Junction conditions in scalar-tensor theories

We analyze junction conditions at a null or non-null hypersurface $Σ$ 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 $Σ$ 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 $Σ$ both in the Jordan and Einstein frames. Our result suggests that the following configurations may be possible; (i) a vacuum thin-shell at null $Σ$ in the Einstein frame, (ii) a vacuum thin-shell at null and non-null $Σ$ in the Jordan frame, and (iii) a non-vacuum $C^1$ regular matching at null $Σ$ in the Jordan frame. Lastly, we clarify the relations between the conditions for $C^1$ regularity and also for vacuum $Σ$ in the Jordan and Einstein frames.

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