Isolating the Lyman Alpha Forest BAO Anomaly

TL;DR

This paper investigates the origin of a 2.9 sigma tension between Lyman alpha forest BAO measurements and the Planck ΛCDM model, finding the discrepancy likely stems from dark energy evolution at redshifts 0.57 to 2.34.

Contribution

It isolates the BAO anomaly by using unanchored measurements, revealing the tension is due to dark energy evolution rather than ΛCDM parameters.

Findings

High-redshift BAO data alone shows 2.9 sigma tension with ΛCDM.

Including CMB acoustic scale increases the tension, ruling out constant dark energy.

Low-redshift BAO data reduces the tension to just over 2 sigma.

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.