My latest paper “Calculating nonadiabatic pressure perturbations during multifield inflation”, written with Adam Christopherson, has now been published in Physical Review D as Phys. Rev. D 85, 063507. The DOI is 10.1103/PhysRevD.85.063507 and if you want to get the (freely available) arXiv version the number is arXiv:1111.6919.
This paper investigates the isocurvature or nonadiabatic perturbations during inflationary expansion with more than one field. We performed numerical simulations using Pyflation version 0.2 which can handle multiple field inflationary models at first order in perturbation theory.
We found that, as expected, the amount of isocurvature varied considerably for the three models we examined. We compared our results to those found using a different definition for the isocurvature, which coincides with ours during slow roll evolution on large scales. As slow roll breaks down near the end of inflation the evolution of the two different forms of isocurvature can vary dramatically.
One reason to be interested in non-adiabatic pressure during the early universe is that it can be a source for the generation of vorticity, which could be observable in the polarization of the cosmic microwave background.