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Gauss-Codazzi and Maxwell initial constraints

Statement

Let (g,F) be a smooth source-free electrovacuum solution of Electrovacuum tensor Cauchy system on a neighborhood of a smooth spacelike embedded hypersurface i:Σ↪M. Let n be its future unit normal, h=i∗g, K(X,Y)=g(∇Xn,Y), and let (e,b) be its observer electric/magnetic data. Then the induced tuple (Σ,h,K,e,b) satisfies all four constraints in Constrained electrovacuum initial data, namely the two Maxwell divergence constraints and

R(h)+(tr⁡hK)2−∣K∣h2=2Λ+κμ0(∣e∣h2+∣b∣h2),

DjKji−Ditr⁡hK=−κμ0(e×hb)i.

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 (g,F), and a smooth spacelike hypersurface with the induced data above.

[A1]

Electrovacuum tensor Cauchy system: G+Λg=κTEM, dF=0, and d(⋆gF)=0.

[F1]

Constrained electrovacuum initial data: K=+12Lnh and the four displayed constraints are the definition of constrained data.

[F2]

Maxwell stress algebra and covariant conservation: TEM(n,n)=(∣e∣h2+∣b∣h2)/(2μ0) and TEM(n,V)=−h(e×hb,V)/μ0.

[F3]

Gauss-Codazzi decomposition in Gaussian normal coordinates and the constraint equations in terms of the induced metric and second fundamental form: for this K convention, G(n,n)=12(R(h)+(tr⁡hK)2−∣K∣h2) and G(n,V)=DjKjiVi−DVtr⁡hK.

[F4]

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 F=−θ0∧E+⋆hB and ⋆gF=θ0∧B+⋆hE, where E=e♭ and B=b♭ are the spatial covectors of Constrained electrovacuum initial data. Pulling back to the slice removes the terms containing θ0.

[F5]

On the oriented Riemannian three-manifold (Σ,h), the Hodge star ⋆h:ΛkT∗Σ→Λ3−kT∗Σ is the unique bundle map satisfying α∧⋆hβ=⟨α,β⟩hvol⁡h. In a positively oriented orthonormal coframe (e1,e2,e3), ⋆he1=e2∧e3, ⋆he2=e3∧e1, and ⋆he3=e1∧e2; by linearity this is ⋆hv♭=ιvvol⁡h. In a positively oriented coordinate chart, vol⁡h=det⁡h dx1∧dx2∧dx3.

[F6]

For the smooth embedding i:Σ↪M, pullback commutes with exterior differentiation: dΣi∗ω=i∗dMω for every smooth differential form ω. In slice coordinates where i(x1,x2,x3)=(0,x1,x2,x3), pullback deletes every term containing dx0; differentiating the remaining coefficients in x1,x2,x3 gives the same terms as pulling back dMω, which proves the identity in these coordinates.

Proof

1.1A1F4F5F6algebra

Fix a point of Σ and choose an oriented slice chart, which is possible by Embedded submanifolds and slice charts. A local smooth extension of n along Σ 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 (n,e1,e2,e3), [F4] gives i∗F=⋆hb♭ and i∗(⋆gF)=⋆he♭. By the coordinate pullback identity [F6], dΣi∗F=i∗dMF and dΣi∗(⋆gF)=i∗dM(⋆gF), so [A1] yields dΣ(⋆hb♭)=0 and dΣ(⋆he♭)=0. To convert these into divergences, [F5] gives ⋆hv♭=ιvvol⁡h. In oriented coordinates, dΣ(ιvvol⁡h)=∂i(det⁡h vi) dx1∧dx2∧dx3. The Levi-Civita formula gives Γiik=12hij∂khij; differentiating the determinant yields ∂klog⁡det⁡h=12hij∂khij. Hence this exterior derivative equals (Divi)vol⁡h. Therefore Dibi=0 and Diei=0 at the chosen point, and hence throughout Σ.

1.2A1F2F3algebra

Apply [F3] to the existing solution in [A1]. The normal-normal projection of G+Λg=κTEM gives 12(R(h)+τ2−∣K∣2)−Λ=κTEM(n,n) because g(n,n)=−1. Substitute [F2] and multiply by 2 to obtain the Hamiltonian constraint.

1.3A1F2F3algebra

For any tangent V, the normal-tangential projection gives G(n,V)=κTEM(n,V) because g(n,V)=0. Substitute [F2]–[F3] to get (DjKji−Diτ)Vi=−(κ/μ0)h(e×hb,V). Since this holds for every tangent V, the two covectors are equal, proving the momentum constraint.

2.1F1step 1.1step 1.2step 1.3∎

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 Σ is nonempty, the induced tuple is constrained electrovacuum initial data in that definition; if Σ is empty, the four equations hold vacuously.

Depends on

Dependency tree · 2 levels

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