The Intergalactic Medium as a Cosmological Probe

Principal Investigator: Matteo Viel


  • Astrophysical Probes of Fundamental Interactions
  • Structures in the Universe

Abstract: This line of research aims at exploiting the cosmological role of the intergalactic medium (IGM) in standard and non-standard cosmological models, by using state-of-the-art data sets and hydro-dynamical simulations of structure formation. The IGM is a unique probe of linear matter perturbations at intermediate and small scales (0.1-100 comoving Mpc/h) and at redshifts z=2-6. In recent years, the IGM has also been used: to probe the geometry of our Universe by measuring Baryonic Acoustic Oscillations in the 3D transmitted flux; to set constraints on the nature of dark matter and the total neutrino mass; to find a 4 sigma evidence for a dipole-like variation in α (the fine-structure constant) across the sky at the level of 10 ppm; to provide constraints on the D/H ratio and on the amount of baryons in the Universe. All these findings have deep and rich implications for structure formation, theoretical particle physics, and experimental dark matter searches, topics that are all supported by the IFPU. Furthermore, there is a large degree of complementarity between the IGM and other cosmological probes which are sensitive to other scales and redshifts like the Cosmic Microwave Background (CMB), weak gravitational lensing, cluster number counts. Under the data point of view, the investigation of the IGM has mainly developed along these three different lines: collection of a large number (hundred of thousands) of low signal-to-noise low resolution quasar spectra; collection of few hundred high-resolution high signal-to-noise quasar spectra; collection of intermediate sets of medium resolution spectra. The first approach has been followed by SDSS-III (BOSS) in order to measure the BAO peak and the neutrino mass, with a sample in which errors are dominated by systematic effects rather than statistical. The second avenue has been pursued by different groups in order to constrain the nature of dark matter over small scales or measure the variations of fundamental constants using heavy element absorption lines; the last approach has been mainly used to extend observations to the high-redshift domain, to reduce the astrophysical and instrumental nuisances and as an overall consistency check between the other approaches. In this respect, the high resolution spectrograph ESPRESSO, which is currently taking data and in which our team is involved, is expected to: 1) soon provide an increase in the accuracy of the measurement of two fundamental constants (fine-structure and proton-to-electron mass ratio) compared to VLT/UVES or Keck/HIRES clarifying the controversy; 2) hit a new regime in probing the small scale structure of metal and Lyman-alpha forest lines. Both these scientific efforts will be instrumental and important in paving our way to the E-ELT, the next milestone, in which the italian community is deeply involved. Concerning cosmological simulations, the strategy in the last few years has been twofold: on the one hand to gradually increase the resolution of the simulations of the cosmic web in order to achieve numerical convergence for the desired quantities (most importantly the flux power spectrum); on the other hand several efforts have been placed in increasing the physical modelling of the IGM by incorporating effects like galactic feedback (Active Galactic Nuclei and winds) and radiative transfer. Additionally, simulations have been instrumental in producing mock data sets with the desired properties, resembling as closely as possible the observed data sets, including observational and instrumental procedures such as continuum fitting, accurate modelling of the spectrograph resolution, etc. Methodologies have also developed significantly in the last few years. In particular, exploring cosmological, astrophysical, and nuisance parameter space has proven to be difficult and time consuming even with standard Bayesian Monte Carlo Markov Chain analysis. Emulators of the Lyman-alpha forest flux power spectrum and/or interpolation techniques that aim at getting a prediction of the desired quantity in the multi-dimensional parameter space using the results of hydro-dynamical simulations have been developed and tested. Also different cosmological models in which either the nature of dark matter is modified or the dark energy is in the form of quintessence has also been compared to the observational data.

Description: The IFPU will allow us to further strengthen existing interactions between researchers, to expand those collaborations further, and to develop along the following lines: 1) the creation of new data sets in which systematic and statistical errors are accurately quantified, in particular taking advantage of new state-of-the-art high resolution spectrographs (e.g. ESPRESSO@VLT). 2) extensive analysis of models beyond LCDM, in particular warm dark matter, sterile neutrinos, fuzzy dark matter models, and quintessence models. 3) new updated measurements of the neutrino mass from high resolution and low resolution data sets using the Lyman-alpha forest. 4) measurement of the astrophysical parameter space obtained from the analysis of HI and metal lines and in particular constraints on the thermal history, radiative transfer effects and galactic feedback, with a strong impact on reionization studies. 5) accurate measurements of the parameters associated with narrow heavy element absorption lines observed in the spectra of distant quasars. 6) tests on homogeneity and isotropy of the Universe using HI data. 7) new measurements of D/H using absorption line systems and derived constraints on Omega_baryon.