Accurate Determination of Hubble Attenuation and Amplification in Expanding and Contracting Cold-Atom Universes

In the expanding universe, relativistic scalar fields are thought to be attenuated by "Hubble friction", which results from the dilation of the underlying spacetime metric. By contrast, in a contracting universe this pseudo-friction would lead to amplification. Here, we experimentally measure with five-fold better accuracy, both Hubble attenuation and amplification in expanding and contracting toroidally-shaped Bose-Einstein condensates, in which phonons are analogous to cosmological scalar fields. We find that the observed attenuation or amplification depends on the temporal phase of the phonon field, which is only possible for non-adiabatic dynamics. The measured strength of the Hubble friction disagrees with recent theory [J. M. Gomez Llorente and J. Plata, {\it Phys. Rev. A} {\bf 100} 043613 (2019) and S. Eckel and T. Jacobson, {\it SciPost Phys.} {\bf 10} 64 (2021)]; because our experiment probes physics outside the scope of this theory -- with large excitations in rings of intermediate thickness -- this indicates the presence of new physics.

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