Could ancient magnetic fields have brought forward cosmic reionization?

From magnetic forces to early galaxies

The proposed connection is indirect. Magnetic forces can disturb ordinary matter after the Universe cools enough for electrons and nuclei to combine. Those density variations also influence dark matter through gravity. Extra concentrations of matter on small scales could then produce more dark matter haloes—the gravitational structures that host galaxies—and therefore more sources of ionizing light.

The question is whether that initial advantage survives the complicated gravitational growth and merging of structures. In this arXiv preprint, Hiten and colleagues follow the proposed effect from initial density fluctuations through simulated halo formation to maps of neutral hydrogen.

Their result is conditional: with the same prescription for producing ionizing photons in every model, the magnetic-field models reionize earlier. It is not evidence that primordial magnetic fields exist.

A controlled comparison

The researchers compare the standard cosmological model, which includes cold dark matter and a cosmological constant, with four alternatives containing additional density fluctuations attributed to primordial magnetic fields. Their field strengths range from 100 to 170 picogauss, where a picogauss is one trillionth of a gauss. All four use the same magnetic spectral index, a parameter describing how magnetic fluctuations are distributed across spatial scales.

They evolve matter under gravity in a simulated cube about 108 megaparsecs across in comoving coordinates, which factor out cosmic expansion. An N-body calculation represents matter with many particles and follows their collective gravitational motion. The team then identifies haloes by grouping nearby particles.

Those haloes supply sources for ReionYuga, an approximate ionization calculation. Each halo receives an ionizing-photon contribution proportional to its mass. Regions become ionized when their modeled photon budget is sufficient to ionize the hydrogen they contain.

The background cosmology, initial random phases and source efficiency remain unchanged across the runs. The efficiency is calibrated so that the standard reference model approaches reionization completion near redshift six and produces an electron-scattering optical depth close to the Planck measurement. Keeping that efficiency fixed isolates the response to the altered starting fluctuations; it does not test every plausible galaxy population.

More early haloes, earlier ionization

The magnetic models retain stronger small-scale matter clustering and contain more early low-mass haloes. With every halo contributing photons under the shared prescription, ionized regions grow and merge sooner.

The reference model reaches its reionization midpoint—when half the volume is ionized—at a redshift of about 7.12. The weakest and strongest magnetic models reach it at about 7.62 and 9.26, respectively. Higher redshift means an earlier cosmic time. Across the four models, the midpoint advances by roughly 0.50 to 2.14 in redshift. These estimates come from interpolation between stored simulation outputs, so their precision is limited by the spacing of those outputs.

The changes also affect the predicted 21-centimetre signal, radiation associated with neutral hydrogen. Its power spectrum describes how strongly the signal varies across different spatial scales. Earlier ionization can increase large-scale contrasts between neutral and ionized regions, then weaken the overall signal as neutral hydrogen disappears. Stronger primordial fields therefore do not simply produce a stronger signal everywhere.

Why this is not a magnetic-field limit

Earlier ionization leaves more free electrons along the paths of cosmic microwave background photons. The calculated Thomson optical depth—a measure of their accumulated scattering probability—rises from about 0.0547 in the reference model to 0.0595–0.0786 in the magnetic models.

The authors express these increases as 0.66–3.28 times the published Planck uncertainty. Those numbers are offsets from their simulated reference, not statistical exclusion significances. Different assumptions about star formation and escaping ultraviolet radiation could change the reionization histories.

There are important physical omissions, too. Magnetic effects enter through the initial density spectrum; the simulations do not subsequently evolve the magnetic field or gas dynamics, and they omit dissipative magnetic heating. The 21-centimetre calculation assumes hydrogen's spin temperature, which controls the line's brightness, is much higher than the microwave background temperature. It therefore does not model the full thermal history.

Numerical resolution adds another limitation. Haloes below about 1.12 billion solar masses are not included, and counts near that threshold are uncertain. Comparisons with analytical halo predictions do not establish numerical convergence.

The study supplies a connected prediction to investigate: altered small-scale clustering, more early sources, and earlier ionization. Distinguishing that chain from changes in ordinary galaxy physics will require joint modeling of magnetic effects, astrophysical uncertainties and observations.