Speaker
Description
All major pulsar timing array (PTA) collaborations---NANOGrav, Parkes PTA, and EPTA---are now seeing indications of a new stochastic process in their latest data sets. If confirmed in the future, this new signal may turn out to be the first glimpse of a stochastic gravitational-wave background at nanohertz frequencies. In this talk, I will review how PTAs search for gravitational waves, discuss the properties of the newly detected signal, and highlight various possible interpretations. First, I will point out that, if the signal is of astrophysical origin, it is expected to arise from the mergers of supermassive black-hole binaries, in which case it would promise to contain invaluable information on galaxy and black-hole evolution. Then, I will turn to the possibility of a cosmological origin and present a range of exotic particle physics processes in the early Universe that may be responsible for the signal, including cosmic strings, phase transitions, and axion-like fields. In this case, PTA observations would allow us to probe the cosmology of the early Universe and particle physics at extremely high energies. Finally, I will conclude with a brief outlook on the future of the field, which is set to see some amazing progress in the coming years.
