Branes, strings and boojums; topological defects in helium-3 and the cosmos
Seminar Room 1, Newton Institute
The order parameter of the superfluid helium-3 condensate exhibits broken symmetries that show analogs with those broken in the various transitions undergone by the Universe after the Big Bang. Fortunately for us, the helium-3 order parameter is also sufficiently complex that the superfluid may exist in several phases, the two most stable being the A and B phases. At Lancaster we have developed techniques to investigate the properties of the interface between the A and B phases in the pure condensate limit, far below the superfluid transition temperature. The order parameter transforms continuously across the A–B boundary, making this interface the most coherent two-dimensional structure to which we have experimental access. It has been argued that this ordered 2-d surface in a 3-d bulk matrix, separating the two phases, can provide a good analog of a cosmological brane separating two distinct quantum vacuum states. In superfluid helium-3 the creation of such 2-branes mu st lead to the formation of point and line defects in the texture of the 3-d bulk, simply as a result of the constraints imposed by the interplay of the order parameter symmetries and the geometry of the container. Furthermore, our experiments have shown that removing the 2-branes from the bulk, in a process analogous to brane annihilation, creates new line defects in large quantities. Such observations may provide insight into the formation of topological defects such as cosmic strings arising from brane interactions in the early Universe. Up to now our experimental techniques have only allowed us to infer the properties of the interface and defects by measuring how they impede the transport of quasiparticle excitations in the superfluid, which is essentially a remote measurement. Our new experiments allow us to directly probe the interface region.