General Overview

The problem of the formation of the galaxies and the large scale structure of the Universe requires a non barionic component (the dark matter) of the matter in the Universe. We are carrying out a research program where we use the correlation functions (spatial and angular) of galaxies and clusters of galaxies, the study of the cosmological nucleosynthesis and of the cosmic background anisotropies, and the time evolution models of structures, in order to identify the microscopical parameters (mass, spin, chemical potential) of the dark matter.

Gravitational Collapse in the Expanding Universe

In the last developments of our project in Cosmology and LSS we have found, in the linear approximation, that a distribution of fermions in the expanding Universe decreases its minimum collapsing mass as the chemical potential of them increases.
The Hot Dark Matter models based upon massive neutrinos have found great difficulties for explaining the large scale structure of the Universe. Classically only large masses of neutrinos (their greatest Jeans masses) can survive to the free streaming damping process. At late stages of the history of the Universe when they become selfgravitating they have not enough time for collapsing before present time. Therefore the creation of galactic structures in a 'top-down' scenario of large scale structure formation it should not allowed.
How those models are again interesting after the first experimental evidences of neutrino mass and because of the discovery of the 'instability shell' below the Jeans length, in which the fermion density perturbations smaller than Jeans length survive to the collisionless damping.
We are now starting to study this phenomenon integrating numerically the exact equations.

The Role of the Chemical Potential in LSS formation

Participants: Massimiliano Lattanzi, Remo Ruffini, Gregory Vereshchagin

We study the evolution of a gas of massive light neutrinos (m_n <10 eV) in an expanding Friedmann Universe and, using the properties of self-similarity of the distribution function, obtain an expression for the neutrino gas energy density at the present time as a function of masses and chemical potentials of the electronic, m and t neutrinos, for a total of six independent parameters. Then, using the constraints imposed by particle physics and cosmological nucleosyntehsis, we express this energy density as a function of three independent parameters only, namely a common mass and the e and m neutrino chemical potentials. Finally we take for the neutrino mass a value of 2 eV and show how, by varying the e and m neutrino chemical potential beetween the values allowed by cosmological nucleosynthesis, we can obtain for the neutrino +antineutrino density the critical value W=1.
Using WMAP data we find joint constraints on the mass and the chemical potential of light neutrinos. We find that WMAP data alone cannot firmly rule out scenarios with a large lepton number; moreover, a small preference for this kind of scenarios is found.

Dark Energy

Participants: Vahe Gurzadyan, Harutyun Khachatryan, Gregory Vereshchagin, Gegham Yegoryan, She-Sheng Xue

We study cosmological implications of the formula for Dark Energy, derived by Gurzadyan and Xue, which fits the observed value of dark energy density parameter. This formula implies possible variation of physical constants such as speed of light and gravitational constant. Some symmetry and underlying invariance within these models is revealed.