Electroweak-Scale Cosmic Magnetism: Magnetogenesis, Baryogenesis, Phase Transitions and Gravitational Waves
Magnetic fields, the baryon asymmetry of the Universe, and stochastic gravitational waves provide complementary probes of physics in the early Universe. Near the electroweak scale, these phenomena are not necessarily independent. The electroweak anomaly allows hypermagnetic helicity to exchange charge with chiral fermions, sphalerons can change baryon and lepton number, and first-order phase transitions in extensions of the Standard Model can drive plasma motion, magnetogenesis, and gravitational waves. In this review, we explain these connections and examine the literature needed to make quantitative predictions. We begin with the properties of primordial magnetic fields and the electroweak plasma, then discuss the mechanisms that produce magnetic fields, the conversion between helicity and baryon number, magnetic backreaction, and the evolution of the field to the present day. We also review the observational constraints and recent developments, including gauge-invariant descriptions of magnetic flux through the crossover and reconnection-controlled field evolution. Particular attention is given to the assumptions behind commonly used formulas and to the distinction between established results and model-dependent estimates. We argue that the most useful way forward is to predict the baryon abundance, present-day magnetic-field spectrum, and gravitational-wave signal from the same microscopic model, with consistent treatment of the intervening plasma dynamics.