Backreactions from loading the stable photon sphere in Weyl conformal gravity

Abstract We investigate the accumulation of null matter at the stable photon sphere in the Mannheim-Kazanas metric, the analogue to the Schwarzschild solution in Weyl’s conformal theory of gravity. In our toy problem in which we consider an infinitely-thin shell, we find that a jump in radial pressure T r r is induced across the shell unless the shell has a radius of either the unstable or stable photon sphere. We then find that upon loading the stable photon sphere, its area remains invariant. Furthermore, at a critical threshold loading limit for this zero-width null matter shell, we are able to produce a metric containing an extremal horizon with an AdS 2 × S 2 geometry completely independent of the cosmological curvature. This hitherto unencountered and therefore unexpected result is a phenomenon unseen in standard nonconformal second-order metrics with nonzero cosmological constants.