  In inflationary cosmology, the rapid expansion of the early universe resulted
in the spontaneous production of cosmological particles from vacuum
fluctuations, observable today in the cosmic microwave background anisotropies.
The analogue of cosmological particle creation in a quantum fluid could provide
insight, but an observation has not yet been achieved. Here we report the
spontaneous creation of analogue cosmological particles in the laboratory,
using a quenched 3-dimensional quantum fluid of light. We observe acoustic
peaks in the density power spectrum, in close quantitative agreement with the
quantum-field theoretical prediction. We find that the long-wavelength
particles provide a window to early times, and we apply this principle to the
cosmic microwave background. This work introduces a new quantum fluid, as cold
as an atomic Bose-Einstein condensate.
