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Boost normalization and affine focusing

Physical scope

Pure mathematical normalization example; no field equation or empirical claim is used.

Statement

Assume Countable Choice and Dependent Choice for the Lorentzian and causal suppliers. Let (M,g,N) be a smooth time-oriented Lorentzian four-manifold, and let S⊂M be a smooth embedded two-manifold without boundary with positive induced metric. Let U⊆S be nonempty and open and carry a smooth normalized future null normal pair (ℓ0,k0), so g(ℓ0,ℓ0)=g(k0,k0)=0 and g(ℓ0,k0)=−2. Let F:U×J→M be a smooth null geodesic hypersurface congruence from U with initial tangent ∂λF(p,0)=ℓ0(p). Write θ0(p):=θ(p,0) for the initial optical expansion along the generator from p. For f∈C∞(U,R), define f^(p,λ):=f(p) on the parameter domain and set ℓ′:=ef^∂λF; the initial companion is k0′=e−fk0. Then θ^0=efθ0, so the sign of the initial expansion is independent of this positive normalization. Along the generator from p, the new affine parameter is λ′=e−f(p)λ; optical expansion, shear and Ricci contraction scale so that the null Raychaudhuri equation is unchanged. Define I+(S):=⋃s∈SI+(s), J+(S):=⋃s∈SJ+(s), and E+(S):=J+(S)∖I+(S), where I+(s) and J+(s) use future timelike and future causal reachability. A focal event and the exclusion from E+(S) of points sufficiently close after the event are independent of the chosen constant-along-generator boost. On any generator with θ0(p)<0 and Ric⁡(ℓ,ℓ)≥0 throughout the comparison interval, the bound λ∗(p):=2/∣θ0(p)∣ rescales to λ∗′=e−f(p)λ∗(p).

Facts & Assumptions

Given: Countable Choice and Dependent Choice; the time-oriented Lorentzian manifold, spacelike surface S, nonempty open section U⊆S, smooth normalized future null normal pair, its smooth null geodesic hypersurface congruence, and the smooth boost factor f extended constantly along each generator as in the Statement.

[F1]

Under a normal-frame boost, χℓ, θℓ and σℓ scale by ef; the optical map and expansion are defined on the positive screen (Null normal pairs and screen geometry).

[F2]

The affine screen equation is θ′=−θ2/2−∣σ∣2−Ric⁡(ℓ,ℓ) and the bound is 2/∣θ(0)∣ when θ(0)<0 and Ric⁡(ℓ,ℓ)≥0, provided the congruence and generator exist through that comparison parameter (Null Raychaudhuri and focal-point bound).

[F3]

The focal event is geometric and every point of the normal generator sufficiently just beyond it lies in I+(S), hence outside E+(S) (Null focal points exclude later horismos generators).

[A1]

f is constant as a function of affine parameter on each generator; without this restriction efℓ need not be affine.

[A2]

Countable Choice and Dependent Choice are assumed, retaining the hypotheses of the focal-boundary supplier used in [F3] (Null focal points exclude later horismos generators).

Proof

Given: [F1]–[F3] and [A1]–[A2].

1.1A1F1algebra

Since ef>0, the boost preserves the future null rays, and g(ℓ′,k′)=efe−fg(ℓ,k)=−2. The boost law in [F1] gives θ^0=efθ0, so θ^0 has the same sign as θ0. The new tangent is ℓ′=d/dλ′, while ℓ=d/dλ; solving dλ/dλ′=ef with both parameters zero at the initial section gives λ′=e−fλ because f is constant along the generator.

2.1F1F2step 1.1algebra

On the open parameter domain where ef(p)λ′∈J, put F^(p,λ′):=F(p,ef(p)λ′). For X∈TpS, differentiation at fixed λ′ gives dpF^(X)=dpF(X)+ef(p)λ′(Xf)ℓ. The added term vanishes in the screen quotient, so at corresponding points A^λ′=Aef(p)λ′. The screen metric is unchanged because the null line is unchanged. Along the fixed generator, f(p) is constant, and differentiation with the screen connection gives A^λ′′=ef(p)Aef(p)λ′′,B^=A^′A^−1=ef(p)B. Taking trace and trace-free part yields θ^=efθ and σ^=efσ. Therefore dθ^/dλ′=e2fdθ/dλ and ∣σ^∣2=e2f∣σ∣2. Tensoriality gives Ric⁡(ℓ′,ℓ′)=e2fRic⁡(ℓ,ℓ). Multiplying the Raychaudhuri equation in [F2] by e2f proves its form is unchanged. For a generator from p with θ0(p)<0 and Ric⁡(ℓ,ℓ)≥0 through the comparison interval, the bound rescales as λ∗′(p)=2∣θ^0(p)∣=e−f(p)2∣θ0(p)∣=e−f(p)λ∗(p).

3.1A2F1F3step 1.1step 2.1algebra∎

By step 2.1, A^λ′=Aef(p)λ′, so its rank-loss event is the same point of M and its parameter value is multiplied by e−f(p). Under [A2], [F3] applies: the geometric statement that points just beyond that event lie in I+(S) is unchanged by relabeling the affine parameter. Thus only the numerical affine label changes; the causal-boundary conclusion does not.

Remarks

This is a normalization calculation for a constant-along-generator boost. No field equation, energy condition, or empirical input is used.

Depends on

Dependency tree · 2 levels

Sources