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Solving the elusive superfluid A-to-B nucleation puzzle to advance cosmology

Samuli Autti  (Lancaster University, UK)

Abstract 

The fundamental theory of how first-order phase transitions are triggered, the homogeneous nucleation theory, has evaded unambiguous experimental validation for a century. Superfluid ³He is perhaps the most obvious platform for testing it, but the predicted lifetime of metastable A phase is astronomical, while the A-B transition is commonly observed in the laboratory. We have constructed two experiments nearly perfectly isolated from their environment [1,2] for the first time producing data with sufficient quality for theory development. We have also built a first-principles numerical simulation of the phase transition dynamics [3], supported by a GEANT-4 simulation of the radiation environment [4,5] and recent measurements of second-order phase transition dynamics [6,7]. Pulling these together, I report progress in resolving the nucleation problem.

Superfluid ³He is increasingly being used to observe and model the behaviour of our Universe. The LISA gravitational wave space mission, due to be launched in the mid 2030’s, will be able to measure the continuous gravitation wave spectrum that would result from a first-order phase transition triggered by homogeneous nucleation in the early Universe. We aim to measure dynamic characteristics such as the speed of the expanding phase boundary of the B phase following its nucleation. This effort will directly inform the simulations of the gravitational wave production.

References

[1] JLTP 215, 477–494 (2024)

[2] arXiv:2503.24288

[3] JLTP 215, 495–524 (2024)

[4] JLTP 215, 465–476 (2024)

[5] Eur. Phys. J. C 84, 248 (2024)

[6] PRR 2, 033013 (2020)

[7] PRL 127, 115702 (2021)