Fundamental Uncertainty in the BAO Scale from Isocurvature Modes

TL;DR

Isocurvature perturbations pose a significant systematic in BAO measurements, affecting dark energy parameter estimates, but also offer new constraints on primordial universe conditions, impacting future galaxy surveys.

Contribution

This paper demonstrates that isocurvature modes significantly bias BAO-based dark energy measurements and shows how BAO data can constrain primordial perturbations.

Findings

Isocurvature modes distort BAO peak positions.

Biases in dark energy parameters can exceed 10σ without correction.

BAO data can tightly constrain primordial isocurvature perturbations.

Abstract

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.