In my opinion, we are experiencing a paradigm-shift for the research of non-Gaussianity.

In 2013, the data release of the Planck experiment did not provide significant evidence about any non-Gaussianity. This discouraged the non-Gaussianity research a lot. Personally I had stopped working on non-Gaussianity for 2 years.

But to put the Planck experiment into history, it is just one milestone of a long journey. The current observational technique is ready for 10x precision improvement (see, e.g. SPHEREx with local fNL~12) and in the very distant future there is potential for a few more orders of magnitude improvements in 21cm experiments, which can in principle reach local fNL~11000.

Most of the previous non-Gaussianity study focus on fNL>1. For example, for local non-Gaussianity, the curvaton scenario naturally get fNL>1 and needs fine tuning to get fNL<1. And for equilateral non-Gaussianity, fNL<1 means that the inflationary sound speed cannot be very different from one, up to small corrections.

Thus, in the era of precision non-Gaussianity, we need new theoretical motivations for fNL<1. Those motivations are also the driving force for making the future experiments happen.

Let us here draw an analogy with the development of particle physics. There are two eras of particle physics. In the past, researchers had focused on the external lines of Feynman diagrams, for example the alpha-scattering, discovery of muon, deep inelastic scattering, and so on. But nowadays, the external lines are well understood and the research interest has moved to understanding the internal lines of Feynman diagrams. The massive states that propagate as the internal lines are the key signals to be searched for on modern colliders.

Similarly, for non-Gaussianity, in the past people have mainly focused on the external lines. For example, the curvaton scenario is the search for different kind of particles as the external line; and the modified sound speed is the kinetic nature of the external line. If we confirm fNL<1 in the future, those searches will eventually lose people’s interest.

On the other hand, the internal lines tells us rich physics. The massive particles propagating as the internal line of the cosmological Feynman diagram tells us about the new particle physics that can be probed during inflation, and also is considered as a direct measurement of the expansion history of the primordial universe. However, the study of those internal lines are very challenging. Great precision with fNL<<1 is needed unless we are exceptionally lucky.

The new era of non-Gaussianity is summarized in the below table:

Era Planck and before After Planck
How to observe? CMB Mainly LSS
fNL > O(1) < O(1)
Physics curvaton, DBI, … EFT, massive states
Feynman Diagrams external line internal line
Toolkit in-in formalism + EdS, O12, nEFT, …

Details of some related research directions can be found at Cosmological Collider Physics and Evolution History of the Primordial Universe.