Axion-Driven Cosmic Magnetogenesis during the QCD Crossover

F. Miniati, G. Gregori, B. Reville, and S. Sarkar
Phys. Rev. Lett. 121, 021301 – Published 9 July 2018

Abstract

We propose a 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 EM field, so when its spatial gradient is misaligned with the thermoelectric field, an electric current is driven. Because of the finite resistivity of the plasma, an electric field appears that is generally rotational. For a QCD axion mass consistent with observational constraints and a conventional efficiency for turbulent dynamo amplification—driven by the same pressure gradients responsible for the thermoelectric fields—a magnetic field is generated on subhorizon scales. After significant Alfvénic unwinding, it reaches a present-day strength of B1013G on a characteristic scale LB20pc. The resulting combination of BLB1/2 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. The amplitude of the pressure gradients may be inferred from the detection of concomitant gravitational waves, while several experiments are underway to confirm or rule out the existence of axions.

  • Received 25 August 2017
  • Revised 15 January 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.021301

© 2018 American Physical Society

Physics Subject Headings (PhySH)

Gravitation, Cosmology & Astrophysics

Authors & Affiliations

F. Miniati1, G. Gregori1,*, B. Reville2, and S. Sarkar1,3

  • 1Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom
  • 2School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, United Kingdom
  • 3Niels Bohr Institute, Blegdamsvej 17, 2100 Copenhagen, Denmark

  • *Corresponding author. gianluca.gregori@physics.ox.ac.uk

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Issue

Vol. 121, Iss. 2 — 13 July 2018

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