Hierarchy of double-time correlations

The hierarchy of correlations is an analytical approximation method which allows us to study non-equilibrium phenomena in strongly interacting quantum many-body systems on lattices in higher dimensions. So far, this method was restricted to equal-time correlators $\langle\hat A_\mu(t)\hat B_\nu(t)\rangle$. In this work, we generalize this method to double-time correlators $\langle\hat A_\mu(t)\hat B_\nu(t')\rangle$, which allows us to study effective light cones and Green functions and to incorporate finite initial temperatures.