QCD and electroweak phase transitions with hidden scale invariance: implications for primordial black holes, quark-lepton nuggets and gravitational waves
We study the cosmological implications of the minimal non-linear realisation of scale invariance within the Standard Model (SM). This framework provides a technically natural explanation for the hierarchy between the Planck scale and the electroweak scale and introduces only a light, feebly coupled dilaton field beyond the SM particles. Although the model is almost indistinguishable from the minimal SM at low energies, its cosmological consequences differ dramatically. In particular, the electroweak Higgs field remains trapped in the symmetric phase until the Universe cools to very low temperatures, $T_c^{(\chi)}\sim 28$ MeV, where the first-order QCD chiral symmetry-breaking phase transition triggers the electroweak phase transition. This scenario offers intriguing possibilities for the production of primordial black holes, low-frequency gravitational waves, and multi-quark and lepton nuggets, which we explore in some detail using simplified approximations.