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New Implications of Lyman-alpha forest for Cosmology

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
 
Final Report Year 2022

Final Report Abstract

The observation of quasar spectra with spectroscopes reveals several lines, called the Lyman-α forests, caused by the absorption of the photons emitted by the quasar along the line of sight when they cross clouds containing neutral hydrogen. There is a mapping between the frequency at which the line appears and the distance to the cloud, that is, its redshift. Thus, each quasar spectrum can be used to map the distribution of matter along the corresponding line of sight. With several quasar spectra, one gets enough information to reconstruct the statistical properties of this distribution of matter, and in particular its twopoint correlation function. The results are expressed as a “one-dimensional (1D) flux power spectrum”. The goal of this project was to obtain robust constrains on neutrino masses, cosmological inflation and extended Dark Matter (DM) models, thanks to an analysis of new Lyman-α (Ly-α) data. More specifically, the goal was to infer cosmological parameter bounds from new measurements of the 1D flux power spectrum, in combination with other cosmological data set. We collaborated actively with the French team of this joint ANR-DFG project in order to infer new bounds on neutrinos, inflation and warm dark matter from Ly-α data from the eBOSS and XQ-100 surveys – in combination with Cosmic Microwave background (CMB) and Baryon acoustic Oscillation (BAO) data. The French team led the analysis of raw data from eBOSS and XQ-100, and presented a new measurement of the 1D flux spectrum. They also supervised the running of a large initial grid of hydrodynamical simulations. We collaborated actively with them on enlarging this grid with additional parameters, and designing a robust data likelihood that takes several astrophysical effects and systematic errors into account. Finally, we led the analysis of the data in terms of Bayesian statistics, while the French team ran a frequentist pipeline for comparison. We collaborated with our French partners on the writing of the central publication of this project, “Hints, neutrino bounds and WDM constraints from SDSS DR14 Lyman-α and Planck full-survey data”, published in JCAP. This long paper describes our analysis and shows our results for neutrino masses, the running of the primordial spectral index, and the mass of DM in the case of Warm Dark Matter. These bounds improved substantially over previous ones, which brought a lot of visibility to our paper. In particular, we obtained a strong bound on the summed neutrino mass: ∑mν < 0.09 eV at the 95% Confidence level (CL) from eBOSS Ly-α + CMB + BAO data. Very interestingly, this places the inverted hierarchy neutrino mass scheme under tension. This neutrino mass bound was the strongest ever obtained when we published our result in 2020. Our result is now confirmed by another analysis based on a different combination of cosmological data (not including Ly-α data), which gives the same number. There is still no stronger bound available. Our main goal was not to obtain strong bounds but robust ones. Our limits on the neutrino and WDM mass are stronger than in previous publications thanks to better data (with ∼4 times more quasar spectra and thus ∼2 smaller statistical error bars) and despite of a more conservative treatment of several systematics. In order to avoid artificial bias in our results, we modelled the Inter-Galactic Medium (IGM) thermal history and the Active Galactic Nuclei (AGN) feedback with a more freedom than in previous works. In our publication, we provide a detailed description of our conservative modelling of systematics and astrophysical effects, and we present the result of several robustness tests. In order to meet all the objectives of the project, we also studied less obvious – but still very interesting – extensions of the standard cosmological model, featuring non-minimal DM scenarios: mixed Cold + Warm DM, DM feebly interacting with baryons, and DM feebly interacting with dark radiation. In “One likelihood to bind them all: Lyman-α constraints on non-standard dark matter”, we have derived bounds on these additional models using some other higher-resolution Ly-α data sets from the MIKE and HIRES surveys. We have met all the objectives of the project through this couple of dense publications, that have received attention and that have been presented in several conferences. The PhD student supported by this project also had time to participate actively to a significant number of additional publications connected to the inference of dark matter properties from cosmological data.

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