Large Scale Structure


  • David Alonso (Oxford)
  • Anze Slosar (BNL)

Measurements of galaxy clustering constrain the cosmic expansion history, the cosmological distance scale, the growth rate of structure, the mass of the neutrinos and the abundance of dark matter. Baryon acoustic oscillations (BAO) in the early Universe imprint a standard ruler in the power-spectrum (or the two-point correlation function), which measures the angular-diameter distance and the Hubble parameter. BAO is a special case of using the full shape of galaxy clustering data to constrain cosmological models. The BAO feature has now been observed in the SDSS, 2dF, and BOSS surveys, using both spectroscopic (e.g., Eisenstein et al. 2005a; Anderson et al. 2012; Xu et al. 2012) and photometrically selected galaxy samples (Padmanabhan et al. 2007; Ho et al. 2012; Seo et al. 2012), and has proven to be a robust probe of dark energy. A second probe of dark energy, and a potential discriminator between dark energy and modified-gravity models, is the growth of large-scale structure in the Universe.

The galaxy distributions measured by LSST will reveal information from both of these classes of probes (see LSST Science Collaborations et al. 2009a, Chapter 13 for details). LSST’s most sensitive measurements of the growth of structure will involve cross-correlations of the galaxy distribution with the shear field measured by lensing (so-called galaxy-galaxy lensing) or with the cosmic microwave background (the integrated Sachs-Wolfe effect). Cross-correlations with external spectroscopic surveys will also lead to powerful tests of modified-gravity models.

Beyond dark energy, the large scale power spectrum is a probe of both neutrino mass and primordial non-Gaussianities in the spectrum of inflationary perturbations. For all these purposes, identifying and removing systematic effects in the maps of the galaxy density, especially on large scales, will be crucial. A careful study of non-linear effects on small scales is equally important to probe, for example, scale-dependent non-Gaussianity. The LSST’s vast coverage will help in making precise measurements of higher-order shear and galaxy statistics, and of cross-correlations between different photometric redshift bins. These will be crucial to test various models for systematic corrections and non-linear structure formation, and in turn provide strong constraints on the mass of neutrinos and primordial non-Gaussianity.


 BAO show up as density ripples in galaxy counts.  (Image credit: Zosia Rostomian, LBNL, BOSS)