Particle theory

Abstract:

We present a mechanism for the generation of magnetic field in the early universe prior to the QCD crossover, assuming that the dark matter is made of pseudoscalar axions. Thermoelectric fields arise due to pressure gradients in the primordial plasma resulting from the difference in charge, energy density and equation of state of the quark and lepton components. The axion field is coupled to the EM field and when its spatial gradient is misaligned with the thermoelectric field an electric current is driven. Due to the finite resistivity of the plasma a generally rotational electric field appears. For a QCD axion mass consistent with observational constraints, a magnetic field is thus generated with strength $B \sim 10^{-11 }$ G and characteristic scale $L_B \sim $ 1 pc at present, and viable values for the combination $BL_B^{1/2}$, which is probed in cosmic voids through $\gamma$-ray observations of distant blazars. The amplitude and spatial/temporal scales of the pressure gradients may in principle be inferred through the detection of the concomitant emission of gravitational waves, while experiments are underway to confirm or rule out the existence of axions, with direct consequences for our predictions.