The universe in a test-tube? The creation of cosmic chords viewed from a superfluid helium droplet

This digest paper describes two experiments of ''experimental cosmology'' realized at the Research Centre for Very Low Temperatures of Grenoble (France), in collaboration with the Lancaster University (UK), and at the University of Technology of Helsinki (Finland). In these two experiments, an helium 3 sample was cooled below its superfluid transition temperature and irradiated with neutrons. The reaction produces protons and tritium with a 764 keV kinetic energy. Each absorbed neutron creates a local heating of liquid helium and destroys the superfluid state inside a 50 microns spherical zone. When these hot zones are cooled again, superfluid zones with different symmetries occur and extend, and finally create vortices during their recombination. Vortices were visualized using sophisticated NMR methods in Helsinki or deduced from the energy of the quasi-particles created during the experiment in Grenoble. These experiments clearly show that an important part of the energy can remain in the form of topological excitations (vortices) after a rapid transition phase. The cosmological impact of this result is that cosmic chords were probably created with a similar mechanism in the primordial universe, if the transition phases suggested by the theoricians really occurred. (J.S.)