The reheating constraints to natural inflation in Horndeski gravity

For the subclass of Horndeski theory of gravity, we investigate the effects of reheating on the predictions of natural inflation. In the presence of derivative self-interaction of a scalar field and its kinetic coupling to the Einstein tensor, the gravitational friction to inflaton dynamics is enhanced. As a result, the tensor-to-scalar ratio $r$ is suppressed. We place the observational constraints on a natural inflation model and show that the model is now consistent with the observational data for some plausible range of the model parameter $\Delta$, mainly due to the suppressed tensor-to-scalar ratio. To be consistent with the data at the $1\sigma$ ($68\%$ confidence) level, a slightly longer $N_k\gtrsim60$ duration of inflation than usually assumed is preferred. Since the duration of inflation, for any specific inflaton potential, is related to reheating parameters, including the duration $N_{re}$, temperature $T_{re}$, and equation-of-state $\omega_{re}$ parameter during reheating, we imposed the effects of reheating to the inflationary predictions to put further constraints. The results show that the duration of inflation $N_k$ is affected by considerations of reheating, mainly by the $\omega_{re}$ and $T_{re}$ parameters. If reheating occurs instantaneously for which $N_{re}=0$ and $\omega_{re}=1/3$, the duration of inflation is estimated to be $N_k\simeq57$, where the exact value is less sensitive to the model parameter $\Delta$ compatible with the CMB data. The duration of inflation is longer (or shorter) than $N_k\simeq57$ for the equation of state larger (or smaller) than 1/3 hence $N_{re}\neq0$. The maximum temperature at the end of reheating is $T_{re}^\text{max}\simeq3\times 10^{15}$ GeV, which corresponds to the instantaneous reheating. The low reheating temperature, as low as a few MeV, is also possible when $\omega_{re}$ is closer to $1/3$.