Next non-Gaussianity frontier: What can a measurement with σ (f_(NL)) ≲ 1 tell us about multifield inflation?

Future galaxy surveys promise to probe local primordial non-Gaussianity at unprecedented precision, σ (f_(NL)) ≲ 1. We study the implications for multifield inflation by considering spectator models, where inflation is driven by the inflaton field, but the primordial perturbations are (partially) generated by a second, spectator field. We perform a Markov chain Monte Carlo likelihood analysis using Planck data to study quantitative predictions for f_(NL) and other observables for a range of such spectator models. We show that models where the primordial perturbations are dominated by the spectator field, while fine-tuned within the broader parameter space, typically predict f_(NL) of order unity. Therefore, upcoming galaxy clustering measurements will constitute a stringent test of whether or not the generation of primordial perturbations and the accelerated expansion in the inflationary universe are due to separate phenomena