Title: General relativity solutions in modified gravity
Speaker: Hayato Motohashi (Yukawa Institute for Theoretical Physics, Kyoto University)
Abstract: Recent gravitational wave observations of binary black hole mergers and a binary neutron star merger by LIGO and Virgo Collaborations associated with its optical counterpart constrain deviation from General Relativity (GR) both on strong-field regime and cosmological scales with high accuracy, and further strong constraints are expected by near-future observations. Thus, it is important to identify theories of modified gravity that intrinsically possess the same solutions as in GR among a huge number of theories. In this talk I will shed light on the three conditions for theories of modified gravity to allow GR solutions. Our analysis is quite general, as it applies a wide class of single-/multi-field scalar-tensor theories of modified gravity in the presence of matter component, and any spacetime geometry including cosmological background as well as spacetime around black hole and neutron star, for the latter of which these conditions provide a necessary condition for no-hair theorem. The three conditions will be useful for further constraints on modified gravity theories as they classify general theories of modified gravity into three classes, each of which possesses unique GR solutions (no-hair theorem), only hairy solutions, and both GR and hairy solutions, for the last of which one of the two solutions may be selected dynamically.