Equilibrium boundary conditions, dynamic vacuum energy, and the big bang

The near-zero value of the cosmological constant Λ in an equilibrium context may be due to the existence of a self-tuning relativistic vacuum variable q. Here, a cosmological nonequilibrium context is considered with a corresponding time-dependent cosmological parameter Λ(t) or vacuum energy density ρ_V(t). A specific model of a closed Friedmann-Robertson-Walker universe is presented, which is determined by equilibrium boundary conditions at one instant of time (t=t_eq) and a particular form of vacuum-energy dynamics (dρ_V/dt∝ρ_M). This homogeneous and isotropic model has a standard big bang phase at early times (t≪t_eq) and reproduces the main characteristics of the present universe (t=t₀<t_eq).