Periodic Reporting for period 1 - COSMO-LYA (A Cosmological Lever Arm for Fundamental Physics)
Reporting period: 2022-09-01 to 2025-02-28
Key open questions in the boundary of cosmology and fundamental physics can be addressed by studying the distribution of matter as a function of scale and time (or redshift): why is the expansion of the Universe accelerating? what are the statistical properties of the primordial universe? what is the mass of the neutrinos?
While the traditional approach has been to use large galaxy catalogues, during the last decade Andreu Font-Ribera and collaborators have pioneered the use of an alternative probe: 3D correlations in the Lyman-α (Lyα) forest, absorption features in the spectra of high-redshift quasars caused by intervening neutral hydrogen. The Lyα forest provides a unique window to study the distribution of matter at earlier times and on smaller scales than those accessible with galaxy catalogues. Using quasar spectra from the Sloan Digital Sky Survey (SDSS), in 2020 Font-Ribera and collaborators were able to measure (with a 2% uncertainty) the expansion of the young Universe, 11 billion years ago. The technique used in this analysis is known as Baryon Acoustic Oscillations (BAO), and it has been the main science driver behind the design of the Dark Energy Spectroscopic Instrument (DESI), an international collaboration with over 70 participating institutions that aims at observing more than 40 million spectra of galaxies and quasars (10 times more than SDSS).
While the main goal of DESI is to measure BAO, the COSMO-LYA project funded by the ERC will allow Font-Ribera and collaborators to fully exploit the cosmological information contained in the quasar spectra collected by DESI. This will result in an very significant increase in the precision with which we can measure several cosmological parameters, such as the expansion of the Universe, the amount of curvature in the Universe, the sum of the neutrino masses and the statistics of the initial conditions of the Universe.
While the traditional approach has been to use large galaxy catalogues, during the last decade Andreu Font-Ribera and collaborators have pioneered the use of an alternative probe: 3D correlations in the Lyman-α (Lyα) forest, absorption features in the spectra of high-redshift quasars caused by intervening neutral hydrogen. The Lyα forest provides a unique window to study the distribution of matter at earlier times and on smaller scales than those accessible with galaxy catalogues. Using quasar spectra from the Sloan Digital Sky Survey (SDSS), in 2020 Font-Ribera and collaborators were able to measure (with a 2% uncertainty) the expansion of the young Universe, 11 billion years ago. The technique used in this analysis is known as Baryon Acoustic Oscillations (BAO), and it has been the main science driver behind the design of the Dark Energy Spectroscopic Instrument (DESI), an international collaboration with over 70 participating institutions that aims at observing more than 40 million spectra of galaxies and quasars (10 times more than SDSS).
While the main goal of DESI is to measure BAO, the COSMO-LYA project funded by the ERC will allow Font-Ribera and collaborators to fully exploit the cosmological information contained in the quasar spectra collected by DESI. This will result in an very significant increase in the precision with which we can measure several cosmological parameters, such as the expansion of the Universe, the amount of curvature in the Universe, the sum of the neutrino masses and the statistics of the initial conditions of the Universe.
The COSMO-LYA group has played a leading role in the analysis of the Early Data Release (EDR) of the Dark Energy Spectroscopic Instrument (DESI), including two publications led by members of the group (Gordon et al. 2023, Ramirez-Perez et al. 2024).
We have also played a leading role in the analysis of the First Data Release of DESI (DR1), including the devlopment of synthetic datasets used to validate the first measurement of Baryon Acoustic Oscillations (BAO, DESI Collaboration 2025).
We have also developed the first neural-network algorithm to make theoretical predictions of the 1D correlations in the Lyman alpha forest as a function of cosmological parameters (Cabayol-Garcia et al. 2023), as well as the first emulator to predict coherently the 1D and 3D correlations (Chaves-Montero et al. 2025).
We have also been working on the impact of different contaminants on the Lyman alpha correlations measured in DESI. For instance, we have conducted a detailed study of the impact of redshift errors on the 3D correlations, soon to be published (Gordon et al. 2025, submitted).
We have also played a leading role in the analysis of the First Data Release of DESI (DR1), including the devlopment of synthetic datasets used to validate the first measurement of Baryon Acoustic Oscillations (BAO, DESI Collaboration 2025).
We have also developed the first neural-network algorithm to make theoretical predictions of the 1D correlations in the Lyman alpha forest as a function of cosmological parameters (Cabayol-Garcia et al. 2023), as well as the first emulator to predict coherently the 1D and 3D correlations (Chaves-Montero et al. 2025).
We have also been working on the impact of different contaminants on the Lyman alpha correlations measured in DESI. For instance, we have conducted a detailed study of the impact of redshift errors on the 3D correlations, soon to be published (Gordon et al. 2025, submitted).
The scientific publications and the software developed by the COSMO-LYA team has already had a major impact in the analysis of the Lyman alpha dataset from the DESI collaboration, and they enable new scientific analyses with a very large discovery potential.