Whispers from the dark side: Confronting light new physics with NANOGrav
  data
  The NANOGrav collaboration has recently observed first evidence of a
gravitational wave background (GWB) in pulsar timing data. Here we explore the
possibility that this GWB is due to new physics, and show that the signal can
be well fit also with peaked spectra like the ones expected from phase
transitions (PTs) or from the dynamics of axion like particles (ALPs) in the
early universe. We find that a good fit to the data is obtained for a very
strong PT at temperatures around 1 MeV to 10 MeV. For the ALP explanation the
best fit is obtained for a decay constant of $F \approx 5\times 10^{17}$ GeV
and an axion mass of $2\times 10^{-13}$ eV. We also illustrate the ability of
PTAs to constrain the parameter space of these models, and obtain limits which
are already comparable to other cosmological bounds.
