New thresholds for Primordial Black Hole formation during the QCD phase   transition
  Primordial Black Holes (PBHs) might have formed in the early Universe as a consequence of the collapse of density fluctuations with an amplitude above a critical value $\delta_{c}$: the formation threshold. Although for a radiation-dominated Universe $\delta_{c}$ remains constant, if the Universe experiences some dust-like phases (e.g. phase transitions) $\delta_{c}$ might decrease, improving the chances of PBH formation. We studied the evolution of $\delta_{c}$ during the QCD phase transition epoch within three different models: Bag Model (BM), Lattice Fit Model (LFM), and Crossover Model (CM). We found that the reduction on the background value of $\delta_{c}$ can be as high as $77\%$ (BM), which might imply a $\sim10^{-10}$ probability of PBHs forming at the QCD epoch. 
J. L. G. Sobrinho, P. Augusto and A. L. Gon\c{c}alves
astro-ph.CO