Cosmology

Cosmic microwave background (CMB) experiments, which currently provide some of the most powerful cosmological data sets, will become much more constraining in the near future. While these measurements promise to teach us more about the nature of dark energy, inflation and neutrino physics, increased precision will require special attention dedicated to the data analysis. In this talk I will focus on the gravitational lensing of the CMB and some of its implications. By introducing a novel analysis technique, applying it to the Planck satellite data and commenting on improvements which will be possible with a CMB Stage 4 experiment, I will first show how we can utilize CMB gravitational lensing to probe self-consistency of the CMB data sets. Then I will overview how gravitational lensing induces non-Gaussian covariances between the CMB data and how these covariances affect constraints on the cosmological parameters.

The observables of the large-scale structure such as galaxy number density generally depends on the density environment (of a few hundred Mpc). The dependence can traditionally be studied by performing gigantic cosmological N-body simulations and measuring the observables in different density environments. Alternatively, we perform the so-called "separate universe simulations", in which the effect of the environment is absorbed into the change of the cosmological parameters. For example, an overdense region is equivalent to a universe with positive curvature, hence the structure formation changes accordingly compared to the region without overdensity. In this talk, I will introduce the "separate universe mapping", and present how the power spectrum
and halo mass function change in different density environments, which are equivalent to the squeezed bispectrum and the halo bias, respectively. I will then discuss the extension of this approach to inclusion of additional fluids such as massive neutrinos. This allows us to probe the novel scale-dependence of halo bias and squeezed bispectrum caused by different evolutions of the background overdensities of cold dark matter and the additional fluid. Finally, I will present one application of the separate universe simulations to predict the squeezed bispectrum formed by small-scale Lyman-alpha forest power spectrum and large-scale lensing convergence, and compare with the measurement from BOSS Lyman-alpha forest and Planck lensing map.

The standard structure formation model is based on the Cold Dark Matter (CDM) hypothesis where non-gravitational dark matter interactions are irrelevant for the formation and evolution of galaxies. Surprisingly, current observations allow for significant departures from the CDM hypothesis,

which could potentially leave signatures of the dark matter particle nature in the properties of galaxies. In this talk, I will describe a framework we have proposed that generalizes the theory of structure formation (in both the linear and non-linear regimes) to include new dark matter physics in order to explore galaxy formation and evolution in the broadest sense.

In this talk I will present new insights on a microscopic holographic theory for de Sitter space. I will focus on the static patch of dS, which describes our universe to a good approximation at late times. We use a conformal map between dS and the BTZ black hole times a sphere to relate the general microscopic properties of dS to those of symmetric product CFTs. In 2d CFT language, de Sitter space corresponds to a thermal bath of long string. The long string phenomenon exhibited by these CFTs implies that the excitation energy decreases at large distances in dS (contrary to the UV/IR relation in AdS/CFT). It also explains the smallness of the vacuum energy of dS. Similar results apply to Minkowski space and AdS below its curvature radius.

The Standard Model of particle physics and its implications for cosmology leave several fundamental questions unanswered, including the strong CP problem and the origins of neutrino masses, dark matter, and dark energy. Previous directions of model building beyond the Standard Model have usually focused on new high-energy physics. As an alternative direction, we have developed a class of low-energy neutrino mass and axion models at a new infrared gravitational scale, which is numerically coincident with the scale of dark energy. In this seminar, I will present this novel class of models and expand on the aspect of gravitational neutrino mass generation. My talk will also cover the wide-ranging cosmological and phenomenological model predictions, in particular the invalidity of the cosmological neutrino mass bound, enhanced neutrino decays, and soft topological defects. 

After more than 12 years of continuous data taking, the Pierre Auger Observatory has collected the largest dataset of ultra-high energy cosmic rays (UHECR) to date. The results obtained in the last years include, for example, precise and accurate measurements of the UHECR flux across a few decades in energy, revealing distinctive spectral features that can bring valuable information on different astrophysical processes like: the transition from galactic to extragalactic fluxes; the different energy loss processes to which ultra-relativistic charged particles are subject during their propagation; the energetics of the production and acceleration of particles at the candidate sources. In this talk, I should however focus on a particular observational probe, that is, the small levels of anisotropy in the flux of UHECR at different angular scales: from the small and intermediate ones, important for the identification of possible point sources, to the large angular scales, usually used to search for signs of the galactic to extragalactic transition. In particular, special attention will be devoted to the first observation of a large scale anisotropy signal at energies above 8x10^18 eV recently reported by the collaboration.