Fundamental Uncertainty in the BAO Scale from Isocurvature Modes

Understand

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.