= Conformal boundary
{title2=$\widetilde g=\Omega^2g,\quad\Omega=0,\quad d\Omega\ne0$}
A boundary obtained by extending the rescaled metric $\widetilde g=\Omega^2g$ smoothly and nondegenerately to a hypersurface on which $\Omega$ vanishes with nonzero derivative. Its causal character is measured using the extended <Lorentzian metric>, not the divergent physical metric. In signature $(+---)$, a timelike normal makes the boundary spacelike, a spacelike normal makes it timelike, and a null normal makes it null. These cases lead to different causal and initial-value properties. The <conformal cylinder of two-dimensional de Sitter spacetime> has spacelike past/future boundaries; the <conformal strip of two-dimensional anti-de Sitter spacetime> has two timelike boundaries.
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