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Baryon Acoustic Oscillations (BAO) allow us to determine the expansionhistory 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 withstatistically independent measurements of $H(z)$ at redshifts of 0.57 and 2.34,respectively. We show that these measurements can be used to test thecosmological constant hypothesis in a model independent manner by means of animproved version of the $Om$ diagnostic. Our results indicate that the SDSSDR11 measurement of $H(z) = 222 \pm 7$ km/sec/Mpc at $z = 2.34$, when taken intandem with measurements of $H(z)$ at lower redshifts, imply considerabletension with the standard $\Lambda$CDM model. Our estimation of the newdiagnostic $Omh^2$ from SDSS DR9 and DR11 data, namely $Omh^2 \approx 0.122 \pm0.01$, which is equivalent to $\Omega_{0m}h^2$ for the spatially flat$\Lambda$CDM model, is in tension with the value $\Omega_{0m}h^2 = 0.1426 \pm0.0025$ determined for $\Lambda$CDM from Planck+WP. This tension is alleviatedin models in which the cosmological constant was dynamically screened(compensated) in the past. Such evolving dark energy models display a pole inthe effective equation of state of dark energy at high redshifts, which emergesas a smoking gun test for these theories.

Model independent evidence for dark energy evolution from Baryon Acoustic Oscillations