5.5 Bose novae: an example of the reverse flow of information?

As we have seen in the previous Sections ?? and 5.4, analogue models have in the past been very useful in providing new, condensed-matter-physics–inspired ideas about how to solve longstanding problems of semiclassical gravity. In closing this section, it is interesting to briefly discuss what perhaps represents, so far, the only attempt to use analogue models in the reverse direction; that is to import well-known concepts of semiclassical gravity into condensed matter frameworks.

The phenomenon we are referring to is the “Bose nova” [175]. This is an experiment dealing with a gas of a few million 85Rb atoms at a temperature of about 3 nK. The condensate is rendered unstable by exploiting the possibility of tuning the interaction (more precisely the scattering length) between the atoms via a magnetic field. Reversing the sign of the interaction, making it attractive, destabilises the condensate. After a brief waiting time (generally called t collapse), the condensate implodes and loses a sizable fraction of its atoms in the form of a “nova burst”. If left to evolve undisturbed, the number of atoms in the burst stabilises and a remnant condensate is left. However, if the condensate interaction is again made repulsive after some time tevolve, before the condensate has sufficient time to stabilise, then the formation of “jets” of atoms is observed, these jets being characterised by lower kinetic energy and a distinct shape with respect to the burst emission.

Interestingly, an elegant explanation of such a phenomenology was proposed in [105Jump To The Next Citation Point, 106Jump To The Next Citation Point], based on the well-known semiclassical gravity analysis of particle creation in an expanding universe. In fact, the dynamics of quantum excitations over the collapsing BEC were shown to closely mimic that for quantum excitations in a time-reversed (collapsing instead of expanding) scenario for cosmological particle creation. This is not so surprising as the quantum excitations above the BEC ground state feel a time-varying background during the collapse, and, as a consequence, one then expects squeezing of the vacuum state and mode mixing, which are characteristic of quantum field theory in variable external fields.

However, the analogy is even deeper than this. In fact, in [105Jump To The Next Citation Point, 106Jump To The Next Citation Point] a key role in explaining the observed burst and jets is played by the concepts of “frozen” versus “oscillating” modes – borrowed from cosmology – (although with a reverse dynamics with respect to the standard (expanding) cosmological case). In the case of Bose novae, the modes, which are amplified, are those for which the physical frequency is smaller than the collapse rate, while modes with higher frequencies remain basically unaffected and their amplitudes obey a harmonic oscillator equation. As the collapse rate decreases, more and more modes stop growing and start oscillating, which is equivalent to a creation of particles from the quantum vacuum. In the case of a sudden stop of the collapse by a new reversal of the sign of the interaction, all of the previously growing modes are suddenly converted into particles, explaining in this way the generation of jets and their lower energy (they correspond to modes with lower frequencies with respect to those generating the bursts).

Although this simple model cannot explain all the details of the Bose novae phenomenology, we think it is remarkable how far it can go in explaining several observed features by exploiting the language and techniques so familiar to quantum cosmology. In this sense, the analysis presented in [105, 106] primarily shows a possible new application of analogue models, where they could be used to lend ideas and techniques developed in the context of gravitational physics to the explanation of condensed matter phenomena.

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