Understand
Primordial electromagnetic fields can strongly affect the cosmic evolution of axions, and vice versa.
- We show that if helical electromagnetic fields are coherently produced in the early universe, their remnants source a field velocity to the coupled axions and enhance the relic abundance of axion dark matter.
- We discuss the implications for the QCD axion and axion-like particles that are coupled to the SM or hidden gauge groups.
- For a QCD axion coupled to hidden photons, we find that the conventional window for the axion decay constant $10^{8}\, \mathrm{GeV} \lesssim f \lesssim 10^{12}\, \mathrm{GeV}$ can be completely closed due to overproduction of axion dark matter by helical electromagnetic fields as little as $\alpha \, \Delta N_{\mathrm{eff}} \gtrsim 10^{-12}$, where $\alpha$ is the gauge coupling and $\Delta N_{\mathrm{eff}}$ is the effective extra relativistic degrees of freedom of the hidden photons.