Among the various scientific interests that members of the LUPM group have identified among the LSST science opportunities, the baryon acoustic oscillations (BAOs) have been singled out as a topic that will leverage local expertise, present opportunities to increase our understanding of the LSST data in depth, and guarantee a natural collaboration with LSST-France colleagues.
BAOs correspond to the acoustic phenomenon in the baryon-photon plasma before recombination. BAOs imprint a particular scale, corresponding to the sound horizon, that can be observed in large-scale structures of the Universe. Using this standard ruler property, BAOs can be used to probe the distance-redshift relation in galaxy catalogues, thus providing a very promising tool to study dark energy model. BAOs can be studied from the statistics (the correlation function or the power spectrum) in the distribution of galaxies. BAOs appear in the correlation function as a small localized bump at the scale of sound horizon. The detection of BAOs at the expected scale enables to confirm the current cosmological model. As for cosmological parameter constraints, enabling the study of dark energy, it is a major goal of modern cosmology. Thus two major applications are clearly identified: BAO detection, and cosmological parameter constraints using the standard ruler property. The first convincing detection of BAOs in large-scale structures was reported in the correlation function analysis of the Sloan Digital Sky Survey (SDSS) and Luminous Red Galaxies (LRG) survey with a 3.4 sigma significance. It was followed by other detections, as in the 2-degree Field Galaxy Redshift Survey (2dFGRS), using power spectrum analysis with a 2.5 \(\sigma\) significance. The power spectrum analysis is also applied with a 3.3 \(\sigma\) detection in the SDSS-LRG. The combined power spectrum of the LRG and the Main samples of SDSS, together with the 2dFGRS survey, is used to obtain respective significances of 3\(\sigma\) and 3.6\(\sigma\). BAO were also detected in the power spectrum of the WiggleZ Dark Energy Survey at a higher redshift z=0.6 with a significance of 3.2\(\sigma\).
The second use of BAOs consists in constraining cosmological parameters. BAOs are very useful because they provide a statistical standard ruler with an absolute size given with small uncertainty by Cosmic Microwave Background (CMB) measurements. So they directly constrain the redshift-distance relation on redshift surveys. Besides, BAOs appear to have the lowest systematic uncertainties among current methods for studying cosmic expansion. Thus BAOs have been used in combination with other cosmological probes to constrain more efficiently cosmological parameters.
Several papers have studied the prospects of measuring the BAOs from photometric surveys. The main advantage of a photometric redshift survey, such as the future LSST survey, is the wide coverage, which reduces the sample variance error, and deep photometry, which leads to more galaxies and lower shot noise. Nevertheless challenges for photometric redshift surveys include redshift errors, dust extinction, galaxy bias, redshift distortion and nonlinear evolution. These uncertainties do not produce strong oscillating features in the power spectrum as the BAOs and can be controlled in the measurements with the LSST.