Maximizing cosmological and reionization information from Lyman-α forest correlations

by Dr Molly Wolfson, Wynne Turner

Monday Sep 29, 2025, 3:00 PM → 4:00 PM Europe/Madrid

C7b/058 - Seminar (IFAE Main Building C7b)

 

Talk 1 [Turner]: One of the biggest open questions in cosmology is the nature of dark energy. Cosmological models can be constrained by measuring the expansion history of the Universe across different epochs. The Lyman-alpha (Lya) forest provides a powerful probe of the expansion history at redshifts 2 < z < 4, where cosmological parameters can be measured from its two-point correlation function through the baryon acoustic oscillation (BAO) scale and the Alcock-Paczyński (AP) effect. However, current analyses are limited by the quasar continuum fitting process, which removes information on large scales and distorts the correlation function on all scales. To address this, I developed the Lya Continuum Analysis Network (LyCAN), a convolutional neural network that predicts the forest continuum solely based on longer-wavelength spectral features. We used LyCAN continuum predictions on DESI DR1 data to perform the most precise measurement to date of the evolution of the effective optical depth. Additionally, since LyCAN predicts independently of the forest region, it preserves large-scale information valuable for cosmology. We performed cosmological forecasts in the idealized case assuming perfect knowledge of the quasar continuum and found that access to the true continuum could enable up to a ~15% improvement on the AP parameter. This improvement is analogous to increasing the Lya forest survey area by roughly 40%. Realizing this improvement will help the Lya forest distinguish between different models of dark energy at high redshift.

 

Talk 2 [Wolfson]: Some of the most important open questions in cosmology are how and when the intergalactic medium (IGM) was reionized by the first luminous sources. Reionization is thought to be complete by redshift z~5, though the thermal and ionization imprints of this epoch may persist at lower redshifts. As ionization fronts propagate during reionization, they heat the IGM and generate temperature fluctuations that deviate from a simple power-law temperature-density relation. At the same time, the ultraviolet background (UVB) evolves rapidly due to the growth and merging of ionized bubbles and exhibits spatial fluctuations prior to their overlap. Both temperature fluctuations and UVB inhomogeneities leave characteristic signatures in the Lyman-α (Lyα) forest. I will discuss measuring the one-dimensional Lyα forest flux power spectrum at large scales, where fluctuations from the IGM’s thermal state and UVB inhomogeneities are expected to contribute. I will present recent progress in quantifying possible excess power at these scales and discuss implications for constraining the timing and nature of reionization-era processes.