We build an effective field theory of multifield inflationary fluctuations based on the adiabatic perturbation and any number of matter fluctuations in the nonadiabatic sector, without imposing extra symmetries on the latter. Focusing on terms with at most two derivatives in the fields’ fluctuations, we argue that taking the decoupling limit—in which gravitational interactions are neglected—is justified in a quasi–de Sitter spacetime with a slow-varying Hubble scale. With these working hypotheses, we find simple forms of multifield mixings (quadratic order) and interactions (cubic order). We explain how to break degeneracies among various terms, and we compare the predictions of the effective field theory to those of nonlinear sigma models of inflation and a more general multifield Lagrangian in the traditional model approach. We stress that several multifield cubic interactions are dictated by nonlinearly realized spacetime symmetries and are therefore given in terms of parameters already present in the quadratic action. We propose various directions to systematically explore the phenomenology generic to multifield inflation and beyond the lamppost of known models.

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• Cosmology
• Effective field theory
• Inflation