Layzer-Irvine equation: New perspectives and the role of interacting dark energy

We derive the Layzer-Irvine equation in the presence of a homogeneous (or quasihomogeneous) dark energy component with an arbitrary equation of state. We extend the Layzer-Irvine equation to homogeneous and isotropic universes with an arbitrary number of dimensions and obtain the corresponding virial relation for sufficiently relaxed objects. We find analogous equations describing the dynamics of cosmic string loops and other p-branes of arbitrary dimensionality, discussing the corresponding relativistic and nonrelativistic limits. Finally, we generalize the Layzer-Irvine equation to account for a nonminimal interaction between dark matter and dark energy, discussing its practical use as a signature of such an interaction.