Density--density correlations and Hawking radiation in ultracold gases

We simulate recent experiments on Bose-condensed gases in which the spontaneous production of analog Hawking radiation was inferred from the observation of density--density correlations. Even when the gas sample is a pure Bose-Einstein condensate, we find that such correlations are present in realistic simulations. This is due to the random shot-to-shot variation in the number of atoms in the sample. This simple effect of ``sample inhomogeneity" is comparable to that of quantum fluctuations, and is comparable to what is observed in experiments.