Study of baryon acoustic oscillations with SDSS DR12 data and
  measurement of $\Omega_\textrm{DE}(a)$
  We define Baryon Acoustic Oscillation (BAO) distances $\hat{d}_\alpha(z,
z_c)$, $\hat{d}_z(z, z_c)$, and $\hat{d}_/(z, z_c)$ that do not depend on
cosmological parameters. These BAO distances are measured as a function of
redshift $z$ with the Sloan Digital Sky Survey (SDSS) data release DR12. From
these BAO distances alone, or together with the correlation angle
$\theta_\textrm{MC}$ of the Cosmic Microwave Background (CMB), we constrain the
cosmological parameters in several scenarios. We find $4.3 \sigma$ tension
between the BAO plus $\theta_\textrm{MC}$ data and a cosmology with flat space
and constant dark energy density $\Omega_\textrm{DE}(a)$. Releasing one and/or
the other of these constraints obtains agreement with the data. We measure
$\Omega_\textrm{DE}(a)$ as a function of $a$.
