Isocurvature perturbations

Thanks to CMB anisotropy measurements (especially from the WMAP and Planck satellites),  we now know that primordial fluctuations are predominantly adiabatic. That is to say that photons, neutrinos, baryons, and dark matter had fluctuations in energy density that were more or less spatially in phase. Isocurvature fluctuations, on the other hand, are fluctuations in the relative energy densities of different species at fixed total density. They are now constrained to be 1% or less of the primordial density fluctuation, depending on precise assumptions. There are 3 well-motivated isocurvature modes (shown below contributing to fluctuations along with an adiabatic component). Generally speaking, isocurvature perturbations are produced by sub-dominant fields during the inflationary era. Detecting them would thus yield insight into physics at the inflationary energy scale, or even the production of baryon number, lepton number, or dark matter in the early universe.