Existence and disappearance of conical singularities in Gleyzes-Langlois-Piazza-Vernizzi theories

In a class of Gleyzes-Langlois-Piazza-Vernizzi theories, we derive both vacuum and interior Schwarzschild solutions under the condition that the derivatives of a scalar field φ with respect to the radius r vanish. If the parameter αH characterizing the deviation from Horndeski theories approaches a nonzero constant at the center of a spherically symmetric body, we find that the conical singularity arises at r=0 with the Ricci scalar given by R=-2αH/r2. This originates from violation of the geometrical structure of four-dimensional curvature quantities. The conical singularity can disappear for the models in which the parameter αH vanishes in the limit that r→0. We propose explicit models without the conical singularity by properly designing the classical Lagrangian in such a way that the main contribution to αH comes from the field derivative φ′(r) around r=0. We show that the extension of covariant Galileons with a diatonic coupling allows for the recovery of general relativistic behavior inside a so-called Vainshtein radius. In this case, both the propagation of a fifth force and the deviation from Horndeski theories are suppressed outside a compact body in such a way that the model is compatible with local gravity experiments inside the solar system.