  Relativistic Bose-Einstein condensates (rBECs) have recently become a
well-established system for analogue gravity. Indeed, while such relativistic
systems cannot be yet realized experimentally, they provide an interesting
framework for mimicking metrics for which no analogue is yet available, so
paving the way for further theoretical and numerical explorations. In this
vein, we here discuss black holes in rBECs and explore how their features
relate to the bulk properties of the system. We then propose the coupling of
external fields to the rBEC as a way to mimic non-metric features. In
particular, we use a Proca field to simulate an aether field, as found in
Einstein-Aether or Horava-Lifshitz gravity. This allows us to mimic a universal
horizon, the causal barrier relevant for superluminal modes in these modified
gravitational theories.
