Cosmological Magnetic Fields: Their Generation, Evolution and Observation

TL;DR

This review analyzes the generation, cosmological evolution, and observational tests of cosmological magnetic fields, focusing on primordial seeds that could survive into the intergalactic medium. It compares inflationary, phase-transition, and perturbative-generation mechanisms, detailing how each produces distinct spectra, helicity, and evolutionary paths under MHD turbulence, viscous damping, and cosmic expansion. The authors synthesize current observational constraints from Faraday rotation, the CMB, large-scale structure, gamma-ray cascades, and UHECRs, and discuss how future surveys (radio, gamma-ray, and UHECR) could discriminate primordial fields from those produced by galactic winds. The work highlights the potential of IGMF measurements in voids to reveal early-Universe physics and to constrain models of magnetogenesis, including scenarios involving helicity, inverse cascades, and the role of phase transitions beyond the Standard Model.

Abstract

We review the possible mechanisms for the generation of cosmological magnetic fields, discuss their evolution in an expanding Universe filled with the cosmic plasma and provide a critical review of the literature on the subject. We put special emphasis on the prospects for observational tests of the proposed cosmological magnetogenesis scenarios using radio and gamma-ray astronomy and ultra high energy cosmic rays. We argue that primordial magnetic fields are observationally testable. They lead to magnetic fields in the intergalactic medium with magnetic field strength and correlation length in a well defined range. We also state the unsolved questions in this fascinating open problem of cosmology and propose future observations to address them.

Figures (20)

• Figure 1: Possible spectral energy distributions of cosmological magnetic fields. At small $k$, the spectra of the fields generated at phase transitions in a causal way follow powerlaw with the slope $n_s=2$. Inflationary mechanisms typically result in the slope $n_s=1$. Inflation could in principle generate a scale-invariant spectrum with $n_s=-3$. At large $k$ all the spectra follow a universal slope formed by turbulence.
• Figure 2: The evolution of the magnetic field spectrum. Top left: incompressible flow, top right: compressible flow, bottom: a fully helical field. The spectra evolve towards smaller $k$. Dashed lines show the kinetic energy spectrum.
• Figure 3: The evolution of the magnetic field amplitude and integral scale for helical fields and for non helical compressible and incompressible flow. Both, the electroweak and QCD phase transitions are indicated. The line on which the tracks end is given by the relation $v_A = \lambda_B/t_{\rm rec}$, with $t_{\rm rec} \sim 200$Mpc.
• Figure 4: Theoretical constraints on the IGMF parameters in the present day Universe.
• Figure 5: Constraints on the IGMF from Faraday rotation measurements.