The Cosmological Constant and its Problems: A Review of Gravitational
  Aether
  In this essay we offer a comprehensible overview of the gravitational aether
scenario. This is a possible extension of Einstein's theory of relativity to
the quantum regime via an effective approach. Quantization of gravity usually
faces several issues including an unexpected high vacuum energy density caused
by quantum fluctuations. The model presented in this paper offers a solution to
the so-called cosmological constant problems.
  As its name suggests, the gravitational aether introduces preferred reference
frames, while it remains compatible with the general theory of relativity. As a
rare feature among quantum gravity inspired theories, it can predict measurable
astronomical and cosmological effects. Observational data disfavor the
gravitational aether scenario at $2.6\text{-}5\,\sigma$. This experimental
feedback gives rise to possible refinements of the theory.
