The temperature of the quantum vacuum
  We propose that the quantum vacuum may be considered as a gas of virtual photons which carry a non-vanishing linear momentum as well as a non-vanishing energy. We study, in particular, the Casimir effect in order to show that these virtual photons should satisfy a Fermi-Dirac statistics, which implies a non-zero temperature of the vacuum state. 
Adonai S. Sant'Anna and Daniel C. Freitas (Department of Mathematics,   Federal University at Parana)