Dynamical Cosmological Constant from A Very Recent Phase Transition

Observation indicates that the expansion of the Universe is accelerating and favours a dynamical cosmological constant, \Lambda(t). We consider the possibility that this is due to a scalar field which has undergone a very recent phase transition. We study a simple class of model, corresponding to a \phi^4 potential with a time-dependent mass squared term. For the models considered the phase transition occurs at a red shift z \leq 1.2. The evolution of the equation of state \omega_{\phi} and energy density \rho_{\phi} with time is distinct from existing dynamical \Lambda models based on slowly rolling fields, with \omega_{\phi} and \rho_{\phi} rapidly changing in a characteristic way following the transition. The \phi energy density is composed of a time-dependent vacuum energy and coherently oscillating condensate component with a negative pressure. The condensate component will collapse to form non-topological soliton lumps, '\phi-axitons', which smoothly populate the Universe.