Globular clusters are among the oldest known objects in the Universe, yet their formation process remains insufficiently understood. Early work hypothesised that globular clusters formed before galaxies themselves. The QCD phase transition in the early universe may provide primordial black hole nuclei for globular clusters. We have explored this hypothesis, considering the accretion and star formation that follow, once 1000 M_ nuclei have formed. When such a nucleus has formed, it remains. Whether these are common in the oldest globular clusters is one decidedly challenging question for the model, which is, as yet, unanswered; another is a possible contribution to the cosmic gravitational radiation background. In the end, based on current evidence, we conclude there are no arguments that can completely refute the model proposed here.
Mould et al. (2026) studied this question.