A Boundary Term for the Gravitational Action with Null Boundaries

Jan 5, 2015
28 pages
Published in:
  • Gen.Rel.Grav. 48 (2016) 7, 94
  • Published: Jun 16, 2016
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Abstract: (Springer)
Constructing a well-posed variational principle is a non-trivial issue in general relativity. For spacelike and timelike boundaries, one knows that the addition of the Gibbons–Hawking–York (GHY) counter-term will make the variational principle well-defined. This result, however, does not directly generalize to null boundaries on which the 3-metric becomes degenerate. In this work, we address the following question: What is the counter-term that may be added on a null boundary to make the variational principle well-defined? We propose the boundary integral of 2g(Θ+κ)2 \sqrt{-g} \left( \Theta +\kappa \right) as an appropriate counter-term for a null boundary. We also conduct a preliminary analysis of the variations of the metric on the null boundary and conclude that isolating the degrees of freedom that may be fixed for a well-posed variational principle requires a deeper investigation.
Note:
  • 47 pages, no figures, title changed
  • Boundary term
  • Einstein–Hilbert action
  • Null surfaces
  • Gibbons–Hawking–York term
  • Variational principle
  • General relativity