  We consider the gravitational generation of the massive Z-boson field of the
standard model, due to the natural breaking of its conformal invariance during
inflation. The electroweak symmetry restoration at the end of inflation turns
the almost scale-invariant superhorizon Z-spectrum into a hypermagnetic field,
which transforms into a regular magnetic field at the electroweak phase
transition. The mechanism is generic and is shown to generate a superhorizon
spectrum of the form B~1/L on a length-scale L regardless of the choice of
inflationary model. Scaled to the epoch of galaxy formation such a field
suffices to trigger the galactic dynamo and explain the observed galactic
magnetic fields in the case of a spatially flat, dark energy dominated Universe
with GUT-scale inflation. The possibility of further amplification of the
generated field by preheating is also investigated. To this end we study a
model of Supersymmetric Hybrid Inflation with a Flipped SU(5) grand unified
symmetry group.
