Primordial vector modes describe vortical fluid perturbations in the early universe .
A regular solution exists with constant non-zero radiation vorticities on super-horizon scales .
Baryons are tightly coupled to the photons , and the baryon velocity only decays by an order unity factor by recombination , leading to an observable CMB anisotropy signature via the Doppler effect .
There is also a large B-mode CMB polarization signal , with significant power on scales larger than l \sim 2000 .
This B-mode signature is distinct from that expected from tensor modes or gravitational lensing , and makes a primordial vector to scalar mode power ratio \sim 10 ^ { -6 } 
detectable .
Future observations aimed at detecting large scale B -modes from gravitational waves will also be sensitive to regular vector modes at around this level .