formulation is particularly powerful for massive neutrinos, where the effective
phase space is three-dimensional rather than two-dimensional, and even moreso
for three different neutrino mass eigenstates. Therefore, it has the potential
to significantly speed up the computation times of cosmological-perturbation
calculations. This approach should also be applicable to models with other
non-cold collisionless relics.
