  The perfect Planck spectrum of the observed cosmic microwave background
radiation indicates that our universe must be in thermal equilibrium. The dark
sector of the universe should also be in the same equilibrium state with dark
matter and dark energy coupled to each other and emits gravitational phonon
blackbody radiation which is the main component of the cosmic background
radiation. In the radiation-dominated era such gravitational radiation should
be the majority species of the cosmic medium. Instead of the ideal fluid
assumed by the standard cosmological model LCDM, the universe has to be taken
as a thermodynamic system consisting of gravitationally connected dark energy
and matter. Besides particle dynamics, statistical thermodynamics is also
necessary for understanding the cosmological constitution and evolution
history. As an alternative to LCDM we constructed a dark-energy-matter-coupled
(DEMC) cosmological model. Based on the relativistic mass-energy relation,
conservation law of energy, Lagrange's equation with variable potential
function, mean-field theory of continuous phase transition, and the symmetry
principle of the kinetic coefficients, we deduced dynamic equations of the
expansion of a DEMC universe with three parameters. These equations reproduce
the observed history of the rate of expansion of our universe.
