  We show local and global well-posedness results for the Hartree equation
$$i\partial_t\gamma=[-\Delta+w*\rho_\gamma,\gamma],$$ where $\gamma$ is a
bounded self-adjoint operator on $L^2(\R^d)$, $\rho_\gamma(x)=\gamma(x,x)$ and
$w$ is a smooth short-range interaction potential. The initial datum
$\gamma(0)$ is assumed to be a perturbation of a translation-invariant state
$\gamma_f=f(-\Delta)$ which describes a quantum system with an infinite number
of particles, such as the Fermi sea at zero temperature, or the Fermi-Dirac and
Bose-Einstein gases at positive temperature. Global well-posedness follows from
the conservation of the relative (free) energy of the state $\gamma(t)$,
counted relatively to the stationary state $\gamma_f$. We indeed use a general
notion of relative entropy, which allows to treat a wide class of stationary
states $f(-\Delta)$. Our results are based on a Lieb-Thirring inequality at
positive density and on a recent Strichartz inequality for orthonormal
functions, which are both due to Frank, Lieb, Seiringer and the first author of
this article.
