On the existence of conformal Killing horizons in LRS spacetimes

Abstract

Let M be a locally rotationally symmetric spacetime, and \(\xi ^a\) a conformal Killing vector for the metric on M, lying in the subspace spanned by the unit timelike direction and the preferred spatial direction, and with non-constant components. Under the assumption that the divergence of \(\xi ^a\) has no critical point in M, we obtain the necessary and sufficient condition for \(\xi ^a\) to generate a conformal Killing horizon. It is shown that \(\xi ^a\) generates a conformal Killing horizon if and only if either of the components (which coincide on the horizon) is constant along its orbits. That is, a conformal Killing horizon can be realized as the set of critical points of the variation of the component(s) of the conformal Killing vector along its orbits. Using this result, a simple mechanism is provided by which to determine if an arbitrary vector in an expanding LRS spacetime is a conformal Killing vector that generates a conformal Killing horizon. In specializing the case for which \(\xi ^a\) is a special conformal Killing vector, provided that the gradient of the divergence of \(\xi ^a\) is non-null, it is shown that LRS spacetimes cannot admit a special conformal Killing vector field, thereby ruling out conformal Killing horizons generated by such vector fields.