Dark matter and gravitational wave signals from a phase transition in the early universe

by Prof. Torsten Bringmann (U. Oslo)

Monday 20 Oct 2025, 10:00 → 12:00 Asia/Tokyo

C203 (Department of Physics, Faculty of Science, U. Toyama)

A strong first-order phase transition offers the intriguing possibility to explain the stochastic gravitational wave background at nHz frequencies that has been observed by pulsar timing arrays. In order to avoid cosmological constraints, such a phase transition must have occurred in a dark sector with non-negligible couplings to the standard model -- opening promising avenues of connecting the gravitational wave signal to the cosmological dark matter abundance. In order to illustrate this, I will consider a classically conformal dark sector with a U(1) gauge symmetry. The spontaneous breaking of this symmetry generates both gravitational waves and sources the mass of a fermionic sub-GeV dark matter candidate, a mass range which has been in the focus of particular phenomenological interest recently. Contact with the standard model is established through kinetic mixing of the U(1) gauge bosons with ordinary photons, a possibility that is actively being searched for at various collider experiments. I will discuss the rich phenomenology of such a scenario, demonstrating that it can successfully generate both the observed nHz gravitational wave background and the cosmological dark matter abundance, while at the same time avoiding all relevant constraints. If time allows, I will also briefly comment on the future LISA mission, and what a putative gravitational wave signal observed at correspondingly much higher frequencies may tell us about the origin of dark matter.