Chi-Yong Lin, Da-Shin Lee, Ray J. Rivers
In a series of papers to demonstrate emergent 'gravity' some authors have explored phonon production in BECs with a tunable speed of sound, in particular to emulate FRW universes. The premiss is that, within such systems, the gapless mode (the phonon) looks Lorentzian for low momentum at least. However, when it comes to phonon production in cold Fermi gases whose speed of sound is controlled by a Feshbach resonance, it is impossible to shake off the the underlying Galilean invariance because of the interplay between gapless and gapped modes. Such phonons as are produced do not follow the pattern anticipated for FRW metrics, at variance with the aims of the programme.
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http://arxiv.org/abs/1205.0133
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