  The Reissner-Nordstrom-de Sitter (RN-dS) spacetime can be considered as a
thermodynamic system. Its thermodynamic properties are discussed that the RN-dS
spacetime has phase transitions and critical phenomena similar to that of the
Van de Waals system or the charged AdS black hole. The continuous phase
transition point of RN-dS spacetime depends on the position ratio of the black
hole horizon and the cosmological horizon. We discuss the critical phenomenon
of the continuous phase transition of RN-dS spacetime with Landau theory of
continuous phase transition, that the critical exponent of spacetime is same as
that of the Van de Waals system or the charged AdS black hole, which have
universal physical meaning. We find that the order parameters are similar to
those introduced in ferromagnetic systems. Our universe is an asymptotically dS
spacetime, thermodynamic characteristics of RN-dS spacetime will help us
understand the evolution of spacetime and provide a theoretical basis to
explore the physical mechanism of accelerated expansion of the universe.
