Gravitational collapse and formation of universal horizons
The invariance under the Lorentz symmetry group is a cornerstone of modern physics, and is strongly supported by observations. In fact, all the experiments carried out so far are consistent with it [1], and no evidence to show that such a symmetry needs to be broken at certain energy scales, although it is arguable that such constraints in the gravitational sector are much weaker than those in the matter sector [2]. Nevertheless, there are various reasons to construct gravitational theories with broken Lorentz invariance (LI) [3]. In particular, our understanding of space-times at Plank scale is still highly limited, and the renomalizability and unitarity of gravity often lead to the violation of LI [4]. One concrete example is the Ho