Whether the weak magnetic fields that pervade the intergalactic medium (IGM) can be produced entirely by astrophysical processes — magnetic-energy injection by supernova and supermassive-black-hole feedback, transport by galactic winds, amplification by turbulent dynamos — or instead require a primordial seed from the early Universe, is a central open question of cosmic magnetism. Cosmological magnetohydrodynamic simulations by Ramesh, Garaldi and Byrohl (2026) show that supernova-driven seeding underproduces the intergalactic field strengths inferred from gamma-ray cascade lower limits at both z=0z=0 and z∼3z\sim3, while their black-hole-driven prescription satisfies present-day limits but remains in mild tension with the z∼3z\sim3 limits of Vovk (2026). Adjacent 2024–2026 results pull in opposite directions: combined gamma-ray and simulation analyses exclude most astrophysical scenarios lacking a space-filling primordial component (Tjemsland et al. 2024), and LOFAR (Low-Frequency Array) rotation measures of cosmic filaments favour primordial magnetogenesis (Carretti et al. 2025), yet the gamma-ray constraints themselves remain contested because plasma instabilities may cool the pair beams on which the method relies. The open problem is therefore: establish whether a purely astrophysical magnetogenesis scenario can be calibrated to simultaneously satisfy standard galaxy-formation observables and intergalactic magnetic-field constraints across cosmic time, or demonstrate that a primordial contribution is required.

Background

Cosmic magnetism — the origin of the microgauss fields of galaxies and of the far weaker fields of intergalactic space — is a long-standing open problem (Widrow 2002; Durrer and Neronov 2013; Subramanian 2016). Two families of explanations compete. Primordial scenarios generate seed magnetic fields in the early Universe, during inflation or cosmological phase transitions; astrophysical scenarios generate them later, inside stars, galaxies and accreting black holes, and rely on outflows to carry magnetized plasma into surrounding space (Bertone et al. 2006; Vazza et al. 2017). In both cases the weak seed fields are thought to be amplified to observed strengths largely by the small-scale dynamo, the turbulent stretching of field lines that converts kinetic energy into magnetic energy. Within galaxies the seeding question is nearly closed: Garaldi et al. (2021) showed that physically motivated astrophysical seeds (the Biermann battery, which generates fields from misaligned pressure and density gradients in ionized plasma, and supernova injection) produce galactic fields indistinguishable from primordial ones by z≲1.5z\lesssim1.5, with only low-density gas and low-mass halos retaining memory of the seed. The battleground has therefore shifted to the intergalactic medium (IGM), the diffuse gas in cosmic filaments and voids between galaxies, together with the circumgalactic medium (CGM), the gaseous halo surrounding a galaxy.

The sharpest constraints on IGM magnetization come from gamma-ray observations of TeV blazars, active galactic nuclei whose relativistic jets point toward us and emit TeV photons. Such photons pair-produce on the extragalactic background light (EBL), the accumulated radiation field of stars and accretion processes; the resulting electron–positron pairs are deflected by intergalactic magnetic fields (IGMF) and inverse-Compton scatter cosmic microwave background photons up to GeV energies, forming an extended pair-echo halo. The absence of these halos and the suppression of GeV flux imply lower limits on the IGMF: Neronov and Vovk (2010) obtained roughly B≳3×10−16B\gtrsim3\times10^{-16} G (for correlation lengths, the distances over which the field direction remains coherent, above about a megaparsec) at z≈0z\approx0, and Vovk (2026) extended the limits to z≈3z\approx3 using 17 years of data from Fermi/LAT (the Large Area Telescope on board the Fermi Gamma-ray Space Telescope), disfavoring the zero-field hypothesis at about 8.6σ\sigma and requiring the field to fill at least about 90% of the volume over z=0.5z=0.5–3. These limits are themselves contested: if plasma instabilities cool the pair beams faster than inverse-Compton scattering does (Broderick, Chang and Pfrommer 2012; Perry and Lyubarsky 2021), the missing GeV emission no longer implies a magnetic field; on the other hand, even a very weak tangled IGMF suppresses those very instabilities (Alawashra and Pohl 2022), and a recent parameterized treatment finds that a lower limit of order 10−1710^{-17} G survives even when instability cooling is fitted to the spectrum of the archetypal blazar 1ES 0229+200 (Dey et al. 2026). Independent radio constraints come from Faraday rotation, the wavelength-squared rotation of the polarization angle of linearly polarized radio emission propagating through a magnetized plasma: LOFAR (the Low-Frequency Array, a low-frequency radio telescope) measurements of background sources imply filament fields of 11–15 nG at z=0z=0 whose steep redshift evolution favours primordial magnetogenesis with a subdominant astrophysical component (Carretti et al. 2025).

Ramesh, Garaldi and Byrohl (2026) run cosmological magnetohydrodynamic (MHD) simulations with the moving-mesh code AREPO on the IllustrisTNG galaxy-formation model in 25 Mpc/h boxes, adding recipes that inject magnetic energy during supernova (SNe) and supermassive black hole (SMBH; black holes of millions to billions of solar masses) feedback events, and compare these with runs initialized with uniform primordial seed fields. Halo magnetic fields at z=0z=0 are largely seeding-insensitive because dynamo amplification erases the memory of the origin, but the IGM retains a strong imprint of the seeding scenario: SNe-only injection underproduces the gamma-ray lower limits at both z=0z=0 and z∼3z\sim3, while their specific SMBH-driven prescription satisfies present-day limits but remains in mild tension with the z∼3z\sim3 limits of Vovk (2026). The paper also reports numerical resolution concerns for halos with M200c≲1011.5 M⊙M_{200c}\lesssim10^{11.5}\,M_\odot (halo mass within the radius enclosing 200 times the critical density) and notes that the robustness of the gamma-ray constraints is debated.

Problem Statement

Determine whether a purely astrophysical magnetogenesis scenario — magnetic energy injected by stellar and supermassive-black-hole feedback, transported through circumgalactic space by galactic winds and large-scale outflows, and amplified by dynamo action — can, when implemented in cosmological magnetohydrodynamic simulations and calibrated against standard galaxy-formation benchmarks (stellar mass function, cosmic star-formation history, black-hole scaling relations), simultaneously reproduce the observational constraints on intergalactic magnetic fields across cosmic time. These constraints comprise the gamma-ray cascade lower limits at z≈0z\approx0 and z≈3z\approx3 (together with their dependence on magnetic correlation length and volume-filling fraction, the fraction of cosmic volume magnetized above a given field strength), the volume-filling and line-of-sight covering fractions of magnetized gas, and the filament field strengths and coherence lengths probed by Faraday rotation.

A complete answer must take one of two forms. Either (i) exhibit a concrete calibration within the purely astrophysical framework that satisfies both the galaxy-formation and intergalactic-magnetic constraint sets across redshift, or (ii) establish, with explicit assumptions — compatible with the galaxy-formation benchmarks above — that genuinely bound the astrophysical model space (feedback energetics and burstiness, wind velocities and extents, seeding times, SMBH prescriptions, and the additional battery channels), that no such calibration exists, implying that a primordial seed contribution is required; a quantified astrophysical-plus-primordial mixture falls under this branch, with the required primordial share fixed by that bound.

Because the gamma-ray cascade limits are the central discriminant, their robustness is contested: the plasma-instability cooling of pair beams — the caveat emphasized in Ramesh et al. (2026) — along with related cascade systematics such as the assumed blazar duty cycles and the extragalactic-background-light attenuation model, affects the inferred lower limits. The common scientific objective uniting these components is a decision on the origin of the volume-filling intergalactic magnetic field — astrophysical, primordial, or a quantified mixture — that is simultaneously consistent with what is independently known about how galaxies form.

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