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Gauss-Codazzi and Maxwell initial constraints
Statement
Let be a smooth source-free electrovacuum solution of Electrovacuum tensor Cauchy system on a neighborhood of a smooth spacelike embedded hypersurface . Let be its future unit normal, , , and let be its observer electric/magnetic data. Then the induced tuple satisfies all four constraints in Constrained electrovacuum initial data, namely the two Maxwell divergence constraints and
This is a necessary-constraint theorem for an existing smooth solution. It is not a sufficiency or local-existence theorem for arbitrary data satisfying these equations.
Facts & Assumptions
Given: A smooth oriented, time-oriented Lorentzian four-manifold, a smooth source-free solution , and a smooth spacelike hypersurface with the induced data above.
Electrovacuum tensor Cauchy system: , , and .
Constrained electrovacuum initial data: and the four displayed constraints are the definition of constrained data.
Observer electric/magnetic reconstruction and invariants and Lorentzian exterior calculus and Hodge star: after locally extending the smooth normal to an observer field, the pointwise decomposition in a positively oriented normal tetrad is and , where and are the spatial covectors of Constrained electrovacuum initial data. Pulling back to the slice removes the terms containing .
On the oriented Riemannian three-manifold , the Hodge star is the unique bundle map satisfying . In a positively oriented orthonormal coframe , , , and ; by linearity this is . In a positively oriented coordinate chart, .
For the smooth embedding , pullback commutes with exterior differentiation: for every smooth differential form . In slice coordinates where , pullback deletes every term containing ; differentiating the remaining coefficients in gives the same terms as pulling back , which proves the identity in these coordinates.
Proof
Fix a point of and choose an oriented slice chart, which is possible by Embedded submanifolds and slice charts. A local smooth extension of along is supplied by The future unit normal, induced metric and second fundamental form of a spacelike hypersurface; after shrinking it remains future timelike and normalization makes it an observer field. In a positively oriented local orthonormal tetrad , [F4] gives and . By the coordinate pullback identity [F6], and , so [A1] yields and . To convert these into divergences, [F5] gives . In oriented coordinates, . The Levi-Civita formula gives ; differentiating the determinant yields . Hence this exterior derivative equals . Therefore and at the chosen point, and hence throughout .
Apply [F3] to the existing solution in [A1]. The normal-normal projection of gives because . Substitute [F2] and multiply by to obtain the Hamiltonian constraint.
For any tangent , the normal-tangential projection gives because . Substitute [F2]–[F3] to get . Since this holds for every tangent , the two covectors are equal, proving the momentum constraint.
The Maxwell divergence identities from 1.1 and the Einstein normal projections from 1.2–1.3 are exactly the four constraint equations in [F1]. If is nonempty, the induced tuple is constrained electrovacuum initial data in that definition; if is empty, the four equations hold vacuously.
Depends on
Electrovacuum tensor Cauchy system
Constrained electrovacuum initial data
Maxwell stress algebra and covariant conservation
Gauss-Codazzi decomposition in Gaussian normal coordinates and the constraint equations in terms of the induced metric and second fundamental form
Lorentzian exterior calculus and Hodge star
Observer electric/magnetic reconstruction and invariants
The future unit normal, induced metric and second fundamental form of a spacelike hypersurface
Embedded submanifolds and slice charts
Dependency tree · 2 levels
Sources
- H. Friedrich and A. D. Rendall, The Cauchy Problem for the Einstein Equations, Lecture Notes in Physics 540 (2000), sections 2.2 and 2.4
- M. Alcubierre, J. C. Degollado and M. Salgado, The Einstein-Maxwell system in 3+1 form and initial data for multiple charged black holes, arXiv:0907.1151v2
- H. S. Reall, Part III General Relativity, Cambridge lecture notes, October 12 2022 revision, sections 10.3-10.4