Cosmo-26

Learning CMB-independent reionisation constraints from high-redshift quasars

Not scheduled

20m

Gorlaeus gebouw (Leiden University)

Talk Methods / Statistical Inference / Machine Learning

Speaker

Timo Kist (Leiden Observatory)

Description

Recent evidence for evolving dark energy from DESI BAO measurements has been shown to be degenerate with the optical depth to reionisation tau_CMB and could be absorbed in a value exceeding Planck's legacy measurement of tau_CMB ~ 0.05. Gaining a true understanding of the Epoch of Reionisation therefore remains one of the pivotal tasks of modern cosmology, and next-generation telescopes such as EUCLID and JWST are opening up the path to the first CMB-independent precision constraints on tau_CMB derived from the Lyman-alpha damping wing signature imprinted by the foreground neutral intergalactic medium (IGM) on the spectra of high-redshift quasars.

We developed a new simulation-based inference framework – coded fully differentiably in the machine learning framework JAX – to disentangle the IGM damping wing from a quasar's unknown intrinsic spectrum and infer its lifetime as well as two physical measures of the local ionisation topology in front of the source: the HI column density and its distance to the first neutral patch. Our pipeline accounts for all relevant modelling uncertainties, caused by IGM transmission fluctuations, quasar continuum reconstruction, and spectral noise. Enabled by a normalising flow model as neural likelihood estimator, our framework is the first that harnesses the full-resolution spectral information, including the highly non-Gaussian pixels blueward of the Lyman-alpha line. By analysing higher-order statistics of synthetic flow-generated spectra, we demonstrate that our flow model has truly learned non-Gaussian information, significantly tightening the resulting parameter constraints.

Based on realistic mock spectra resembling the distribution of upcoming quasars found by Euclid, we show that we will soon be able to robustly constrain the evolution of the IGM neutral fraction at the < 5% level between 6 < z < 10. We present the first such constraints covering the redshift range 5.75 < z < 7.7 based on 41 archival spectra and 27 new high-quality JWST/NIRSpec spectra from our currently running 94-hour Cycle 4 program, constraining both the global timing and the local ionisation topology near these objects and placing a robust Planck-independent constraint on tau_CMB.