We calculate the expectation values of the stress–energy bitensor defined at two different spacetime points x, x' of a massless, minimally coupled scalar field with respect to a quantum state at finite temperature T in a flat N-dimensional spacetime by means of the generalized zeta-functionmethod. These correlators, also known as the noise kernels, give the fluctuations of energy and momentum density of a quantum field which are essential for the investigation of the physical effects of negative energy density in certain spacetimes or quantum states. They also act as the sources of the Einstein–Langevin equations in stochastic gravity which one can solve for the dynamics of metric fluctuations as in spacetime foams. In terms of constitutions these correlators are one rung above (in the sense of the correlation—BBGKY or Schwinger-Dyson—hierarchies) the mean (vacuum and thermal expectation) values of the stress–energy tensor which drive the semiclassical Einstein equation in semiclassical gravity. The low- and the high-temperature expansions of these correlators are also given here: at low temperatures, the leading order temperature dependence goes like T^N while at high temperatures they have a T^2 dependence with the subleading terms exponentially suppressed by e−T. We also discuss the singular behavior of the correlators in the x' → x coincident limit as was done before for massless conformal quantum fields.
Journal Of Physics A: Mathematical and theoretical 45, 374013(15pages)