Gravitational polarization tensor of thermal λφ4theory

The low-momentum structure of the gravitational polarization tensor of an ultrarelativistic plasma of scalar particles with λ${\mathrm{\phi }}^4$ interactions is evaluated in a two-loop calculation up to and including order ${\lambda }^{3/2}$. This turns out to require an improved perturbation theory which resums a local thermal mass term as well as nonlocal hard-thermal-loop vertices of scalar and gravitational fields. © 1996 The American Physical Society.