The growth rate of cosmic structure from peculiar velocities at low and
  high redshifts
  Peculiar velocities are an important probe of the growth rate of mass density
fluctuations in the Universe. Most previous studies have focussed exclusively
on measuring peculiar velocities at intermediate ($0.2 < z < 1$) redshifts
using statistical redshift-space distortions. Here we emphasize the power of
peculiar velocities obtained directly from distance measurements at low
redshift ($z \lesssim 0.05$), and show that these data break the usual
degeneracies in the \Omega_{m,0} -- $\sigma_{8,0}$ parameter space. Using only
peculiar velocity data, we find $\Omega_{m,0} = 0.259\pm0.045$ and
$\sigma_{8,0} = 0.748\pm0.035$. Fixing the amplitude of fluctuations at very
high redshift using observations of the Cosmic Microwave Background (CMB), the
same data can be used to constrain the growth index $\gamma$, with the
strongest constraints coming from peculiar velocity measurements in the nearby
Universe. We find $\gamma = 0.619\pm 0.054$, consistent with LCDM. Current
peculiar velocity data already strongly constrain modified gravity models, and
will be a powerful test as data accumulate.
