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Abstract:

We discuss the systematic uncertainties in the recovery of dark energyproperties from the use of baryon acoustic oscillations as a standardruler. We demonstrate that while unknown relativistic components in theuniverse prior to recombination would alter the sound speed, theinferences for dark energy from low-redshift surveys are unchanged solong as the microwave background anisotropies can measure the redshiftof matter-radiation equality, which they can do to sufficient accuracy.The mismeasurement of the radiation and matter densities themselves (asopposed to their ratio) would manifest as an incorrect prediction forthe Hubble constant at low-redshift. In addition, these anomalies doproduce subtle but detectable features in the microwave anisotropies.

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