Standard rulers, candles, and clocks from the low-redshift Universe
  We measure the length of the Baryon Acoustic Oscillation (BAO) feature, and
the expansion rate of the recent Universe, from low-redshift data only, almost
model-independently. We make only the following minimal assumptions:
homogeneity and isotropy; a metric theory of gravity; a smooth expansion
history, and the existence of standard candles (supernov\ae) and a standard BAO
ruler. The rest is determined by the data, which are compilations of recent BAO
and Type IA supernova results. Making only these assumptions, we find for the
first time that the standard ruler has length $103.9 \pm 2.3\, h^{-1}$ Mpc. The
value is a measurement, in contrast to the model-dependent theoretical
prediction determined with model parameters set by Planck data ($99.3 \pm 2.1
\, h^{-1}$ Mpc). The latter assumes $\Lambda$CDM, and that the ruler is the
sound horizon at radiation drag. Adding passive galaxies as standard clocks or
a local Hubble constant measurement allows the absolute BAO scale to be
determined ($142.8\pm 3.7$ Mpc), and in the former case the additional
information makes the BAO length determination more precise ($101.9\pm 1.9 \,
h^{-1}\,$Mpc). The inverse curvature radius of the Universe is weakly
constrained and consistent with zero, independently of the gravity model,
provided it is metric. We find the effective number of relativistic species to
be $N_{\rm eff} = 3.53\pm 0.32$, independent of late-time dark energy or
gravity physics.
