  We describe how magnetic fields in Friedmann universes can experience
superadiabatic growth without departing from conventional electromagnetism. The
reason is the relativistic coupling between vector fields and spacetime
geometry, which slows down the decay of large-scale magnetic fields in open
universes, compared to that seen in perfectly flat models. The result is a
large relative gain in magnetic strength during the pre-galactic era that can
lead to astrophysically interesting B-fields, even if our universe is only
marginally open today.
