Abstract.
Last year pulsar timing arrays unveiled the first detection of a stochastic gravitational wave background at nano-Hertz frequencies. The background could potentially arise from a population of merging supermassive black holes or – arguably even more exciting – an event in the early cosmos. In this talk aimed at non-experts on cosmology, I will discuss the possibility that the newly measured signal stems from a phase transition that happened within the first second after the Big Bang, when the primordial plasma was still several billion Kelvin hot. The specific focus of the talk will be under which conditions phase transitions in a dark sector can serve as an explanation compatible with constraints from precision cosmology. I will conclude with a comment on the question of the likelihood of a new physics explanation.
(The slides will be available here)