The next non-Gaussianity frontier: what can a measurement with $\sigma(f_{\rm NL}) \lesssim 1$ tell us about multifield inflation?

TL;DR

Future galaxy surveys with extremely precise measurements of local primordial non-Gaussianity will critically test multifield inflation models, especially those involving spectator fields, by potentially detecting or constraining $f_{NL}$ at the order of unity.

Contribution

This study uses MCMC likelihood analysis with Planck data to predict $f_{NL}$ in spectator multifield inflation models, highlighting their potential to be tested by upcoming galaxy surveys.

Findings

Spectator models can produce $f_{NL}$ of order unity.

Upcoming surveys will test the spectator field dominance in primordial perturbations.

Fine-tuned models predict observable non-Gaussianity.

Abstract

Future galaxy surveys promise to probe local primordial non-Gaussianity at unprecedented precision, $\sigma(f_{\rm NL}) \lesssim 1$. 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 an MCMC likelihood analysis using Planck data to study quantitative predictions for $f_{\rm NL}$ 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 $f_{\rm NL}$ 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.