Cosmological Experiments in He3 - He4 Mixtures

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Cosmological phase transitions can be a copious source of topological defects. The same happens in condensed matter systems, and experiments on these systems can help in determining the mechanism for defect formation. The similarities between the superfluid transitions in liquid helium and cosmological models containing topological defects is especially strong. As a result, in recent years there has been considerable activity to simulate phase transitions of the early universe using liquid He-3 and He-4.Previous experiments in both He-3 and He-4 have shown that defects, in this case vortices, are indeed created, and that the vortex line density is consistent with estimates based on the mechanism proposed by Zurek, now known as the Kibble-Zurek (K-Z) mechanism. This mechanism makes specific predictions about the scaling of the vortex line density with a) the rate at which the transition takes place and b) the amplitude of the correlation length of the order parameter. In earlier experiments there was little control over the former, while the latter, determined solely by the fluid, could not be changed appreciably either. Prof. Mulder's group will carry out a new experiment, operating close to the tricritical point in He-3 and He-4 mixtures, will have the ability to change both over a wide range. This should result in a definitive test of the K-Z mechanism.This project is jointly supported by the Division of Physics, the Division of Materials Research and the MPS Office of Multidisciplinary Activities.