Effective field theory and decoupling in multifield inflation: An illustrative case study

Abstract

We explore the effects of heavy degrees of freedom on the evolution and perturbations of light modes in multifield inflation. We use a simple two-field model as an example to illustrate the subtleties of integrating out massive fields in a time-dependent background. We show that when adiabaticity is violated due to a sharp turn in field space, the roles of massive and massless field are interchanged, and furthermore the fields are strongly coupled; thus the system cannot be described by an effective single field action. Further analysis shows that the sharp turn imparts a non Bunch-Davis component in each perturbation mode, leading to oscillatory features in the power spectrum, and a large resonantly enhanced bispectrum.

On the importance of heavy fields during inflation

“…For instance, in ref. [69] it is claimed that the effective field theory (3.6) is only valid in the regime where turns are such that |θ| H. 10 The main argument made there is that the ratio η ⊥ =θ/H corresponds to the coupling determining the kinetic energy transfer between the light curvature mode with the heavy fields. A large value of η ⊥ would therefore imply large transfer of energy from curvature perturbations to the heavy mode, exciting the heavy modes and rendering the effective field theory invalid.…”

“…However, general field theoretical arguments due allow for large sizable corrections to the low energy EFT [57,58]. In the specific case of multi field models, there are special circumstances where the background inflationary dynamic is such that the interchange of kinetic energy between curvature perturbations and heavy degrees of freedom may be dramatically enhanced [26,[62][63][64][65][66][67][68][69][70]. For example, if the inflationary trajectory is subject to a sharp turn in such a way that the heavy scalar fields stay normal to the trajectory (see Figure 1 for an illustration) then unsuppressed interactions -kinematically coupling curvature perturbations with heavy fields-are unavoidably turned on.…”

Effective field theory in time-dependent settings

“…Some effort has already been made to apply the ideas of effective field theories to inflationary backgrounds [17][18][19][20]. And some works too have specifically examined the influence of a heavy field on the correlation functions of the inflaton [21][22][23][24], using classical field theory. The picture that is emerging is a very intriguing one.…”

Inflation and String Theory

“…1 These fields fluctuation quantum-mechanically during inflation and therefore have to be included in the computation of the primordial perturbations. The phenomenology of these models of quasisingle-field inflation [490] has been explored in [118,207,208,239,255,265,267,[491][492][493][494][495][496][497].…”