Nature of Phase Transitions and Metastability in Scalar-Tensor Theories.

Compact stars above a critical stellar mass develop large scalar fields in some scalar-tensor theories. This scenario called spontaneous scalarization has been an intense topic of study since it passes weak-field gravity tests naturally while providing clear observables in the strong-field regime. The underlying mechanism for the onset of scalarization is often depicted as a second-order phase transition. Here, we show that a first-order phase transition is in fact the most common mechanism. This means metastability and transitions between locally stable compact object configurations are much more likely than previously believed, opening vast new avenues for observational prospects.