How the Universe Got Magnetized Before the First Stars express
Right after the Big Bang, space was a hot plasma — a mix of charged particles and light. In such a medium, electric currents couldn't flow on their own: a 'potential difference' was needed to push electrons relative to photons. That's exactly what theories of cosmological magnetogenesis seek — the birth of the first magnetic fields.
The authors built a complete chain of equations, from particle collisions to the familiar Ohm's law, and tested three mechanisms. The first, discovered in 2005, works like a comb: the velocity difference between electrons and photons twists currents. The second was found only now: where photon density varies in space, an extra push on electrons arises. The third is a 'tailwind' from large-scale matter flows.
Numerical cosmological simulations (CAMB) at the epoch of recombination, when the Universe was 380,000 years old, showed that the new term adds nearly as much field as the old mechanism. Together, they boost the result by a factor of 1.5. And if large-scale matter flows circulated in the Universe, even at the limit allowed by Planck satellite data, a field of ten billionths of a gauss emerges on megaparsec scales. That's millions of times above the threshold where the galactic dynamo amplifies weak 'seeds' into the magnetic fields of stars and nebulae.
Thus, the cosmic plasma, which had never known a single star, was already weaving an invisible magnetic web, the scaffold for future galaxies.
Original abstract
as written by the authors arXiv:2606.16680
We derive and compare three mechanisms of cosmological magnetogenesis: the Thomson-scattering velocity-difference mechanism of Takahashi et al.\ (2005), a new density-gradient source identified here for the first time, and the Harrison bulk-flow mechanism of Cembranos et al.\ (2020). Starting from the coupled Maxwell-Boltzmann equations, the complete kinetic theory chain is derived in a single document -- from the BBGKY hierarchy and Thomson collision term, through the generalised Ohm's law, to the second-order magnetic induction equation. The Ohm's law correction terms are each bounded by $$m_e/m_p\approx5.4\times10^{-4}$$, confirming the standard single-fluid approximation to better than $$0.1\%$$. At second order in cosmological perturbations, products of first-order scalar source vorticity, we identify a coupling between the photon density contrast $$\delta_\gamma \equiv \delta\rho^{(1)}_\gamma / \bar\rho_\gamma$$ and the electron-photon velocity difference $$(u_e-u_\gamma)^{(1)}$$ that was implicitly present in previous treatments but never isolated. Numerical evaluation with CAMB~v1.6.6 at $$z=1100$$ shows that this term contributes at $${\approx}0.97\times B_{\rm Tak}$$, giving a scattering-mechanism total $${\approx}1.4\times$$ the Takahashi result. The Harrison mechanism at the Planck bulk-flow limit ($$\beta<8.5\times10^{-4}$$) yields $$B\approx5.7\times10^{-24}$$~G at 1~Mpc today and dominates for $$\beta\gtrsim2\times10^{-3}$$, mildly above the Planck limit. All seed fields exceed the galactic dynamo threshold by many orders of magnitude.
The magnetic field born in the early Universe's plasma may be older than the oldest light we see today, the cosmic microwave background.