Carlo Tasillo (Uppsala U.) The Nano-Hertz Gravitational Wave Background: Black Holes or Cosmological Phase Transitions?

Thursday 13 Nov 2025, 11:00 → 12:00 Europe/Stockholm

AlbaNova A5:1041 - CoPS grupprum (AlbaNova Main Building)

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.

I will outline what it takes for a phase transition to explain the PTA signal and 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 and directions of related ongoing research projects.

Place: CoPS room/A5:1041 and Zoom