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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, Λ(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 ϕ^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 ω_ϕ and energy density ρ_ϕ with time is distinct from existing dynamical Λmodels based on slowly rolling fields, with ω_ϕ and ρ_ϕ rapidly changing in a characteristic way following the transition. The ϕenergy 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, 'ϕ-axitons', which smoothly populate the Universe.