Origin of cosmic-web magnetism: discriminating primordial versus astrophysical magnetogenesis through joint radio observations and simulations

The megaparsec-scale filaments of the cosmic web are magnetised — radio observations imply volume-averaged fields of order tens of nanogauss — yet the origin of this magnetisation is unknown: primordial magnetogenesis during inflation or an early phase transition, or seeding by astrophysical activity (supernova-driven winds, active galactic nuclei) followed by transport and amplification during structure formation. Vazza et al. (2021) argued that existing constraints — synchrotron stacking detections, Faraday-rotation limits, and the non-detection of blazar cascade halos — strain purely astrophysical scenarios and tentatively favour a volume-filling primordial component, with the bluest tested spectrum ($P_B(k)\propto k^{+1}$, $B_{\rm Mpc}\approx 0.04$ nG) giving the best match; they identified six coupled uncertainties blocking a definitive answer: unresolved small-scale dynamo action, ideal-MHD-only modeling, unknown shock electron-acceleration efficiencies, radio foregrounds, degeneracies with ultra-high-energy-cosmic-ray, fast-radio-burst, and axion-like-particle probes, and the unexplored wider parameter space of inflationary magnetogenesis. The problem is to determine which magnetogenesis scenario(s) are compatible with the magnetisation of cosmic-web filaments, through the joint development of cosmological MHD simulations that control the sub-grid dynamo and non-ideal physics, calibrated particle-acceleration efficiencies, and foreground-robust radio observations, cross-validated against independent probes. Post-2021 work has strengthened the case against astrophysical-only seeding and produced first cosmic-web-scale radio detections, but no unambiguous scenario identification or primordial spectral-shape confirmation exists; the question remains open.

Background

The cosmic web and its magnetic fields. On scales of megaparsecs, matter in the Universe forms the cosmic web: galaxies and gas are concentrated in sheets and filaments — tunnel-like overdensities up to tens of megaparsecs long — that surround near-empty voids and meet at galaxy clusters, the most massive gravitationally bound objects. The gas in filaments is the warm-hot intergalactic medium (WHIM): a tenuous, million-degree plasma at a few to a few hundred times the mean cosmic baryon density. Magnetic fields of nanogauss strength ($1\,\mathrm{nG}=10^{-9}\,\mathrm{G}$) or more are expected to permeate this gas, but their origin is one of the oldest open questions in astrophysical cosmology.

Competing magnetogenesis scenarios. Two families of scenarios compete. In primordial magnetogenesis, fields are generated in the very early Universe — during cosmological inflation or at phase transitions — and are conventionally parametrized by a power-law power spectrum $P_B(k)\propto k^{\alpha}$ with amplitude $B_{\rm Mpc}$ (the field smoothed on a comoving scale of $1$ Mpc); they then evolve passively and are amplified by the compression and turbulence accompanying structure formation. In astrophysical seeding, fields are produced by ordinary astrophysical activity — stellar dynamos expelling flux via supernova-driven galactic winds, and jets and outflows from accreting supermassive black holes (active galactic nuclei, AGN) — and are then transported outward into intergalactic space. Because galaxy and cluster interiors are heavily processed, both scenarios predict similar microgauss fields there; filaments and voids, being closer to pristine, preserve the amplitude and topology (spatial coherence and orientation) of the seed, which is why the low-density cosmic web is the decisive arena for discriminating the scenarios.

Radio observables. Three kinds of observation carry the information. (i) Synchrotron emission: structure-formation shocks accelerate electrons to relativistic energies (diffusive shock acceleration), and these electrons emit low-frequency radio waves spiralling in the filament field. The Low-Frequency Array (LOFAR), a distributed radio telescope observing at the lowest radio frequencies accessible from the ground, detected a 'ridge' of such emission in the filament connecting the merging clusters Abell 0399 and Abell 0401 (Govoni et al. 2019), and stacking (averaging) of large samples of galaxy pairs in all-sky radio and X-ray maps detected mean synchrotron plus thermal emission from intercluster bridges with $B\sim 30$–$60$ nG (Vernstrom et al. 2021). (ii) Faraday rotation: a linearly polarized radio wave passing through magnetised plasma has its polarization angle rotated by an amount proportional to the rotation measure, $\mathrm{RM}=0.812\int n_e B_\parallel\,dl$ (in $\mathrm{rad\,m^{-2}}$ for electron density $n_e$ in $\mathrm{cm^{-3}}$, field $B$ in $\mu$G, path length in pc); the scatter and redshift-dependence of extragalactic RMs probe the line-of-sight intergalactic field. (iii) Related probes: blazars — active galactic nuclei whose relativistic jets point nearly toward us — emit TeV gamma rays that pair-produce on the cosmic background light, and the resulting electron–positron pairs re-radiate an extended GeV glow through inverse-Compton scattering unless intergalactic magnetic fields deflect them, so the non-detection of such cascade halos lower-limits the intergalactic field; the deflection of ultra-high-energy cosmic rays (UHECR), fast-radio-burst (FRB) dispersion and rotation measures, and the oscillation of photons into axion-like particles (ALPs) in intergalactic fields all depend on cosmic magnetism, but each is degenerate with its own source-model uncertainties (source distributions, duty cycles, composition, progenitor environments).

The simulation state of the art and its limits. Vazza et al. (2021) reviewed this problem using the MAGCOW suite: uniform-grid ideal-MHD (magnetohydrodynamic) simulations of an $85^3\,\mathrm{Mpc^3}$ cosmological volume with the ENZO code (a widely used open-source cosmological grid code) at $83.3$ kpc/cell comoving resolution, comparing a uniform $1$ nG primordial seed, a negligible seed amplified by a sub-grid dynamo model, astrophysical seeding by star-formation and AGN feedback, and primordial spectra from cosmic-microwave-background (CMB) constraints with $\alpha=-1,0,+1$ and $B_{\rm Mpc}=1.87,0.35,0.042$ nG respectively (models CMB2/3/4). The small-scale (turbulent) dynamo — random amplification of seed fields by turbulent stretching on scales below the 'MHD scale' at which the field back-reacts on the flow — cannot be resolved at this resolution, so the simulations employ an approximate run-time prescription driven by the gas vorticity, with assumed conversion efficiencies and a field topology taken parallel to the vorticity. Their conclusions: present constraints appear to exclude purely astrophysical scenarios and primordial models maximally amplified in filaments, while the 'bluest' spectrum CMB4 ($\alpha=+1$) gives the best — still tentative — match. The authors explicitly flag what blocks a definitive answer: unresolved sub-MHD-scale dynamo physics in filaments; the restriction to ideal single-fluid MHD, which neglects resistive dissipation and the weakly collisional (particle-behaviour-dominated) plasma physics of rarefied regions; the largely unknown efficiency with which structure-formation shocks accelerate relativistic electrons, and the unconstrained role of turbulent re-acceleration on scales beyond cluster 'bridges'; the dominance of foregrounds (Galactic rotation-measure fluctuations, faint unresolved radio sources, instrumental systematics) on the $\geq 10$-arcmin scales where filament signals are sought; degeneracies with the UHECR/FRB/ALP probes; and the unexplored wider parameter space of inflationary magnetogenesis.

Problem Statement

Determine the origin of the magnetisation of the cosmic web: identify, or quantitatively exclude, the magnetogenesis scenario(s) — primordial (with which spectral shape $P_B(k)\propto k^{\alpha}$ and amplitude $B_{\rm Mpc}$, including the blue $\alpha\approx+1$ spectrum found tentatively compatible by Vazza et al. 2021) versus astrophysical seeding by galaxies and AGN, or a mixture — that are consistent with the observed magnetisation of cosmic-web filaments, as probed by low-frequency synchrotron emission and Faraday rotation interpreted through cosmological MHD simulations.

A valid answer must account for each uncertainty source Vazza et al. (2021) identify as blocking the discrimination: (a) the treatment of small-scale dynamo amplification below the resolved MHD scale in and around filaments; (b) the impact of non-ideal and weakly-collisional plasma physics in rarefied filament gas; (c) the acceleration efficiency of relativistic electrons at structure-formation shocks and the role of turbulent re-acceleration; (d) the separation of filament radio signals from Galactic and unresolved-source foregrounds; (e) consistency with the independent magnetic-field probes (CMB limits on primordial fields, UHECR deflections, FRB rotation and dispersion measures, ALP-induced spectral modifications); and (f) coverage of the wider parameter space of inflationary magnetogenesis beyond the tested reference spectra. Accounting for a source means showing that the quantified conclusion survives it, and the demonstration is graded per source: for the nuisance parameters (a)-(d), by showing the conclusion is unchanged when the source is varied over its plausible range; for the cross-probe check (e), by showing the conclusion is consistent with each independent magnetic-field probe, so that no single probe-specific systematic drives it; and for (f), by showing the conclusion holds over the wider inflationary parameter space beyond the tested reference spectra. Resolving the uncertainty itself is not required, and existing sub-grid-dynamo, non-ideal, acceleration and foreground prescriptions may be re-used for these demonstrations; the joint development of new simulations named in the opening paragraph is the longer-term programme the problem motivates, not a per-answer deliverable. Completion is a quantified identification or exclusion — for example confidence regions or exclusion bounds in the $(B_{\rm Mpc},\alpha)$ primordial plane and in astrophysical-seeding parameter space — demonstrated to be robust under these uncertainties. The simulation, calibration, and observational components (a)-(d) and (f) are not independent deliverables: each supplies a necessary link in the single inference chain from radio measurement to magnetogenesis scenario, and the cross-probe checks (e) guard against a probe-specific systematic driving the conclusion.

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