Further Results on the Smoothability of Cauchy Hypersurfaces and Cauchy Time Functions |
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Authors: | Antonio N. Bernal Miguel Sánchez |
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Affiliation: | (1) Departamento de Geometría y Topología, Universidad de Granada. Facultad de Ciencias, Avda. Fuentenueva s/n, 18071 Granada, Spain |
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Abstract: | Recently, folk questions on the smoothability of Cauchy hypersurfaces and time functions of a globally hyperbolic spacetime M, have been solved. Here we give further results, applicable to several problems: (1) | Any compact spacelike acausal submanifold H with boundary can be extended to a spacelike Cauchy hypersurface S. If H were only achronal, counterexamples to the smooth extension exist, but a continuous extension (in fact, valid for any compact achronal subset K) is still possible. | (2) | Given any spacelike Cauchy hypersurface S, a Cauchy temporal function (i.e., a smooth function with past-directed timelike gradient everywhere, and Cauchy hypersurfaces as levels) with is constructed – thus, the spacetime splits orthogonally as in a canonical way. | Even more, accurate versions of this last result are obtained if the Cauchy hypersurface S were non-spacelike (including non-smooth, or achronal but non-acausal). |
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Keywords: | Causality global hyperbolicity Cauchy hypersurface smoothability time and temporal functions Geroch’ s theorem submanifolds quantum fields on curved spacetimes |
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