Stable isotropic cosmological singularities in quadratic gravity

We show that, in quadratic Lagrangian theories of gravity, isotropic cosmological singularities are stable to the presence of small scalar, vector, and tensor inhomogeneities. We study the effects of the quadratic Ricci term on the dynamics of the Universe at early times. Unlike in general relativity, a particular exact isotropic solution is shown to be the stable attractor on approach to the initial cosmological singularity. This solution is also known to act as an attractor in Bianchi universes of types I, II, and IX, and the results of this paper reinforce the hypothesis that small inhomogeneous and anisotropic perturbations of this attractor form part of the general cosmological solution to the field equations of quadratic gravity. Implications for the existence of a gravitational entropy are also discussed.