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.

16 pages, no figure, minor typos corrected, accepted for publication in Gen. Relativ. Gravit

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