Axion-Driven Cosmic Magnetogenesis during the QCD Crossover

We have proposed a novel mechanism for the generation of a magnetic field in the early universe during the QCD crossover assuming that dark matter is made of axions. Thermoelectric fields arise at pressure gradients in the primordial plasma due to the difference in charge, energy density and equation of state between the quark and lepton components. The axion field is coupled to the electro-magnetic field, so when its spatial gradient is misaligned with the thermoelectric field, an electric current is driven. Due to the finite resistivity of the plasma an electric field appears that is generally rotational. This seed field, while initially small, in then amplified by turbulent dynamo, driven by the same pressure gradients responsible for the thermoelectric fields, and a magnetic field is generated on sub-horizon scales. Because of the expansion of the Universe, the magnetic field unwinds and reaches a present day strength of B ~ 10^-13 G on a characteristic scale L _B ~ 20 pc. The resulting combination of B L_B^1/2 is significantly stronger than in any astrophysical scenario, providing a clear test for the cosmological origin of the field through γ-ray observations of distant blazars which probe cosmic voids. Such observations will be made with the Cherenkov telescope array.