Abstract.
In the past decades, observational cosmology has established our cosmological history up to temperatures at the MeV-scale with percent-level precision. Still, the early Universe at higher temperatures, i.e. before Big Bang Nucleosynthesis (BBN), has remained observationally inaccessible. The most promising probe pushing to these early times are gravitational waves (GWs), with possible sources including first-order phase transitions in the early Universe. GWs at low frequencies could be observed by pulsar timing arrays (PTAs), in particular in the form of a stochastic background. The PTA NANOGrav has found evidence for such a background that may be a cosmological signal. In this talk, I will discuss if a phase transition in a dark sector can explain the NANOGrav data taking into account constraints from BBN and the cosmic microwave background (CMB).
(The slides will be available here)