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arXiv · 2609.26754

To USR or not to USR: An effective description of two-field dynamics

Samuel Sánchez López·Eemeli Tomberg
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Abstract

Single-field models of inflection point inflation experience a phase of ultra-slow roll (USR), leading to amplified curvature perturbations. Recently, Lorenzoni et al. modified the scenario by adding a light spectator field, showing that it ruins USR by setting a floor for the first slow-roll parameter $ε$. Mixing with isocurvature perturbations still amplifies the curvature power spectrum. We offer an alternative point of view, arguing that the inflaton still undergoes effective USR. The spectator lifts the potential by a constant amount, and the inflaton perturbations follow single-field dynamics in the lifted inflection point potential, decoupling from the spectator perturbations. Mode mixing only affects the power spectrum dip, making it milder. We demonstrate this effective single-field behavior numerically for example models. Our results offer a simple way to study the models' parameter dependence.

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Samuel Sánchez López, Eemeli Tomberg. 2026-09-22. To USR or not to USR: An effective description of two-field dynamics. https://arxiv.org/abs/2609.26754

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