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Boost normalization and affine focusing
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 be a smooth time-oriented Lorentzian four-manifold, and let be a smooth embedded two-manifold without boundary with positive induced metric. Let be nonempty and open and carry a smooth normalized future null normal pair , so and . Let be a smooth null geodesic hypersurface congruence from with initial tangent . Write for the initial optical expansion along the generator from . For , define on the parameter domain and set ; the initial companion is . Then , so the sign of the initial expansion is independent of this positive normalization. Along the generator from , the new affine parameter is ; optical expansion, shear and Ricci contraction scale so that the null Raychaudhuri equation is unchanged. Define , , and , where and use future timelike and future causal reachability. A focal event and the exclusion from of points sufficiently close after the event are independent of the chosen constant-along-generator boost. On any generator with and throughout the comparison interval, the bound rescales to .
Statement references
Facts & Assumptions
Given: Countable Choice and Dependent Choice; the time-oriented Lorentzian manifold, spacelike surface , nonempty open section , smooth normalized future null normal pair, its smooth null geodesic hypersurface congruence, and the smooth boost factor extended constantly along each generator as in the Statement.
Under a normal-frame boost, , and scale by ; the optical map and expansion are defined on the positive screen (Null normal pairs and screen geometry).
The affine screen equation is and the bound is when and , provided the congruence and generator exist through that comparison parameter (Null Raychaudhuri and focal-point bound).
The focal event is geometric and every point of the normal generator sufficiently just beyond it lies in , hence outside (Null focal points exclude later horismos generators).
is constant as a function of affine parameter on each generator; without this restriction need not be affine.
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].
Since , the boost preserves the future null rays, and . The boost law in [F1] gives , so has the same sign as . The new tangent is , while ; solving with both parameters zero at the initial section gives because is constant along the generator.
On the open parameter domain where , put . For , differentiation at fixed gives The added term vanishes in the screen quotient, so at corresponding points . The screen metric is unchanged because the null line is unchanged. Along the fixed generator, is constant, and differentiation with the screen connection gives Taking trace and trace-free part yields and . Therefore and . Tensoriality gives . Multiplying the Raychaudhuri equation in [F2] by proves its form is unchanged. For a generator from with and through the comparison interval, the bound rescales as
By step 2.1, , so its rank-loss event is the same point of and its parameter value is multiplied by . Under [A2], [F3] applies: the geometric statement that points just beyond that event lie in 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
- E. Gourgoulhon and J. L. Jaramillo, A 3+1 perspective on null hypersurfaces and isolated horizons, Physics Reports 423 (2006), arXiv:gr-qc/0503113v2: §§2.6-2.7 and 5.1, 5.5-5.8
- Edward Witten, Light Rays, Singularities, and All That, Clay Mathematics Institute lecture notes, full 106-page PDF: §§5.4-5.5 and 8.5