The Unity of Cosmological Attractors

Abstract: Recently, several broad classes of inflationary models have been discovered whose cosmological predictions are stable with respect to significant modifications of the inflaton potential. Some classes of models are based on a non-minimal coupling to gravity. These models, which we will call $\xi$-attractors, describe universal cosmological attractors (including Higgs inflation) and induced inflation models. Another class describes conformal attractors (including Starobinsky inflation and T-models) and their generalization to $\alpha$-attractors. The aim of this paper is to elucidate the common denominator of these models: their attractor properties stem from a pole of order two in the kinetic term of the inflaton field in the Einstein frame formulation, prior to switching to the canonical variables. We point out that $\alpha$- and universal attractors differ in the subleading corrections to the kinetic term. As a final step towards unification of $\xi$ and $\alpha$ attractors, we introduce a special class of $\xi$-attractors which is fully equivalent to $\alpha$-attractors with the identification $\alpha = 1+{1\over 6\xi}$. There is no theoretical lower bound on $r$ in this class of models.