The length of the low-redshift standard ruler
  Assuming the existence of standard rulers, standard candles and standard
clocks, requiring only the cosmological principle, a metric theory of gravity,
a smooth expansion history, and using state-of-the-art observations, we
determine the length of the "low-redshift standard ruler". The data we use are
a compilation of recent Baryon acoustic oscillation data (relying on the
standard ruler), Type 1A supernov\ae\ (as standard candles), ages of early type
galaxies (as standard clocks) and local determinations of the Hubble constant
(as a local anchor of the cosmic distance scale). In a standard $\Lambda$CDM
cosmology the "low-redshift standard ruler" coincides with the sound horizon at
radiation drag, which can also be determined --in a model dependent way-- from
CMB observations. However, in general, the two quantities need not coincide. We
obtain constraints on the length of the low-redshift standard ruler: $r^h_{\rm
s}=101.0 \pm 2.3 h^{-1}$ Mpc, when using only Type 1A supernov\ae\ and Baryon
acoustic oscillations, and $r_{\rm s}=150.0\pm 4.7 $ Mpc when using clocks to
set the Hubble normalisation, while $r_{\rm s}=141.0\pm 5.5 $ Mpc when using
the local Hubble constant determination (using both yields $r_{\rm s}=143.9\pm
3.1 $ Mpc).
  The low-redshift determination of the standard ruler has an error which is
competitive with the model-dependent determination from cosmic microwave
background measurements made with the {\em Planck} satellite, which assumes it
is the sound horizon at the end of baryon drag.
