Speaker: Andre Fuzfa, GAMASCO, University of Namur (FUNDP) and LUTH, Observatory of Paris.
Abstract: Coupling dark matter (DM) to dark energy (DE) is one of the most promising way to build a unified description of the invisible sector of cosmology. However, such DM-DE couplings make the mass of the DM particles varying, therefore breaking the universality of free fall (Galileo's equivalence principle). Doing so, the strong equivalence principle, stating the universality of gravitational binding energy, does not hold anymore, particularly where DM is profuse like in the large-scale universe. Such mass-varying DM particles therefore constitute an Abnormally Weighting type of Energy (AWE Hypothesis) and their cosmological abundance induces modifications of gravity on large-scales, modifications that can explain cosmic acceleration. I will present here how this AWE hypothesis can be naturally achieved in terms of the explicit symmetry breaking of a global U(1) symmetry in particle physics. In this context, the U(1)-charged complex scalar usually splits into a mass-varying pseudo-Nambu-Goldstone boson (axion-like DM particles) while the vacuum expectation value of the complex scalar has to be stabilised, usually through the use of the potential (spontaneous symmetry breaking mechanism). We show here that the cosmological relaxation toward the equivalence principle can play this crucial stabilising role and explain in addition the observed cosmic acceleration. Stabilisation of the new scalar is achieved through non-minimal gravitational couplings. We will show several remarkable cosmological predictions of this idea and emphasize some interesting applications on neutrino physics for the gravitational symmetry breaking of lepton number symmetry.