How loud was the dark Big Bang? Dark sector phase transitions in the light of PTAs and BBN

The past decades were a great success story for precision cosmology. Still, at temperatures above the MeV-scale we lack experimental probes of our theories. To test new models for the Early Universe at times prior to Big Bang Nucleosynthesis (BBN) we need a new messenger that is able to pass the hot and dense plasma of elementary particles, the most promising one being gravitational waves. The pulsar timing array (PTA) NANOGrav has found evidence for a signal at frequencies that would correspond to temperatures just above the MeV scale, if it is from gravitational waves of a cosmic origin. In this talk, I will discuss the possibility whether the measured signal could be due to a dark sector phase transition when taking into account constraints from BBN and the cosmic microwave background.