Helical Magnetogenesis from Chiral Spacetime Torsion: Explaining Intergalactic Magnetic Fields via 5D Geometry

We address the long-standing problem of the origin of large-scale magnetic fields in cosmology. Observations reveal coherent Intergalactic Magnetic Fields (IGMFs) with strengths of ~ 10^-15 G over Mpc scales, a phenomenon that standard astrophysical dynamo theories struggle to explain due to causality constraints. We propose that these fields are not generated dynamically post-recombination, but are a primordial relic inherited from the rotational dynamics of the pre-inflationary manifold. We demonstrate that a Chern-Simons topological term in the 5D effective action, driven by a non-trivial spacetime torsion, sources a chiral instability in the primordial electromagnetic field. This instability acts as an exponential amplifier for seed magnetic fields, converting the kinetic energy of the bulk geometry into a stable, large-scale, maximally helical magnetic field. We provide a quantitative estimate yielding a present-day field strength of B_0 approx 5 x 10^-10 G, sufficient to seed galactic dynamos and explain IGMF lower bounds. Crucially, we predict that this primordial field must possess a non-zero magnetic helicity, leading to specific parity-violating signatures in the Cosmic Microwave Background (CMB), such as non-zero TB and EB cross-correlations, providing a falsifiable test for future experiments like LiteBIRD.