Model independent evidence for dark energy evolution from Baryon
  Acoustic Oscillations
  Baryon Acoustic Oscillations (BAO) allow us to determine the expansion
history of the Universe, thereby shedding light on the nature of dark energy.
Recent observations of BAO's in the SDSS DR9 and DR11 have provided us with
statistically independent measurements of $H(z)$ at redshifts of 0.57 and 2.34,
respectively. We show that these measurements can be used to test the
cosmological constant hypothesis in a model independent manner by means of an
improved version of the $Om$ diagnostic. Our results indicate that the SDSS
DR11 measurement of $H(z) = 222 \pm 7$ km/sec/Mpc at $z = 2.34$, when taken in
tandem with measurements of $H(z)$ at lower redshifts, imply considerable
tension with the standard $\Lambda$CDM model. Our estimation of the new
diagnostic $Omh^2$ from SDSS DR9 and DR11 data, namely $Omh^2 \approx 0.122 \pm
0.01$, which is equivalent to $\Omega_{0m}h^2$ for the spatially flat
$\Lambda$CDM model, is in tension with the value $\Omega_{0m}h^2 = 0.1426 \pm
0.0025$ determined for $\Lambda$CDM from Planck+WP. This tension is alleviated
in models in which the cosmological constant was dynamically screened
(compensated) in the past. Such evolving dark energy models display a pole in
the effective equation of state of dark energy at high redshifts, which emerges
as a smoking gun test for these theories.
