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.