  Quantum fields in curved spacetime exhibit a wealth of effects like Hawking
radiation from black holes. While quantum field theory in black holes can only
be studied theoretically, it can be tested in controlled laboratory
experiments. In experiments, a fluid going from sub- to supersonic speed
creates an effectively curved spacetime for the acoustic field, with a horizon
where the speed of the fluid equals the speed of sound. The challenge to test
predictions like the Hawking effect in such systems lies in the control of the
spacetime curvature and access to the field spectrum thereon. Here, we create
tailored stationary effective curved spacetimes in a polaritonic quantum fluid
of light in which either massless or massive excitations can be created, with
smooth and steep horizons and various supersonic fluid speeds. Using a recently
developed spectroscopy method we measure the spectrum of collective excitations
on these spacetimes, crucially observing negative energy modes in the
supersonic regions, which signals the formation of a horizon. Control over the
horizon curvature and access to the spectrum on either side demonstrates the
potential of quantum fluids of light for the study of field theories on curved
spacetimes, and we discuss the possibility of investigating emission and
spectral instabilities with a horizon or in an effective Exotic Compact Object
configuration.
