Complete Affine Connection in the Causal Boundary: Static, Spherically Symmetric Spacetimes

TL;DR

This paper investigates possible affine connections on the future null infinity boundary of static, spherically symmetric spacetimes, revealing a gauge-dependent family of connections and identifying conditions for completeness.

Contribution

It introduces two methods for analyzing affine connections on the boundary, demonstrating gauge freedom and providing a complete connection choice in static, spherically symmetric spacetimes.

Findings

Multiple possible connections depend on gauge choices.

Choosing a constant function yields a complete connection.

Other boundaries like timelike infinity have unique, complete connections.

Abstract

The boundary at $\Cal I^+$, future null infinity, for a standard static, spherically symmetric spactime is examined for possible linear connections. Two independent methods are employed, one for treating $\Cal I^+$ as the future causal boundary, and one for treating it as a conformal boundary (the latter is subsumed in the former, which is of greater generality). Both methods provide the same result: a constellation of various possible connections, depending on an arbitrary choice of a certain function, a sort of gauge freedom in obtaining a natural connection on $\Cal I^+$; choosing that function to be constant (for instance) results in a complete connection. Treating $\Cal I^+$ as part of the future causal boundary, the method is to impute affine connections on null hypersurfaces going out to $\Cal I^+$, in terms of a transverse vector field on each null hypersurface (there is much gauge freedom on choice of the transverse vector fields). Treating $\Cal I^+$ as part of a conformal boundary, the method is to make a choice of conformal factor that makes the boundary totally geodesic in the enveloping manifold (there is much gauge freedom in choice of that conformal factor). Similar examination is made of other boundaries, such as timelike infinity and timelike and spacelike singularities. These are much simpler, as they admit a unique connection from a similar limiting process (i.e., no gauge freedom); and that connection is complete.