VORTEX FORMATION IN NEUTRON-IRRADIATED SUPERFLUID HE-3 AS AN ANALOG OF COSMOLOGICAL DEFECT FORMATION

TOPOLOGICAL defects formed during a rapid symmetry-breaking phase tran sition in the early Universe(1,2) could be responsible for seeding lar ge-scale structure, for the anisotropy of the microwave background rad iation, and for the predominance of matter over antimatter(3,4). The t heory describing this cosmological phase transition is formally analog ous to that describing the transition to the superfluid state in liqui d He-3, so that in principle the process of cosmological defect format ion can be modelled in the laboratory. Here we report the results of a n experiment in which the 'primordial fireball' is mimicked using a ne utron-induced nuclear reaction (n + He-3 --> p + He-3 + 0.76 MeV) to h eat small regions of superfluid He-3 above the superfluid transition t emperature. These bubbles of normal liquid cool extremely rapidly, and we find that their transition back to the superfluid state is accompa nied the formation of a random network of vortices (the superfluid ana logue of cosmic strings). We monitor the evolution of this defect stat e by rotating the superfluid sample, allowing vortices to escape from the network and thus be probed individually. Our results provide clear confirmation of the idea that topological defects form at a rapid sec ond-order phase transition, and give quantitative support to the Kibbl e-Zurek mechanism(5,6) of cosmological defect formation.