How vector fields during inflation leave a distinct fingerprint in the early universe

During inflation, adding vector fields coupled to the inflaton changes how quantum fluctuations grow and interact. This analysis maps out the full shape of three-point correlations (the bispectrum) across different coupling strengths, finding that strong vector fields produce a distinctive flattened signal rather than the local type expected from standard inflation. Crucially, this signature persists even when the universe looks isotropic, giving future CMB and large-scale structure surveys a clean target to distinguish vector-driven inflation.