A Model of Dark Energy and Dark Matter

A dynamical model for the dark energy is presented in which the ``quintessence'' field is the axion, $a_Z$, of a spontaneously broken global $U(1)_{A}^{(Z)}$ symmetry whose potential is induced by the instantons of a new gauge group $SU(2)_Z$. The $SU(2)_Z$ coupling becomes large at a scale $\Lambda_Z \sim 10^{-3} eV$ starting from an initial value $M$ at high energy which is of the order of the Standard Model (SM) couplings at the same scale $M$. A perspective on a possible unification of $SU(2)_Z$ with the SM will be briefly discussed. We present a scenario in which $a_Z$ is trapped in a false vacuum characterized by an energy density $\sim (10^{-3} eV)^4$. The lifetime of this false vacuum is estimated to be extremely large. Other estimates relevant to the ``coincidence issue'' include the ages of the universe when the $a_Z$ potential became effective, when the acceleration ``began'' and when the energy density of the false vacuum became comparable to that of (baryonic and non-baryonic) matter. Other cosmological consequences include a possible candidate for the weakly interacting (WIMP) Cold Dark Matter as well as a scenario for leptogenesis. A brief discussion on possible laboratory detections of some of the particles contained in the model will also be presented.