Inflaton perturbations through an ultra-slow-roll transition and Hamilton-Jacobi attractors

Abstract We examine the behaviour of the gauge invariant scalar field perturbations in an analytic inflationary model that transitions from slow-roll to an ultra-slow-roll (USR) phase. We find that the numerical solution of the Mukhanov-Sasaki equation is well described by Hamilton-Jacobi (HJ) theory, as long as the appropriate branches of the Hamilton-Jacobi solutions are invoked: modes that exit the horizon during the slow-roll phase evolve into the USR as described by the first HJ branch, up to a subdominant 𝒪 ( k 2 / H 2 ) correction to the Hamilton-Jacobi prediction for their final amplitude that we compute, indicating the influence of neglected gradient terms. Modes that exit during the USR phase are described by a separate HJ branch once they become sufficiently superhorizon, obtained by the shift ( ϵ 1 , ϵ 2 ) ≃ (0,-6+Δ) → ( ϵ 1 , ϵ 2 ) ≃ (0,-Δ) and corresponding to a slow-roll solution (very close to de Sitter) supported by the same potential. This transition is similar to the conveyor belt concept put forward in our previous work Phys. Rev. D 104 (2021) 083505 and suggests that the limit ϵ 2 → -6 is unphysical as an asymptotic value for the background/long wavelength solution. We further discuss implications for the validity of the stochastic equations arising from the Hamilton-Jacobi formulation. Our work suggests that if Hamilton-Jacobi attractors are appropriately used, they can successfully describe the dynamics of long wavelength inflationary inhomogeneities for potentials with USR regions.