The Negative Baryon Acoustic Oscillation Shift in the Lyα Forest from Cosmological Simulations

We present the first measurement of the Ly alpha forest baryon acoustic oscillations (BAO) shift parameter from cosmological simulations. In particular, we generate a suite of 1000 accurate effective field-level bias-based Ly alpha forest simulations of volume V=(1h(-1)Gpc)(3) at z = 2, both in real and redshift space, calibrated upon two fixed-and-paired cosmological hydrodynamic simulations. To measure the BAO, we stack the 3D power spectra of the 1000 different realizations, compute the average, and use a model accounting for a proper smooth-peak component decomposition of the power spectrum, to fit it via an efficient Markov Chain Monte Carlo scheme estimating the covariance matrices directly from the simulations. We report the BAO shift parameters to be alpha=0.9969(-0.0014)(+0.0014) and alpha=0.9905(-0.0027)(+0.0027) in real and redshift space, respectively. We also measure the bias b(lya) and the BAO broadening parameter Sigma(nl), finding b(lya)=-0.1786-0.0001+0.0001 and Sigma nl=3.87-0.20+0.20 in real space, and b(lya)=-0.073-0.004+0.005 and Sigma(nl)=6.55(-0.22)(+0.23 )in redshift space. Moreover, we measure the linear Kaiser factor beta(lya)=1.39(-0.18)(+0.24 )from the isotropic redshift space fit. Overall, we find evidence for a negative shift of the BAO peak at the similar to 2.2 sigma and similar to 3.5 sigma levels in real and redshift space, respectively. This work sets new important theoretical constraints on the Ly alpha forest BAO scale and offers a potential solution to the tension emerging from previous observational analysis, in light of ongoing and upcoming Ly alpha forest spectroscopic surveys, such as DESI, the Prime Focus Spectrograph Survey, and WEAVE-QSO.