New Implications of Lyman-Alpha forests for Cosmology – NILAC

The project revolves around 5 tasks.

Task 1 takes care of the project coordination. Because the collaboration includes two institutions (CEA-Saclay in France and TWTH in Germany), we plan to have regular meetings to exchange on the advances of the project.

Task 2: Lya data analysis
The aim of this task is to measure the 1D (i.e. along quasars line-of-sight) Lya flux power spectrum in 13 redshift bins ranging from z=2.2 to 4.6. We will use 170 000 (BOSS) and about 40 000 (eBOSS) z>2.1 quasars, complemented by the 1D Lya power spectrum at smaller scales from 100 quasars at redshifts between 3.5 and 4.5, observed by XQ-100 on the VLT. We will further extend the measurement to higher redshift (and small scales) using the first data from the DESI project. Valuable cosmological results can be derived from any combination of these subsets.

Task 3: Neutrino masses
To constrain neutrino masses, we use dedicated hydrodynamical simulations that vary cosmological parameters in presence of a free-streaming component. We build a second-order likelihood from these simulations, adding nuisance parameters to model systematic effects, whether of instrumental or astrophysical origin. We also study the impact of baryonic feedback on galactic scales using a separate set of dedicated simulations.

Task 4: inflation
We will define a parameterization of the primordial power spectrum allowing for a departure from a scale-invariant spectral index on small scales.On the analysis side, we will study jointly Lya with Planck CMB data.

Task 5: constraints on dark matter particles
The major goal of this task is to address the question of dark matter (DM) under a new angle. Task 5 is the main scientific objective of the developments undertaken in Tasks 2 and 3. It is split into two parts: task 5.1 where we will produce extended grids of hydrodynamical simulations to tackle this issue, and task 5.2 where the grids will be used to derive constraints on each scenario.

The project team derived the most precise measurement to-date of the 1D Lya flux power spectrum, improving upon previously published result (from the DR9 release in 2013), both in precision (achieving a reduction by a factor of two) and in redshift coverage. We also performed a thorough investigation of the identified sources of systematic uncertainties that affect the measurement. The resulting power spectrum is in excellent agreement with the one from the DR9 data. This new measurement was presented in a paper, now published in JCAP.

This measurement was used to compute the linear 3D matter power spectrum at redshift 0. It was also the main input to an important collaborative study of the two partners of this project leading to the most stringent constraint to-date on the sum of the neutrino masses, setting an upper limit of 0.11 eV at the 95% confidence level using the combination of Lyman-a and Planck (temperature and polarisation) data. This limit tightens to 0.09 eV when adding CMB lensing and BAO data. The paper presenting these results was submitted to JCAP and is currently under review by the referee.

There have been two outreach highlights during this first period.
1. irfu.cea.fr/Phocea/Vie_des_labos/Ast/ast.php: Le spectrographe Desi ouvre ses 5000 yeux sur le cosmos pour traquer l’energie noire, (October 28, 2019) announces the first light of the DESI instrument. Following this event, the DESI instrument started its commissioning period.
2. irfu.cea.fr/Phocea/Vie_des_labos/Ast/ast.php: L’étau se resserre autour des neutrinos du cosmos (December 20, 2O19) presents the measurement of the 1D Lyman-a power spectrum analysis and its cosmological implications in terms of constraint on the neutrino mass and on the nature of warm dark matter. This highlight is a summary of papers 1 and 3 below.

During this 18-month period, the team published three papers related to the NILAC project:
1. Lyman-a data power spectrum: The one-dimensional power spectrum from the SDSS DR14 Ly forests, Chabanier, Palanque-Delabrouille, Yèche et al., JCAP 07, 017 (2019)
2. Matter power spectrum: from Ly forest to CMB scales, Chabanier, Millea & Palanque-Delabrouille, MNRAS 489, 2247 (2019)
3. Hints, neutrino bounds and WDM constraints from SDSS DR14 Lyman-a and Planck full-survey data, Palanque-Delabrouille, Yèche, Schoneberg, Lesgourgues, Walther et al., arXiv:1911.09073

Cosmology has entered an era of high precision, thanks to Cosmic Microwave Background (CMB) and Large Scale Structure (LSS) observations. The ?CDM model has been confirmed as a valid description of cosmological data on the largest scales, within experimental errors. Still, some issues remain unsolved or not completely understood, 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-a (thereafter Lya) forest. On the one hand, we will analyze the largest sample of medium-resolution quasar spectra available to date (from the BOSS and eBOSS experiments just completed by the end of this proposal), complemented by the most recent set of high-resolution quasar spectra (XQ-100 from VLT as well as first data from DESI), to measure with percent-level precision the Lya 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 Lya 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 (WDM) 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 Lya 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, which predicts M? = 0.11eV given the latest data of neutrino oscillations, for neutrino masses.

Finally, Lya forest provides a very good opportunity to check the shape of the matter power spectrum on sub-CMB scales. Recent results indicate a small tension between CMB and Lya 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.