  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 $10^{-20} M_\odot< M < 10^{-10}
M_\odot$.
