Junction conditions in scalar-tensor theories

Understand

We analyze junction conditions at a null or non-null hypersurface $\Sigma$ in a large class of scalar-tensor theories in arbitrary $n(\ge 3)$ dimensions.

• After showing that the metric and a scalar field must be continuous at $\Sigma$ as the first junction conditions, we derive the second junctions conditions from the Einstein equations and the equation of motion for the scalar field.
• Subsequently, we study $C^1$ regular matching conditions as well as vacuum conditions at $\Sigma$ both in the Jordan and Einstein frames.
• Our result suggests that the following configurations may be possible; (i) a vacuum thin-shell at null $\Sigma$ in the Einstein frame, (ii) a vacuum thin-shell at null and non-null $\Sigma$ in the Jordan frame, and (iii) a non-vacuum $C^1$ regular matching at null $\Sigma$ in the Jordan frame.