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 ( supernovæ ) 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 _ { eff } = 3.53 \pm 0.32 , independent of late-time dark energy or gravity physics .