Large Scale Structure
Theories of the origin and the evolution of the universe predict the statistical properties of the dark matter distribution on the largest scales. So if we could measure the dark matter clustering we could learn about the Universe composition, its evolution and even about inflation. In particular the shape of the power spectrum contains information about cosmological parameters such as matter density , baryon density, n, dn/d lnk. Higher-order correlations can be used to test the nature of the initial conditions such as non gaussianity. While we cannot easily observe the dark matter distribution directly and the galaxy distribution could be a biased tracer of the dark matter distribution, higher-order correlations can be used to measure this bias. I have developed the bispectrum technique and I have applied it to the 2 degree field (2dF) galaxy redshift survey to constrain the bias parameter: 2dF galaxies are faithful tracers of the dark matter distribution. The bispectrum analysis of the Baryon Oscillation Spectroscopic Survey (BOSS) galaxies shows instead that these are highly biased tracers with a complex relation to the dark matter distribution. I have used galaxy surveys data (2dF and SDSS) in combination with other data sets to constrain cosmological parameters. I have also investigated the potential of large scale structures to constrain the nature of the initial conditions. I have been part of the BOSS survey (Sloan Digital Sky Survey III) and I am now involved with Euclid and DESI.
Cosmic Microwave background
The big bang theory predicts that the early universe was hot and dense and an almost perfect blackbody. Thus the universe should be filled by this radiation called the cosmic microwave background (CMB). The CMB was emitted 130000 years after the big bang, thus by looking at the CMB we see the very early universe. The statistical properties of the small CMB temperature fluctuations can tell us about the early universe, the universe age, geometry and composition. I was involved with the analysis of WMAP data. These data confirmed the standard cosmological model. The dark matter is cold and collisionless, the sum of neutrino masses has to be less than 1.8 eV. The dark energy is consistent with being a cosmological constant. The primordial power spectrum has a spectral slope slightly red (ns ~0.96), as predicted in the simplest inflationary scenarios. The data allow one to put some constraints on specific inflationary models.
Recently the Planck mission produced improves maps of the CMB, both in temperature and polarization. While these data broadly speaking confirm the standard cosmological model , some puzzling discrepancies may be surfacing. I am interested in wether that is a signature of low level systematics or the first hint of new physics. The polarization signal of the CMB is the next frontier, as it can offer key information about early Universe and high energy physics. many order of magnitude above the energy scale tested by particle accelerators on Earth. Unfortunately foreground emission, mostly form our own galaxy, dominates the signal and must be cleaned. This the the great challenge ahead.
Recent observations indicate that something is making the expansion of the Universe accelerate. What is it? A cosmological constant seems to fit the data well so far, but it is important to explore other options such as slowly rolling scalar fields (quintessence) and possible modifications of the law of gravity on very large scales, tricking us into thinking that the Universe is accelerating. We should devise new observations and ways to interpret current observations to learn more about the nature of dark energy. The study of dark energy is interesting, to me, for two reasons. An accelerated expansion (similar to the present day one but much much faster) might have happened shortly after the big bang (a theory known as inflation) and generated the primordial perturbations. Can the two accelerations be driven by the same physics? The other reason is that in both inflation and dark energy I think the breakthrough will come from a close collaboration with theoretical particle physicists.
What is the nature of the initial conditions? knowledge of their detailed statistical properties will help explain the mechanism that originates them. The nature of initial conditions leaves specific but subtle signatures in the late-time Universe.