Heavy fields, reduced speeds of sound, and decoupling during inflation

Abstract

We discuss and clarify the validity of effective single field theories of inflation obtained by integrating out heavy degrees of freedom in the regime where adiabatic perturbations propagate with a suppressed speed of sound. We show by construction that it is indeed possible to have inflationary backgrounds where the speed of sound remains suppressed and slow-roll persists for long enough. In this class of models, heavy fields influence the evolution of adiabatic modes in a manner that is consistent with decoupling of physical low and high energy degrees of freedom. We emphasize the distinction between the effective masses of the isocurvature modes and the eigenfrequencies of the propagating high energy modes. Crucially, we find that the mass gap that defines the high frequency modes increases with the strength of the turn, even as the naive heavy (isocurvature) and light (curvature) modes become more strongly coupled. Adiabaticity is preserved throughout, and the derived effective field theory remains in the weakly coupled regime, satisfying all current observational constraints on the resulting primordial power spectrum. In addition, these models allow for an observably large equilateral non-Gaussianity.

Inflation and String Theory

“…As discussed in §2.1.5, a heavy mode of frequency ω can be integrated out in this way only when the adiabatic conditionω/ω 2 < 1 holds: more rapid evolution leads to excitation of the heavy modes. (Analyses of the effects of heavy fields include [493,496,505,506]. See also the discussion of resonance in §5.…”

Backreaction and stochastic effects in single field inflation

“…As we assume Ψ is a relatively heavy field, the oscillations of Ψ are suppressed by the large mass m Ψ , and this in turn will induce corrections to spectrum of Φ that are suppressed by powers of m Ψ . Similar scenarios have been discussed in many works, for example [93][94][95][96].…”

Moduli backreaction and supersymmetry breaking in string-inspired inflation models

“…Once W 1 breaks supersymmetry, however, there is a mixing between the 5 As in [27,29] we consider setups in which the heavy fields trace their inflaton-dependent minima instantaneously and adiabatically. In this sense our approach is different from the one taken in [30][31][32][33][34][35], where the authors consider sharp turns of inflationary trajectories in the valleys of heavy fields. Although our models generically do not exhibit such features, it may be interesting to study a combination with the effects that we find.…”