Gravitational-wave probes of axion-like-particles
Geraldine Servant (DESY Hamburg)

It is usually assumed that the axion starts oscillating around its minimum from its initially frozen position. However, the early dynamics of the Peccei-Quinn field can induce a large kinetic energy in the axion field, which starts rotating during its early evolution, before it gets trapped in one minimum. This can modify the equation of state of the universe by triggering a kination era before the axion acquires its mass.

This imprints a smoking-gun gravitational-wave peak on the primordial gravitational-wave background in the sensitivity bands of LISA, ET, and CE. Future gravitational-wave observatories will thus offer a new window on axion models.

I will present explicit realisations. I will also discuss how this dynamics opens up the allowed parameter space for ALP dark matter and other observational consequences.