Testing dark energy using pairs of galaxies in redshift space
  The distribution of angles subtended between pairs of galaxies and the line
of sight,which is uniform in real space, is distorted by their peculiar
motions, and has been proposed as a probe of cosmic expansion. We test this
idea using N-body simulations of structure formation in a cold dark matter
universe with a cosmological constant and in two variant cosmologies with
different dark energy models. We find that the distortion of the distribution
of angles is sensitive to the nature of dark energy. However, for the first
time, our simulations also reveal dependences of the normalization of the
distribution on both redshift and cosmology that have been neglected in
previous work. This introduces systematics that severely limit the usefulness
of the original method. Guided by our simulations, we devise a new, improved
test of the nature of dark energy. We demonstrate that this test does not
require prior knowledge of the background cosmology and that it can even
distinguish between models that have the same baryonic acoustic oscillations
and dark matter halo mass functions. Our technique could be applied to the
completed BOSS galaxy redshift survey to constrain the expansion history of the
Universe to better than 2%. The method will also produce different signals for
dark energy and modified gravity cosmologies even when they have identical
expansion histories, through the different peculiar velocities induced in these
cases.
