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Static supertranslation shear without Bondi news
A formal boundary-data example with an exact flat seed and finite shear action; it distinguishes shear from news and does not claim a full transformed vacuum metric or observation.
Example
Let be the unit round metric on , its Levi-Civita derivative, and its area form. On and , with a length, the exact flat Bondi metric is ; its shear, news and mass aspect are . Let . Let on , set for a nonzero length , and use the finite active boundary action , . It gives , where , and . Treat these as formal boundary data with and impose the local coefficient constraint . Then , , and on every retarded-time interval, where for . No full finite-transformed vacuum metric is asserted. A nonzero shear value alone therefore does not imply nonzero news or a nonzero shear difference.
Example references
Facts & Assumptions
Given: The exact flat Bondi end and boundary data in the Example, the finite active boundary shear action of Bondi supertranslations and their action on the shear, and the stipulated local coefficient constraint whose vacuum-expansion version is established in Bondi mass-aspect constraint coefficient.
The angular metric is positive definite, is its Levi-Civita derivative, and the news is (A smooth vacuum Bondi expansion).
In the fixed Bondi frame the finite active boundary action is and on its stated domain (Bondi supertranslations and their action on the shear).
The local mass-aspect equation is imposed on the formal boundary data; it is the coefficient proved under the stronger smooth-vacuum hypotheses of Bondi mass-aspect constraint coefficient.
Countable Choice is the inherited smooth Lorentzian sphere-bundle convention; full Choice is not used (The Axiom of Countable Choice ()).
Verification
Given: [F1]–[F3] and [A1].
The restriction is smooth on the sphere because it is the restriction of a polynomial. Put . Its trace is zero. At , , and , so ; hence is nonzero.
With , the metric becomes , the Minkowski metric in spherical spatial coordinates. Thus its Bondi data are exactly . Since is independent of , [F2] gives and . We use only these as formal boundary data; we do not assert that they are the coefficients of a finite transformed vacuum metric.
Set for the formal data. With , [F3] is satisfied and , so . The shear is independent of , hence on every interval. These conclusions follow directly from the stipulated data and constraint; neither vacuum mass-loss nor the memory proposition is invoked.
Depends on
Dependency tree · 2 levels
Sources
- Andrew Strominger, Lectures on the Infrared Structure of Gravity and Gauge Theory, Princeton University Press (2018), full lecture notes arXiv:1703.05448: §5.2.1, Eqs. (5.2.4)-(5.2.6), and §6, Eqs. (6.0.4)-(6.0.5)
- Thomas Mädler and Jeffrey Winicour, Bondi-Sachs Formalism, arXiv:1609.01731 (2016), full 31-page PDF: §4, Eqs. (78)-(80), finite retarded-time supertranslations and asymptotic shear