Quantum vacuum radiation in optical glass

A recent experimental claim of the detection of analogue Hawking radiation in an optical system Phys. Rev. Lett. 105, 203901 (2010) has led to some controversy Phys. Rev. Lett. 107, 149401 (2011). While this experiment strongly suggests some form of particle creation from the quantum vacuum (and hence it is per se very interesting), it is also true that it seems difficult to completely explain all features of the observations by adopting the perspective of a Hawking-like mechanism for the radiation. For instance, the observed photons are emitted parallel to the optical horizon, and the relevant optical horizon is itself defined in an unusual manner by combining group and phase velocities. This raises the question: Is this really Hawking radiation, or some other form of quantum vacuum radiation? Naive estimates of the amount of quantum vacuum radiation generated due to the rapidly changing refractive index---sometimes called the dynamical Casimir effect---are not encouraging. However we feel that naive estimates could be misleading depending on the quantitative magnitude of two specific physical effects: ``pulse steepening'' and ``pulse cresting''. Plausible bounds on the maximum size of these two effects results in estimates much closer to the experimental observations, and we argue that the dynamical Casimir effect is now worth additional investigation.