Clumping of CDM from the cosmological QCD transition

The cosmological QCD transition affects primordial density perturbations. If the QCD transition is first order, the sound speed vanishes during the transition and density perturbations fall freely. For scales below the Hubble radius at the transition the primordial Harrison-Zel'dovich spectrum of density fluctuations develops large peaks and dips. These peaks grow with wave number for both the radiation fluid and for cold dark matter (CDM). The peaks in the radiation fluid are wiped out during neutrino decoupling. For cold dark matter that is kinetically decoupled at the QCD transition (e.g. axions) these peaks lead to the formation of CDM clumps of masses $10^{-20} M_\odot< M < 10^{-10} M_\odot$