New Implications of Lyman-alpha forest for Cosmology

• Applicant: Professor Dr. Julien Lesgourgues
• Subject Area: Astrophysics and Astronomy ; Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: from 2017 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 391265003

Cosmology has entered into an era of high precision, thanks to Cosmic Microwave Background (CMB) and Large Scale Structure (LSS) observations. While the Lambda-CDM model has been confirmed as a valid description of cosmological data on the largest scales, some issues remain unsolved, like the nature of Dark Matter, the mechanism responsible for the acceleration of the expansion, the exact role and behavior of neutrinos in cosmology, the energy scale of inflation and its connection with a fundamental theory. The objective of this proposal is to address these issues both on the experimental and on the theoretical fronts, via the information embedded in Lyman- alpha forests. On the one-hand, we will analyze the largest sample of medium-resolution quasar spectra available to date (from the BOSS and eBOSS projects), complemented by the most recent set of high- resolution quasar spectra (from VLT), to measure with percent-level precision the Lyman-alpha flux power spectrum at redshifts between 2.0 and 4.6, and over scales ranging from sub-Mpc to hundreds of Mpc. On the other hand, we will run state-of-the-art hydrodynamical simulations that incorporate the ingredients of various cosmological models that we want to test, and we will confront them to the Lyman- alpha data to determine the parameters that best reproduce the observations. With data to high redshift, we will have access to early stages of the non-linear evolution of structures, best suited to capture the sharp cut-off that warm dark matter impacts on the matter power spectrum. With the large lever-arm in scales, we will also be able to test the smoother cut-off that appears, for instance, in mixed-dark matter or resonantly-produced sterile neutrino scenarios, and we will constrain a variety of models with non-standard dark matter such as interacting dark matter or axions. The measurement of the power spectrum on sub-Mpc scales will allow us to address the intrinsic degeneracy between the temperature properties of the intergalactic medium and the cosmological parameters such as scalar spectral index or neutrino mass. Beyond the question of the nature of dark matter, we will therefore use our unique Lyman-alpha data sample, complemented by the latest Planck measurement of the cosmic microwave background anisotropies, to obtain the tightest constraint on neutrino mass, reaching the limit where we can confirm or exclude the minimal Inverted Hierarchy scenario for neutrino masses (predicting Mnu = 0.11eV given the latest data of neutrino oscillations). Finally, Lyman-alpha forests provide a very good opportunity to check the shape of the matter power spectrum on sub- CMB scales. Recent results indicate a small tension between Lyman- alpha and CMB measurements, which, if confirmed, would have interesting consequences for inflation theories, or point to new physics such as modified gravity or interactions between dark matter and other components.

• DFG Programme: Research Grants
• International Connection: France
• Cooperation Partner: Nathalie Palanque-Delabrouille, Ph.D.