  Small fractions of isocurvature perturbations correlated with the dominant
adiabatic mode are shown to be a significant primordial systematic for future
Baryon Acoustic Oscillation (BAO) surveys, distorting the standard ruler
distance by broadening and shifting the peak in the galaxy correlation
function. Untreated this systematic leads to biases that can exceed $10\sigma$
in the dark energy parameters even for Planck-level isocurvature constraints.
Accounting for the isocurvature modes corrects for this bias but degrades the
dark energy figure of merit by at least 50\%. The BAO data in turn provides
extremely powerful new constraints on the nature of the primordial
perturbations. Future large galaxy surveys will thus be powerful probes of the
earliest phase of the universe in addition to helping pin down the nature of
dark energy.
