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KN Killing generators, surface gravity, area and potential
Statement
Let and in the selected ingoing KN model. For either set , , and where is the smooth rotation field on . The surface is a null hypersurface and is its future null normal and Killing generator. Under the specified normalization in Causal solutions, Killing horizons, escape and metric extensions, At the double root . These are local geometric calculations on the specified model; they assert no maximality or uniqueness.
Facts & Assumptions
Given: The exact ingoing KN metric and potential with and .
Regular ingoing KN metric, field and sphere axes: the product metric and potential are smooth on , with , and the future timelike field is . On a polar chart, , , and ; .
Killing fields and conserved currents, and on-shell conservation of the Hilbert stress-energy tensor: a smooth field with zero metric Lie derivative is Killing; along an affinely parameterized geodesic, its metric pairing with the tangent is constant.
Classification of future and past causal vectors by a timelike field: a nonzero null vector is future-directed when its pairing with the selected future timelike field is negative.
Explicit rational and real power regimes: each nonnegative real has a unique nonnegative square root, with .
Causal solutions, Killing horizons, escape and metric extensions, Definition (b)-(c): the null conormal criterion, nonempty dense regular Killing-generator portion, ambient signed normalization , and intrinsic positive induced area density apply to the specified horizon and section.
is the global smooth generator of rotations of the round sphere; on polar charts it equals . The section is the sphere at fixed . Parameters and radial coordinates have length units; angles are dimensionless, and have inverse-length units.
Proof
The quadratic factors as , with . Its nonnegative root satisfies : if , then , contrary to . Hence both roots are positive. The metric coefficients are independent of and invariant under rotations, so [F2] makes , , and their constant linear combination Killing.
In a polar chart, substitute into the metric at . The result is The multiplier is positive. Since , this identity gives and shows is tangent and normal to the level . It also gives , so the level is null. Moreover , whence ; [F3] makes the generator future. The generator has component one everywhere, so its nonzero-generator part is the whole horizon, nonempty and dense. This verifies the Killing-horizon conditions in [L1]. The formula is smooth at the axes because it is an identity of global tensors agreeing on the dense polar charts.
Direct contraction on a polar chart gives At the second squared factor vanishes, as does its first radial derivative. Also . Therefore Comparing with [L1] and step 2.1 yields . Differentiating the factorization of gives , including zero at the double root.
The metric induced on the fixed- sphere is Its area density is , so integration over , gives . Contracting the potential in [F1] gives which is independent of .
Remarks
The current vocabulary supplies the exact Killing-horizon, signed surface-gravity and area definitions. The coordinate calculations above verify those interfaces for the displayed smooth metric, including both roots and the double-root limit.
Depends on
Regular ingoing KN metric, field and sphere axes
Causal solutions, Killing horizons, escape and metric extensions
Explicit rational and real power regimes
Killing fields and conserved currents, and on-shell conservation of the Hilbert stress-energy tensor
Classification of future and past causal vectors by a timelike field