Primordial non-Gaussianity constraints on dissipative inflation

Abstract

Dissipative effects appear in many early-Universe scenarios, yet their universal observational signatures and systematic confrontation with data remain largely unexplored. We employ the Open Effective Field Theory of Inflation (Open EFToI) to consistently incorporate dissipative and stochastic effects while preserving scale invariance. Dissipation enhances specific interaction channels of the Goldstone mode, generating distinctive primordial non-Gaussian signatures, beyond those generically produced by standard EFToI. In the weak-dissipation regime, this includes folded bispectrum shapes observationally more favoured than both the equilateral and orthogonal templates. Using the Modal bispectrum pipeline with the Planck CMB data, we obtain the likelihood and derive the first model-independent bounds on early-Universe dissipation. We find a marginalised upper bound on the dissipation scale $γ\leq 384\,H$ and a lower bound on the sound speed $c_s \geq 0.38$ at $95\%$ confidence level. The maximum likelihood for best-fit models reveals a degeneracy between $γ$ and $c_s$. These results open a model-independent window for probing departures from minimal inflation and discriminating between early-Universe scenarios with stochastic noise and dissipative effects.

Figures (4)

• Figure 1: Likelihood \ref{['eqn:-2logL']} in the $(\gamma, c_s)$ plane used to constrain the dissipation $\gamma$ and sound speed $c_s$. The likelihood is normalized to unity at its maximum to facilitate visual comparison.
• Figure 2: Shape correlation of the Open EFToI bispectrum with the standard equilateral, orthogonal and local templates, as used in Planck PNG analysis. The shaded regions show the envelope obtained by varying $c_s$ between $0.01$ and $1$. At large dissipation, the signal correlates with the equilateral template at the $90\%$ level. At low dissipation, the signal peaks in a strongly folded configuration whose correlations with standard templates depend on $c_s$ and are not generically orthogonal.
• Figure 3: Corner plot of the posterior distributions for the dissipation rate $\gamma$ and sound speed $c_s$, assuming uniform priors. Thin dotted lines mark the $2\sigma$ exclusion limits of the marginalized posteriors, yielding $\gamma < 383.6H$ and $c_s > 0.38$ at 95% confidence level.
• Figure 4: Absolute signal-to-noise ratio (SNR) as a function of the sound speed $c_s$ and dissipation $\gamma$, computed from the shape of the predicted bispectrum. The SNR decreases for increasing $\gamma$ and decreasing $c_s$. The differences with the likelihood in Fig. \ref{['fig:likelihood']} reflect that Eq. \ref{['eq:SNR']} depends only on the bispectrum shape, while the likelihood Eq. \ref{['eqn:likelihood-modal']} also incorporates the amplitude through $f_{\rm NL}^{\rm th}$.