The early universe is $\textit{ACT}$-ing $\textit{warm}$

TL;DR

The paper argues that the observed flat primordial spectrum and low tensor-to-scalar ratio challenge simplest cold-inflation models, advocating warm inflation (WI) as a principled alternative protected by symmetries. It develops the warm little inflaton (WLI) framework, combining fermionic and scalar dissipative channels to yield a dissipative coefficient $\Upsilon$ and a rich phenomenology captured by $G(Q)$ in the power spectrum, implemented with the WI2easy code for high-precision predictions. Through analyses of the WLIF and WLIS limiting cases and the full WLI model, the authors show that strong dissipation ($Q>1$) can fit ACT data, with WLIS achieving substantial $Q$ leads to tiny $r$ and $n_s$ near unity, while WLIF remains viable in weaker regimes; the full model aligns with observations across a broad parameter space and predicts a positive running $\alpha_s$ at large $Q$. The results support WI as a natural path to reconcile inflation with data, while underscoring the need for first-principles microphysics of dissipation and future observational tests (e.g., bispectrum and tensor modes) to constrain the dissipative regime.

Abstract

The recently released data from the $\textit{Atacama Cosmology Telescope}$ (ACT) confirms that the primordial scalar spectrum is extremely flat. This, together with current upper bounds on the tensor-to-scalar ratio, implies that the simplest models of inflation coming from particle physics (for instance, a minimally-coupled scalar with monomial potentials) need additional ingredients in order to make them compatible with observations. Instead of invoking arbitrary new couplings or new interactions that are not protected symmetries, we argue that dissipation of the inflaton field with the radiation bath should be added as a new physical principle. Accordingly, we show that warm inflation provides the correct paradigm to explain the current observations, given very natural choices of dissipative terms. The model analyzed here has mirror and $Z_4$ symmetries, which explicitly protect the inflaton potential from large quantum and thermal corrections. We use a recent precision numerical code designed for warm inflationary perturbations, improving on the determination of the cosmological observables previously obtained for such models.

Figures (7)

• Figure 1: The one-sigma and two-sigma constraints from Planck (orange shaded regions) and the combined P-ACT-LB results (violet shaded region) for $n_s$ as a function of $Q_*$ (panels a, b) and for $n_s \times r$ (panels c, d). Panels a and c are the results for the WLIF models (with a quartic inflaton potential), while panels b and d are for the WLIS model (with a quadratic inflaton potential). The shaded region on the left of panels a and b indicates the region for which $r>0.036$.
• Figure 2: Similar to the Fig. \ref{['fig1']}, but now showing the running $\alpha_s$ as a function of the dissipation ratio $Q_*$ for the WLIF and WLIS models.
• Figure 3: The $G(Q)$ function generated through WI2easyRodrigues:2025neh for a dissipation coefficient eq. (\ref{['UpsWLIFS']}) for $b=1,\, 10,\, 20$, and for the potential (\ref{['pot2']}). We present the result for both nonthermal $n_*=0$ and fully thermalized $n_*=n_{\rm BE}$. The inset plot zoom in around the region $10^{-4} \lesssim Q \lesssim 1$.
• Figure 4: The same as in fig. \ref{['fig1']}, but showing the results for the WLI model, with a quadratic inflaton potential and for the cases of $b=1,\,10$ and $20$. Panels a, b and c show $n_s$ as a function of $Q_*$, while panels d, e and f show $n_s \times r$.
• Figure 5: The running $\alpha_s$ for the WLI model as a function of the dissipation ratio $Q_*$ at Hubble crossing and with the dissipation coefficient (\ref{['UpsWLIFS']}).