Perturbative region on non-Gaussian parameter space in single-field inflation

We calculate one-loop correction to the two-point functions of curvature perturbation in single-field inflation generated by cubic self-interaction. Incorporating observed red-tilted spectrum perturbation, relevant takes a finite value and inversely proportional spectral tilt. Requiring be much smaller than tree-level contribution leads an upper bound on primordial non-Gaussianity. While observationally allowed region non-Gaussian parameter space is found entirely included region, where contribution, appreciably large has larger 1% or even 10% latter. If future observations conclude non-Gaussianity falls such then it would important incorporate higher-order corrections order achieve precise cosmology. In some extreme cases, comparable magnitude might indicate breakdown cosmological theory context inflation.