Horizons and singularity in Clifton's spherical solution of f(R) vacuum

Due to the failure of Birkhoff's theorem, black holes in f(R) gravity theories in which an effective time-varying cosmological "constant" is present are, in general, dynamical. Clifton's exact spherical solution of R^(1+\delta) gravity, which is dynamical and describes a central object embedded in a spatially flat universe, is studied. It is shown that apparent black hole horizons disappear and a naked singularity emerges at late times.