The turbulent small-scale dynamo amplifies a weak seed magnetic field into the observed microgauss fields of galaxies and clusters, but simulations by Seta & Federrath (2020) show that the dynamo erases all memory of the seed's strength and structure, so evolved turbulent fields cannot reveal their origin. Two source-faithful questions therefore remain open. (1) Survival: do magnetic fields generated in the early Universe (during inflation or cosmological phase transitions) survive turbulent decay, turbulent diffusion, galactic winds, and flux expulsion through structure formation, reaching forming galaxies and clusters with amplitude sufficient to act as dynamo seeds? (2) Identifiability: in low-turbulence regions where dynamo erasure is ineffective, chiefly cosmic voids, can observable properties — field strength, spectral shape, coherence length, volume-filling factor, correlation with the cosmic web — discriminate a primordial relic origin from late-time astrophysical magnetization by winds, jets, or leakage of galactic fields? Literature from 2022 to 2026 leaves both parts actively contested, including an unresolved 2025–2026 dispute over whether astrophysical mechanisms can explain the gamma-ray-implied void fields at all.

Background

Galaxies and galaxy clusters carry magnetic fields of order a few μG\mu\mathrm{G}, and the standard explanation for their origin is the turbulent (small-scale) dynamo: random turbulent motions of a conducting fluid stretch magnetic field lines faster than Ohmic diffusion can relax them, producing exponential growth of a random field once the magnetic Reynolds number Rm=UL/η\mathrm{Rm}=UL/\eta (the ratio of advective transport to magnetic diffusion, with η\eta the magnetic diffusivity, UU a characteristic velocity, and LL a characteristic length) exceeds a critical value of order a few hundred. The dynamo needs only a seed field of nonzero amplitude — but the strength, geometry, and above all the physical origin of that seed remain unknown. Two families of origin are debated. Primordial (early-Universe) magnetogenesis produces fields during inflation (the hypothesized epoch of exponential expansion of space immediately after the big bang) or during cosmological phase transitions in the very early hot Universe; astrophysical mechanisms produce them later, for example via the Biermann battery (weak fields generated by charge separation between electrons and ions), plasma instabilities in counter-streaming flows, or magnetized ejecta from stars, supernovae, and active galactic nuclei.

Seta & Federrath (2020) asked explicitly whether the seed leaves any trace in the amplified field. Simulating driven isothermal turbulence with three seed configurations — a uniform field, a random field with a power-law spectrum, and a random field with a parabolic spectrum — they found that the kinematic growth rate, the saturated amplitude, the spectrum, and the spatial structure of the dynamo-generated field are statistically independent of the strength and structure of the seed. Their conclusion is that in any actively turbulent region all memory of the seed field is lost, so the evolved field cannot be used to trace seed origin (they also showed that even below the dynamo threshold, tangling of a large-scale field by turbulence maintains a weak small-scale component that grows only linearly).

Two consequences frame the open problem. First, whether primordial fields survive to act as seeds at all is, as the paper states, still not clear: primordial fields decay through magnetohydrodynamic turbulence and turbulent diffusion, and in galaxies additionally through winds and flux expulsion (turbulent transport of magnetic flux out of the region, which is, however, less efficient than diffusive decay). Second, memory loss holds only where turbulence is strong. In the cosmic voids — the near-empty regions that occupy most of the volume of the present-day Universe — dynamo action is negligible, and cosmological simulations show that the field there retains the imprint of its seed. Voids are also the one regime with a direct lower bound on intergalactic fields: TeV gamma rays from blazars (active galactic nuclei whose relativistic jets point nearly toward us) are absorbed on the extragalactic background light and regenerate as electron–positron pairs whose emission should appear as GeV-scale cascade halos around the source; the non-detection of these halos by the Fermi-LAT gamma-ray telescope implies that intervening void-level fields of roughly 10−1710^{-17}–10−15 G10^{-15}\ \mathrm{G} deflect the cascade electrons out of the line of sight (Neronov & Vovk 2010). In denser regions the complementary probe is the Faraday rotation measure — the wavelength-squared rotation of the polarization angle of radio radiation, proportional to the line-of-sight integral of electron density times magnetic field. Whether those void fields are primordial relics or were deposited later by galactic winds, jets, or leakage of galactic fields is exactly what current observations cannot decide.

Problem Statement

Two linked questions, stated as open in Seta & Federrath (2020) and unresolved by subsequent work:

(1) Survival as seeds. Do magnetic fields generated in the early Universe — during inflation or during cosmological phase transitions — survive magnetohydrodynamic decay, turbulent diffusion, galactic winds, and flux expulsion during structure formation, arriving at forming galaxies and galaxy clusters with amplitude sufficient to act as the seed fields that the small-scale turbulent dynamo then amplifies into the observed microgauss fields within the available time?

(2) Identifiability of origin in low-turbulence regions. In regions where dynamo action is weak or absent, chiefly the cosmic voids, the evolved field is predicted to retain imprints of its seed. Can observable properties of void and intergalactic magnetic fields — strength, spectral shape (for example a white-noise versus scale-invariant spectrum), coherence length, volume-filling factor, and correlation with the cosmic web — discriminate a primordial relic origin from late-time astrophysical magnetization by galactic winds, black-hole jets, or leakage of galactic dipole fields into voids, at the level of current or planned gamma-ray and radio observations?

The common objective is to determine whether present-day cosmic magnetic fields descend from early-Universe magnetogenesis and whether any observational window on seed origin survives.

References

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  2. Andrii Neronov and Ievgen Vovk, 'Evidence for Strong Extragalactic Magnetic Fields from Fermi Observations of TeV Blazars', Science 328, 73–75 (2010), DOI: 10.1126/science.1184192, https://doi.org/10.1126/science.1184192
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