Andrew Long (KICP, United States)
It is customary and often necessary to study cosmological phase transitions, such as inflation and thermal symmetry-breaking phenomena, through the relics they leave behind. A relic magnetic field, for instance, could have been sustained by the hot plasma of the early universe and provide the seed for galactic-scale fields seen today. Evidence for this primordial magnetic field may be found in the voids between the galaxy clusters where the field survives unprocessed by structure formation. Ongoing efforts by gamma ray telescopes hope to reveal a magnetically-broadened electromagnetic cascade halo around TeV gamma ray sources such as blazars. I will discuss how measurements of the halo morphology can be used to probe not only the magnetic field strength, but also its helicity and coherence length. Helical magnetic fields, having a larger field amplitude in either left- or right-circular polarization modes, are predicted by models of pseudoscalar inflation as well as models of baryogenesis, and in the latter scenario, the sign of the helicity is determined by the sign of the baryon asymmetry. Thus a detection of helical magnetic fields in intergalactic space would open a new chapter in early universe cosmology, and efforts are already underway to make such a measurement possible.