The Effective Quintessence from String Landscape

The quintessence-like potential of vacuum energy can meet requirement from quantum gravity as well as the accelerating expansion of the universe. Since string theory only admits vacuum with negative energy density, one needs to lift the anti-de Sitter vacuum to the meta-stable de Sitter vacuum with positive vacuum energy density to explain the accelerating expansion of the universe. However the two criterion about the swampland conjecture destroys the effort. Based on the possible large scale Lorentz violation, we define an effective cosmological constant which depends not only on the bare cosmological constant known as the vacuum energy density but also on the Lorentz violation effect. We find the evolution of the effective cosmological constant exhibits the behavior of quintessence potential driving universe when the bare cosmological constant is from string landscape regime while the effective cosmological constant exhibits local minimum during evolution which certainly is caused by a meta-stable de Sitter potential in the swampland regime. The critical value of of the transition for the effective cosmological constant is approximately zero. The frozen large scale Lorentz violation can uplift the AdS vacua to an effective quintessence-like one in this sense.