What is the best way to measure baryonic acoustic oscillations?

Oscillations in the baryon–photon fluid prior to recombination imprint different signatures on the power spectrum and correlation function of matter fluctuations. The measurement of these features using galaxy surveys has been proposed as means to determine the equation of state of the dark energy. The accuracy required to achieve competitive constraints demands an extremely good understanding of systematic effects which change the baryonic acoustic oscillation (BAO) imprint. We use 50 very large volume N-body simulations to investigate the BAO signature in the two-point correlation function. The location of the BAO bump does not correspond to the sound horizon scale at the level of accuracy required by future measurements, even before any dynamical or statistical effects are considered. Careful modelling of the correlation function is therefore required to extract the cosmological information encoded on large scales. We find that the correlation function is less affected by scale-dependent effects than the power spectrum. We show that a model for the correlation function proposed by Crocce & Scoccimarro, based on renormalized perturbation theory, gives an essentially unbiased measurement of the dark energy equation of state. This means that information from the large-scale shape of the correlation function, in addition to the form of the BAO peak, can be used to provide robust constraints on cosmological parameters. The correlation function therefore provides a better constraint on the distance scale (∼50 per cent smaller errors with no systematic bias) than the more conservative approach required when using the power spectrum (i.e. which requires amplitude and long-wavelength shape information to be discarded).