Cosmological constraints from the eBOSS Lyman-α forest using the PRIYA simulations

Abstract We present new cosmological parameter constraints from the eBOSS Lyman-α forest survey. We use a new theoretical model and likelihood based on the PRIYA simulation suite. PRIYA is the first suite to resolve the Lyman- α forest in a (120 Mpc/h) 3 volume, using a multi-fidelity emulation technique. We use PRIYA to predict Lyman- α forest observables with ≲ 1% interpolation error over an 11 dimensional (9 simulated, 2 in post-processing) parameter space. We identify an internal tension within the flux power spectrum data. Once the discrepant data is removed, we find the primeval scalar spectral index measured at a pivot scale of k 0 = 0.78 Mpc -1 to be n P = 1.009 +0.027 -0.018 at 68% confidence. This measurement from the Lyman- α forest flux power spectrum alone is in reasonable agreement with Planck, and in tension with earlier eBOSS analyses. The amplitude of matter fluctuations is σ 8 = 0.733 +0.026 -0.029 at 68% confidence, in agreement with Dark Energy Survey weak lensing measurements and other small-scale structure probes and in tension with CMB measurements from Planck and ACT. The effective optical depth to Lyman- α photons from our pipeline is in good agreement with earlier high resolution measurements. We find a linear power at z = 3 and k = 0.009 s/km of Δ 2 L = 0.302 +0.024 -0.027 with a slope n eff = -2.264 +0.026 -0.018 . Our flux power spectrum only chains prefer a low level of heating during helium reionization. When we add IGM temperature data we find n P = 0.983 ± 0.020 and σ 8 = 0.703 +0.023 -0.027 . Our chains prefer an early and long helium reionization event, as suggested by measurements from the helium Lyman- α forest. In the near future we will use our pipeline to infer cosmological parameters from the DESI Lyman-α data.