A New Strong-Field Effect in Scalar Tensor Gravity: Spontaneous Violation of the Energy Conditions

TLDR: In this article, it was shown that the spontaneous scalarization phenomenon is linked to another strong-field effect: a spontaneous violation of the weak energy condition, which causes the scalar field inside a neutron star to rapidly become inhomogeneous once the star's mass increases above some critical value.

Abstract: A decade ago, it was shown that a wide class of scalar-tensor theories can pass very restrictive weak-field tests of gravity and yet exhibit nonperturbative strong-field deviations away from general relativity. This phenomenon, called "spontaneous scalarization," causes the (Einstein frame) scalar field inside a neutron star to rapidly become inhomogeneous once the star's mass increases above some critical value. For a star whose mass is below the threshold, the field is instead nearly uniform (a state that minimizes the star's energy) and the configuration is similar to the general relativity one. Here we show that the spontaneous scalarization phenomenon is linked to another strong-field effect: a spontaneous violation of the weak energy condition.