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Exact spherical horizon charge
Statement
Fix and real with , and take the ingoing extremal RN metric and coordinates specified by the spherical test-scalar definition. Let be any admissible neutral spherical solution of , set , and define its spherical horizon charge by
Then is constant for all . If this constant is nonzero, the regular ingoing-chart transverse derivative is nonzero and does not tend to zero along the horizon. If is the scalar-field unit, then . This is a local fixed-background statement and asserts no global decay or nonlinear instability.
Statement references
Facts & Assumptions
Given: , , , and an admissible real spherical neutral solution of on the selected ingoing RN model.
Spherical scalar waves in regular extremal RN coordinates: the background is with and fixed Maxwell field . The vector field generates the null level with zero signed surface gravity. The reduced profile lifts to , , and the wave operator on this lift is the trace of its Hessian, equivalently ; it is . All radial derivatives are ingoing-chart derivatives.
The derivative of the metric density and the chart formula for the divergence of a vector field: for a smooth vector field , , where . Its coordinate proof uses only first derivatives of and the product rule, so the same formula holds for vector fields. Since is and is smooth, is and the extension applies.
Existence and uniqueness of the metric-selected connection of a nondegenerate metric, and its extension to tensor fields: the metric-selected connection is metric compatible and torsion free; in particular and its tensor derivatives have the stated coordinate formula.
A function continuous on an interval whose derivative vanishes at every interior point of is constant on ; consequently two such functions with the same derivative differ by a constant: a continuous real function on an order-convex interval whose derivative vanishes at every interior point is constant on that interval.
The Axiom of Countable Choice (): the tangent-bundle smooth structure used by [F1] and [F3] assumes . This is the only choice assumption inherited here; no full axiom of choice is used.
Proof
In a local sphere chart with angular metric components , the ingoing metric block and its inverse are The first block has determinant , so the full metric density is . Put . The spherical lift gives , , and ; this vector field is , so [F2] applies. By [F1], [F3] and metric compatibility, . Applying the density formula and cancelling the angular density, which is independent of , gives Expanding the derivatives and using equality of the continuous mixed partials yields
Since , one has and . Differentiating the latter expression gives . Divide the equation of step 1.1 by to obtain
From it follows by direct differentiation that . Substituting into step 2.1 leaves . The assumed regularity makes continuous and differentiable with .
The real line is an order-convex interval, so [F4] applied to and step 3.1 shows that for every pair . If this common value is nonzero, the defining equality shows that the ingoing transverse derivative is the same nonzero number for every and therefore cannot tend to zero.
Remarks
The charge is the derivative of in the regular ingoing chart. It is not the derivative of alone and is not computed in a singular static chart. In Aretakis Part I's convention , this normalization is , since . The result is proved directly from the local wave equation; no decay estimate or initial-data theorem is assumed.
Depends on
A function continuous on an interval whose derivative vanishes at every interior point of is constant on ; consequently two such functions with the same derivative differ by a constant
The Axiom of Countable Choice ()
Spherical scalar waves in regular extremal RN coordinates
The derivative of the metric density and the chart formula for the divergence of a vector field
Existence and uniqueness of the metric-selected connection of a nondegenerate metric, and its extension to tensor fields
Dependency tree · 2 levels
Sources
- Stefanos Aretakis, Stability and Instability of Extreme Reissner–Nordström Black Hole Spacetimes for Linear Scalar Perturbations I, Communications in Mathematical Physics 307 (2011), arXiv:1110.2007
- Stefanos Aretakis, Stability and Instability of Extreme Reissner–Nordström Black Hole Spacetimes for Linear Scalar Perturbations II, Annales Henri Poincaré 12 (2011), arXiv:1110.2009
- Harvey Reall, What happens at the horizon of an extreme black hole?, complete 34-page lecture presentation (2013)
- Martin Taylor, Lectures on the nonlinear stability of the Schwarzschild family of black holes, complete lecture notes (Spring 2022)