Pure Natural Inflation Passes the ACT

Abstract

Pure natural inflation is a compelling effectively single-field model of inflation stemming from a top-down approach to the acceleration mechanism. In this short letter we show that such model is compatible with the latest CMB constraints from the Atacama Cosmology Telescope. Under both the instantaneous reheating hypothesis and standard assumptions for reheating, we rule in a non-trivial fraction of the parameter space. We apply our analysis also to a phenomenological extension of the model and chart its viable parameter space.

Figures (5)

• Figure 1: Plot of the pure natural potential Eq. (\ref{['eqn:PureNaturalPot']}) for several values of $p$.
• Figure 2: Trajectory traced in the $r$-$n_s$ plane as the parameter $F$ is varied in the pure natural inflation model. The light/dark orange contours correspond to Planck+BICEP constraints, while the purple regions stem from the ACT results. Solid lines are computed from Eq. (\ref{['eqn:Nrh']}) assuming instantaneous reheating, while the dashed lines assume an inflationary evolution of 50 e-folds. In both cases, for each $p$, the range of $F$ spans from $0.1 M_{\rm p}$ to $75M_{\rm p}$, the latter value providing an excellent approximation for the predictions of the chaotic inflation scenario. These are identified by the black square or star, depending on the duration of inflation.
• Figure 3: The $r-n_s$ plane is plotted here using a logarithmic scales for the $r$ axis in order to capture a wider range of values of the parameter space. The lowest value of $r$ corresponds to the lowest allowed value for $F$ and comes from the requirement that $\phi_*/F< 0.90/\gamma(p)$ where 90% is a somewhat arbitrary threshold to make sure we are not too close to the actual bound Nomura_2018. Dashed/Solid lines whose endpoint is a square/star are obtained as in Fig. \ref{['Fig:PlanckActContoursLinear']}.
• Figure 4: Results for small negative values of the parameter $p$. The upper panel tracks the evolution of observables as $F$ decreases from $75\; M_{\rm p}$ to $10^{-4}\; M_{\rm p}$, while the lower panel provides a detailed view for $F \geq 0.25 \;M_{\rm p}$ to highlight the range of parameter space consistent with $1\sigma$ observational constraints. Squares and stars follow the same conventions as in Fig. \ref{['fig:pExtNinsta']}, denoting results for $N=50$ and instantaneous reheating, respectively. Intermediate points represent non-instantaneous reheating via the perturbative decay channel described in Sec. \ref{['PostInfl']}: clovers denote the maximum coupling allowed by the backreaction constraint, while pikes correspond to $1\%$ of this maximum value.
• Figure 5: Zoomed-in version of Fig. 3 for positive values of the parameter $p$ in the range $p\in (0.05,2)$. The intermediate points correspond to the $(r,n_s)$ predictions obtained from the value of $N_k$ associated with a non-instantaneous reheating phase, as described in this section. Clover symbols, as in Fig. 4, denote the maximum coupling value allowed by the backreaction constraint, while pike symbols correspond to the coupling choice $\lambda = 0.01$.