Axion-Driven Cosmic Magnetogenesis during the QCD Crossover

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 B∼10^{-13} G on a characteristic scale L_{B}∼20 pc. The resulting combination of BL_{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. 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.

Neutrino-electron magnetohydrodynamics in an expanding universe

“…Contrary to the usual Biermann term, the neutrino battery does not vanish if the plasma remains barotropic during the QCD crossover (as one would expect if the QCD is not, in fact, a first-order phase transition [57]), since the turbulent fluctuations in the thermodynamic properties of the plasma generated at the phase transition will not be perfectly correlated with those of the neutrino field. The relevance of this result also lies in the fact that it does not require physics outside of the Standard Model to explain primordial magnetogenesis, such as axion-photon coupling [58], or other processes that break conformal invariance (see, e.g., [55] and references therein). Assuming a lepton asymmetry of the order of ∼10 −9 n e , we can estimate the electric field produced by the neutrinos as…”

“…where L H is the particle horizon, which generates a magnetic seed at the QCD crossover of B QCD ∼ 10 −46 G. Despite being very small, this initial seed can undergo significant amplification before being damped by cosmological expansion through a small-scale dynamo produced by the turbulence at the QCD. In fact, as suggested by lattice simulations [59][60][61], if turbulent velocity fluctuations of the order of δu ∼ 1= ffiffiffiffiffiffiffi 3g à p (with g à the effective number of degrees of freedom) are excited on scales of l ∼ 0.1L H , the Reynolds number at the crossover is expected to be large (Re ≃ δu × l=λ mfp;ν ≳ 10 4 , where λ mfp;ν is the neutrino mean free path), and the magnetic seed grows exponentially on a timescale comparable to the viscous-scale eddy turnover time ∼Re −1=2 t QCD ≪ t QCD , reaching the equipartition field strength (in comoving units) with the turbulent energy of ∼0.1 μG [58]. If the magnetic field is then frozen in in the expanding plasma, this equipartition value would correspond to a magnetic field strength at recombination of ∼10 −3 nG [58], close to the 5 × 10 −3 − 0.1 nG range which results from constraints from the cosmic microwave background anisotropy and current magnetic fields in galaxy clusters [62][63][64][65].…”

Modified Friedmann Equations via Conformal Bohm–de Broglie Gravity

“…Moreover, small ultralight scalars in the range 10 −21 eV m a 10 −17 eV have been invoked to explain the cosmological origin of magnetic fields (Choi et al 2018) (see also Ref. Miniati et al (2018)).…”

Simulating relic gravitational waves from inflationary magnetogenesis

“…Hindmarsh et al (2015Hindmarsh et al ( , 2017 have produced numerical simulations of GW generation by assuming a first order phase transition (Kamionkowski et al 1994;Ellis et al 2019Ellis et al , 2020. Even if the phase transition is not a first order one, as initially assumed, it is still possible to produce primordial turbulence from magnetic fields that could be generated during the electroweak epoch (see, e.g., Joyce & Shaposhnikov 1997;Cornwall 1997;Bhatt & Pandey 2016;Miniati et al 2018). The existence of large-scale magnetic fields in the early universe is motivated by indirect evidence of their presence in the intergalactic regime from the non-detection of GeV photons in blazar observations (Neronov & Vovk 2010;Taylor et al 2011;Ackermann et al 2018).…”