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Weighted Einstein Hilbert Variation in Ten Dimensions

Statement

Let U⊆R10 be an open oriented coordinate domain in a smooth ten-manifold. For s in an open interval about 0, let g(s) be a family of smooth nondegenerate Lorentzian metrics of inertia (9,1,0), jointly smooth in (s,x), with g(0)=g and g(s)=g outside one fixed compact set K⊂U. Put qMN:=∂sg(s)MN∣s=0. For each s use the connection with coefficients ΓPMN(s)=12g(s)PQ(∂Mg(s)NQ+∂Ng(s)MQ−∂Qg(s)MN): for vector fields, (∇XY)P=XM(∂MYP+ΓPMNYN). On scalars set ∇Xf=X(f) and extend to covariant tensors by duality and the product rule. This is the torsion-free metric-compatible (Levi-Civita) connection, as verified below. All unmarked quantities are evaluated at s=0. Fix the curvature convention RPQMN=∂MΓNQP−∂NΓMQP+ΓMRPΓNQR−ΓNRPΓMQR, RQN=RPQPN, and R=gQNRQN. Write w(s)=∣det⁡(g(s)MN)∣, w=w(0), GMN=RMN−12gMNR, and □f=gMN∇M∇Nf. Let f∈C∞(U) be independent of s, and define the bulk first variation by

δSf[g](q):=dds∣s=0∫U(w(s)fR(s)−wfR) d10x.

The density difference is supported in K, so this integral is finite even when either unvaried action integral is not. Every smooth compactly supported symmetric qMN is admissible: the inverse-metric path g(s)MN=gMN+sqMN remains Lorentzian for sufficiently small ∣s∣, since the support of q is compact and the Lorentzian matrices form an open set. The bulk first variation is

δ∫UwfR d10x=∫Uw[fGMN+gMN□f−∇M∇Nf]qMN d10x.

This is a local bulk identity for compactly supported variations; no boundary condition on the unvaried region is needed.

Facts & Assumptions

Given: The ten-dimensional chart, metric, connection, curvature convention, scalar f, and compactly supported smooth symmetric variation qMN in the Statement.

[F1]

The derivative of the determinant along an invertible C¹ matrix curve: (det⁡A)′=det⁡A  tr⁡(A−1A′): for an invertible differentiable matrix curve, δdet⁡g=det⁡(g)tr⁡(g−1δg); differentiating gMPgPN=δNM gives δgMN=−gMPgNQqPQ.

[F2]

Affine connection on a smooth manifold supplies the affine-connection linearity and Leibniz rules; Christoffel symbols of an affine connection supplies the convention ∇∂M∂N=ΓPMN∂P. Neither definition supplies metric compatibility or zero torsion. The coefficients used here are defined in the Statement, and their required properties are proved in step 1.1.

[L1]

If VM is a smooth compactly supported vector field on U, then ∫U∂M(wVM) d10x=0: extend wVM by zero to a closed coordinate box containing its support. The extension and its derivatives are smooth, hence integrable by Every continuous function on a closed nondegenerate rectangle in Rm is Riemann integrable. Apply Riemann--Fubini on product rectangles, with lower and upper section integrals and content-zero exceptional sections to integrate each coordinate derivative along its coordinate lines; The second fundamental theorem: if G is differentiable on [a,b] with G′=f and f is integrable, then ∫abf=G(b)−G(a) gives zero because the extension vanishes near both endpoints. From [F1] and the Christoffel formula in the Statement, ΓMMP=w−1∂Pw, so w∇MVM=∂M(wVM).

Proof

1.1givenF1F2algebra

The inverse metric is jointly smooth: each entry is a cofactor divided by the nonzero determinant. Thus the displayed coefficients are smooth and the formula for ∇XY directly satisfies the affine-connection rules in [F2]. Their symmetry in M,N gives zero torsion, and substitution gives ∂MgNQ=ΓPMNgPQ+ΓPMQgNP, hence ∇g=0, for every s. Conversely, cyclically permuting this identity and using symmetry recovers the displayed coefficients, proving uniqueness on U. Put hMN=δgMN=−gMPgNQqPQ by [F1]; then δw=−12wgMNqMN. Write CPMN=δΓPMN. Differentiating metric compatibility gives ∇MhNQ=CPMNgPQ+CPMQgNP; combining its three cyclic versions and using CPMN=CPNM yields CPMN=12gPQ(∇MhNQ+∇NhMQ−∇QhMN).

2.1F2step 1.1algebra

Differentiating the stated coordinate formula for RPQMN and grouping the connection terms into covariant derivatives gives δRQN=∇PδΓNQP−∇NδΓPQP. Contracting with gQN and inserting step 1.1 yields δR=RMNqMN−∇M∇NqMN+□(gMNqMN). The first identity follows by cancellation of the quadratic-connection terms in the curvature derivative; the second uses ∇g=0.

3.1givenL1step 1.1step 2.1algebra∎

Joint smoothness and the common compact support permit differentiation under the integral: on a closed coordinate box containing K, the difference quotients of the zero-extended density difference converge uniformly to its s-derivative by the one-variable fundamental theorem of calculus and uniform continuity of that derivative. Varying wfR, the density and algebraic curvature terms are wf(RMN−12gMNR)qMN. The two remaining terms from step 2.1 integrate by parts twice, with no boundary contribution because q has compact support: ∫wf[−∇M∇NqMN+□(gMNqMN)]=∫w[gMN□f−∇M∇Nf]qMN. By [L1], compactly supported coordinate divergences integrate to zero; the Christoffel trace identity in [L1] converts the covariant divergence to a coordinate divergence. The scalar Hessian is symmetric because the connection is torsion-free, so the twice-integrated coefficient has the displayed index order. Adding these coefficients proves the formula.

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