  We derive constraints on cosmological parameters and tests of dark energy
models from the combination of baryon acoustic oscillation (BAO) measurements
with cosmic microwave background (CMB) and Type Ia supernova (SN) data. We take
advantage of high-precision BAO measurements from galaxy clustering and the
Ly-alpha forest (LyaF) in the BOSS survey of SDSS-III. BAO data alone yield a
high confidence detection of dark energy, and in combination with the CMB
angular acoustic scale they further imply a nearly flat universe. Combining BAO
and SN data into an "inverse distance ladder" yields a 1.7% measurement of
$H_0=67.3 \pm1.1$ km/s/Mpc. This measurement assumes standard pre-recombination
physics but is insensitive to assumptions about dark energy or space curvature,
so agreement with CMB-based estimates that assume a flat LCDM cosmology is an
important corroboration of this minimal cosmological model. For open LCDM, our
BAO+SN+CMB combination yields $\Omega_m=0.301 \pm 0.008$ and curvature
$\Omega_k=-0.003 \pm 0.003$. When we allow more general forms of evolving dark
energy, the BAO+SN+CMB parameter constraints remain consistent with flat LCDM.
While the overall $\chi^2$ of model fits is satisfactory, the LyaF BAO
measurements are in moderate (2-2.5 sigma) tension with model predictions.
Models with early dark energy that tracks the dominant energy component at high
redshifts remain consistent with our constraints. Expansion history alone
yields an upper limit of 0.56 eV on the summed mass of neutrino species,
improving to 0.26 eV if we include Planck CMB lensing. Standard dark energy
models constrained by our data predict a level of matter clustering that is
high compared to most, but not all, observational estimates. (Abridged)
