Universal horizons and black hole spectroscopy in gravitational theories with broken Lorentz symmetry

Understand

The violation of Lorentz invariance (LI) in gravitational theories, which allows superluminal propagations, dramatically alters the causal structure of the spacetime and modifies the notion of black holes (BHs).

• Instead of metric horizons, now universal horizons (UHs) define the boundaries of BHs, within which a particle cannot escape to spatial infinities even with an infinitely large speed.
• Then, a natural question is how the quasi-normal modes (QNMs) of a BH are modified, if one considers the UH as its causal boundary.
• In this paper, we study in detail this problem in Einstein-aether theory, a vector-tensor theory that violates LI but yet is self-consistent and satisfies all observations to date.