Seminar: The Nano-Hertz Gravitational Wave Background: Black Holes or Cosmological Phase Transitions?

Pulsar timing arrays (PTAs) have recently uncovered evidence for a nano-Hertz gravitational wave background. While supermassive black hole binaries are the leading astrophysical explanation, they may not tell the full story. A strong first-order phase transition in the early universe could provide the missing piece – if it also passes the stringent tests from cosmology, colliders, and dark matter searches.

In this talk aimed at non-experts in BSM phenomenology, I will first discuss the working principle of PTAs, and outline what it takes to explain the PTA signal either through an astrophysical or cosmological background of gravitational waves. I will then focus on one predictive scenario [arXiv:2502.19478] that links the signal to the observed dark matter abundance and can be tested with the next generation of beam-dump experiments. I will close with an outlook on PTA science in the coming decade.