Intergalactic Magnetic Fields from First-Order Phase Transitions
  We study the generation of intergalactic magnetic fields in two models for
first-order phase transitions in the early Universe that have been studied
previously in connection with the generation of gravitational waves (GWs): the
Standard Model supplemented by an $|H|^6$ operator (SM+$H^6$) and a classically
scale-invariant model with an extra gauged U(1) $B - L$ symmetry (SM$_{B-L}$).
We consider contributions to magnetic fields generated by bubble collisions and
by turbulence in the primordial plasma, and we consider the hypotheses that
helicity is seeded in the gauge field or kinetically. We study the conditions
under which the intergalactic magnetic fields generated may be larger than the
lower bounds from blazar observations, and correlate them with the
observability of GWs and possible collider signatures. In the SM+$H^6$ model
bubble collisions alone cannot yield large enough magnetic fields, whereas
turbulence may do so. In the SM$_{B-L}$ model bubble collisions and turbulence
may both yield magnetic fields above the blazar bound unless the B$-$L gauge
boson is very heavy. In both models there may be observable GW and collider
signatures if sufficiently large magnetic fields are generated.
