the condensate. When averaging over GP simulations with a 10-20% variation in the number of atoms, and no quantum fluctuations, a correlation function similar to the measured one is produced. The addition of quantum fluctuations improves the agreement, but their effect is sub-dominant. That is, we find that the observed correlation arises by modulation of the standing wave, and is unrelated to the intrinsic quantum correlations that would be present without the standing wave. In particular, we find that the quantum fluctuations do not trigger any instability.

Returning now to the results of the simulations reported here, let us summarize our findings. As the potential step is swept through the condensate, a growing hydrodynamic standing wave (Fig. \[fig:GP1\]) arises between the horizons, which appears similar to the one observed in the experiment. This wave is Bogoliubov-Čerenkov radiation (BCR) [@Carusotto], generated at the WH [@Coutant:2012zh; @Busch:2014hla; @Mayoral2011] \[see Figs. \[fig:exp\](d) and (e)\]. It is also known as a zero-frequency undulation, and is reminiscent of a ship’s bow wave.

Three independent lines of evidence all indicate the BCR nature of this standing wave. First, the growth rate of the standing wave (Fig. \[fig:GP\_growth\]) matches very closely the square of the background density, 