Dissipative effects in the Effective Field Theory of Inflation

TL;DR

Cheung, Creminelli, Fitzpatrick, Kaplan, Senatore, and collaborators extend the EFT of single-clock inflation to include dissipative effects by coupling the metric to composite operators that encode an auxiliary dissipative sector. In the regime γ≫H, the curvature perturbation becomes noise-dominated, leading to potentially large enhancements of the power spectrum and non-Gaussianities, with f_NL scaling roughly as γ/(c_s^2 H) for many realizations. They present a general EFT framework, derive linear and nonlinear response relations, and perform explicit matching to local trapped inflation and warm inflation as concrete examples. The work provides a versatile toolkit to connect early-universe dissipation with observable signatures in the cosmic microwave background and large-scale structure, while expanding the space of viable inflationary models beyond the standard slow-roll paradigm.

Abstract

We generalize the effective field theory of single clock inflation to include dissipative effects. Working in unitary gauge we couple a set of composite operators in the effective action which is constrained solely by invariance under time-dependent spatial diffeomorphisms. We restrict ourselves to situations where the degrees of freedom responsible for dissipation do no contribute to the density perturbations at late time. The dynamics of the perturbations is then modified by the appearance of `friction' and noise terms, and assuming certain locality properties for the Green's functions of these composite operators, we show that there is a regime characterized by a large friction term γ>> H in which the ζ-correlators are dominated by the noise and the power spectrum can be significantly enhanced. We also compute the three point function <ζζζ> for a wide class of models and discuss under which circumstances large friction leads to an increased level of non-Gaussianities. In particular, under our assumptions, we show that strong dissipation together with the required non-linear realization of the symmetries implies |f_NL| ~ γ/(c_s^2H) >> 1. As a paradigmatic example we work out a variation of the `trapped inflation' scenario with local response functions and perform the matching with our effective theory. A detection of the generic type of signatures that result from incorporating dissipative effects during inflation, as we describe here, would teach us about the dynamics of the early universe and also extend the parameter space of inflationary models.

Figures (9)

• Figure 1: Top: Dependence on $\gamma$ of power spectrum given in Eq. (\ref{['pow1gamma']}) normalized as $p_{\pi}(\gamma)=6 N_c^2(kc_s)^3P_{\pi}(k)/(\pi \nu_{{\cal O}})$, for $z_0\to+\infty$; Bottom: The power spectrum $P_{\pi}(k)$ for $|kc_s\eta|\to 0$ as a function of $z_0$, for $\gamma/H=0$ (solid), $\gamma/H=3$ (Dashed), and $\gamma/H=9$ (Dotted).
• Figure 2: The shape $F(x_2,x_3)=x_2^2x_3^2 \frac{F(1,x_2,x_3)}{F(1,1,1)}$ given by Eq. (\ref{['fdeltagooshape']}) for $\gamma= 4 H$ (top) and $\gamma=40 H$ (bottom). To avoid showing equivalent configurations twice, the function is set to zero outside the region $1-x_2\leq x_3\leq x_2$.
• Figure 3: The shape $F(x_2,x_3)=x_2^2x_3^2 \frac{F(1,x_2,x_3)}{F(1,1,1)}$ for $\gamma= 10 H$ corresponding to the last term in Eq. (\ref{['pieqnl']}).
• Figure 4: The shape $F(x_2,x_3)=x_2^2x_3^2 \frac{F(1,x_2,x_3)}{F(1,1,1)}$ obtained from Eq. (\ref{['NGNoise']}) for $\gamma\simeq 10 H$. To avoid showing equivalent configurations twice, the function is set to zero outside the region $1-x_2\leq x_3\leq x_2$.
• Figure 5: The wavy lines correspond to $\pi$ and the dark interactions are insertions of ${\cal O}\pi$ couplings. The line connecting the dots represents Feynman's time order product of Eq. (\ref{['tfeynm']}).