Isolating the Lyman Alpha Forest BAO Anomaly

Abstract

A 2.5-3 sigma discrepancy has been reported between the baryonic acoustic oscillation peak (BAO) in the Lyman $\alpha$ forest at $z\sim 2.34$ and the best fit Planck $\Lambda$CDM cosmology. To isolate the origin of the tension, we consider unanchored BAO, in which the standard BAO ruler is not calibrated, eliminating any dependence on cosmology before redshift $z\sim 2.34$. We consider BOSS BAO measurements at $z\sim 0.32$, 0.57 and 2.34, using the full 2-dimensional constraints on the angular and line of sight BAO scale, as well as isotropic BAO measurements by 6dF and SDSS at $z\sim 0.106$ and $z\sim 0.15$. We find that the $z>0.43$ data alone is in 2.9 sigma of tension with $\Lambda$CDM with or without the Planck best fit values of the mass fraction $\Omega_m$ and the BAO scale $r_d H_0$, indicating that the tension arises not from the $\Lambda$CDM parameters but from the dark energy evolution itself at 0.57<z<2.34. This conclusion is supported when the acoustic scale measured by the CMB is included, which further increases the tension and excludes a solution with a constant dark energy equation of state. Including the low $z$ BAO data, which is itself consistent with $\Lambda$CDM, reduces the tension to just over 2 sigma, however in this case a CPL parametrization of the dark energy evolution yields only a modest improvement.