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 _ { 0 m } h ^ { 2 } for the spatially flat \Lambda CDM model , is in tension with the value \Omega _ { 0 m } 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 .