Magnetic Helicity Through a Bounce: Conservation, Inverse Cascade, and the Field That Survives

Magnetic helicity, the linkage of magnetic flux, is nearly conserved in a highly conducting plasma even while magnetic energy decays. That single fact decides which primordial fields survive a long cosmic history. This paper follows a field created at a torsion-bound bounce, carrying a fraction 1.9e-6 of the turnaround density on the turnaround horizon scale, through the radiation era under the standard decay laws for helical and non-helical fields. The comoving coherence scale at the turnaround is only 1.55e+06 metres, yet helicity-driven inverse cascade grows the helical field's coherence to 2.47e+02 parsecs comoving while preserving a present strength of 2.01e-06 nanogauss. The same initial energy in a non-helical field ends at 1.35e-09 nanogauss on 1.66e-01 parsecs. Helicity is therefore not a refinement of bounce magnetogenesis; it is the difference between a field that reaches the present universe and one that largely does not. The evolution is computed from stated decay laws, not measured.