  We consider thermodynamics of the Universe within a quasiparticle approach
where the collective dynamics of a system is governed by the thermal mass of
the constituents. The spacetime dependence of this thermal mass leads to a
negative contribution to the pressure and a positive contribution to the energy
density, similar to the effect due to dark energy. We propose a mechanism based
on thermodynamic arguments to quantify this contribution from the thermal
vacuum of the system. For a sufficiently large spacetime variation of the
thermal mass, the effective pressure can become negative and could mimic a dark
energy equation of state. We validate our framework using results from
renormalizable interacting scalar field theory and demonstrate an application
on QCD axion.
