Thermal condensate structure and cosmological energy density of the
  Universe
  The aim of this paper is the study of thermal vacuum condensate for scalar
and fermion fields. We analyze the thermal states at the temperature of the
cosmic microwave background (CMB) and we show that the vacuum expectation value
of the energy momentum tensor density of photon fields reproduces the energy
density and pressure of the CMB. We perform the computations in the formal
framework of the thermo field dynamics. We also consider the case of neutrinos
and thermal states at the temperature of the neutrino cosmic background.
Consistency with the estimated lower bound of the sum of the active neutrino
masses is verified. In the boson sector, non trivial contribution to the energy
of the universe is given by particles of masses of the order of $10^{-4}eV$
compatible with the ones of the axion-like particles. The fractal self-similar
structure of the thermal radiation is also discussed and related to the
coherent structure of the thermal vacuum.
