COSMOLOGICAL MAGNETIC FIELD MEASUREMENTS

Andrii Neronov – University of Geneva (Switzerland)

I will review the methods of measurements and constraints on cosmological magnetic fields using gamma-rays, radio, CMB and gravitational wave data. A specific focus will be on recent NANOGrav evidence for the existence of a stochastic gravitational wave background in the 1-100 nHz frequency range. I will explore the possibility that such background could have been produced by magneto-hydrodynamic (MHD) turbulence at the QCD scale. In this interpretation NANOGrav measurement suggests the magnetic field parameters: comoving field strength close to microGauss and a correlation length close to 10% of the Hubble radius at the QCD phase transition epoch. Magnetic field with such parameters evolving through cosmic ages can influenced recombination and produce noticeable effect on CMB, and partially relieve the Hubble tension. This magnetic field at present epoch can be marginally detectable with CTA observations of extended emission around nearest blazars.