Researchers have revisited the bispectrum resulting from cosmic inflation when kinetically coupled vector fields are incorporated. This work is crucial for understanding the imprints that an inflationary universe, driven by non-scalar fields, could leave on the cosmic microwave background (CMB). Inflation, the period of exponential expansion of the early universe, is the dominant explanation for the homogeneity, isotropy, and flatness of the cosmos, as well as for the origin of primordial density fluctuations that seeded galaxies. However, the exact nature of the inflationary field (inflaton) remains a mystery, and models including vector fields offer interesting alternatives to standard scalar models.

The study focused on organizing the dynamics in terms of a parameter $h$, which quantifies the contribution of the vector field's kinetic energy relative to that of the scalar field. The bispectrum was evaluated in the strong vector field regime ($h \gg 1$), and a low-energy effective field theory (EFT) was developed for this regime. In this scenario, the entropic perturbation becomes heavy and can be integrated out, while the remaining curvature mode exhibits an imaginary sound speed and undergoes transient growth before crossing the horizon. This contrasts with the $h \ll 1$ regime, where the transfer from the vector sector persists out of the horizon and produces an enhanced local-type contribution scaling as $h^2N_K^3$.

The results reveal that, in addition to the already known enhanced flattened signals scaling as $h^3$, there are new enhanced flattened signals scaling as $h^2$, and a sharp local projection scaling as $h$. The competition between these contributions leads to a signal dominated by the local component for intermediate values of $h$, and a signal dominated by the flattened component for larger values of $h$. These non-Gaussianity predictions in the bispectrum are important because they would allow distinguishing inflationary dynamics supported by vector fields, even in an exactly isotropic background. The detection of such patterns in the CMB would be strong evidence for these alternative models of inflation.