Distortions of the Harrison-Zel'dovich spectrum from the QCD transition

We investigate the effect of the cosmological QCD transition on the evolution of primordial density perturbations. If the phase transition is first order, the sound speed vanishes during the transition and density perturbations fall freely. The Harrison-Zel'dovich spectrum of density fluctuations develops large peaks on scales below the Hubble radius at the transition. These peaks above the primordial spectrum grow with wavenumber and produce cold dark matter clumps of masses less than $10^{-10} M_\odot$. At the horizon scale the amplification of overdensities is of order unity, and thus no $1 M_\odot$ black hole production is possible for a COBE normalized scale-invariant spectrum.