Testing Gravitational Theories with Broken Lorentz Symmetry by Gravitational Wave and Black Hole Observations

Einstein-aether theory is a particular type of the general vector-tensor theory, and the vector field is always time-like, so it defines a preferred direction and locally breaks the Lorentz symmetry. Yet, it is self-consistent (free of ghosts, instabilities, and so on), and consistent with all observations carried out so far. Its Cauchy problem is also well posed. In addition, due to the presence of the aether, three different species of gravitons, the spin-0, spin-1, and spin-2, exist, and gravitational waves (GWs) have six non-vanishing polarization modes. These are dramatically different from those of Einstein’s General Relativity and provide excellent opportunity to test the theory in the strong-field regime.

In this talk, I shall present our recent studies on testing Einstein-aether theory by GW and black hole observations. In particular, I shall show that the GW observations already reduce the four free coupling constants of the theory to two, and future observations of GWs and black holes shall further constrain the theory.