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Gravitational wave amplification of seed magnetic fields

We discuss how gravitational waves could amplify seed magnetic fields to strengths capable of supporting the galactic dynamo. We consider the interaction of a weak magnetic field with gravity wave distortions in almost FRW cosmologies and find that the magnitude of the original field is amplified proportionally to the wave induced shear anisotropy and, crucially, proportionally to the square of the field's initial scale. The latter makes our mechanism particularly efficient when operating on superhorizon sized magnetic fields, like those produced during inflation. In that case, the achieved amplification can easily boost magnetic strengths, which may still lie relatively close to the galactic dynamo lower limits, well within the currently accepted range.