Interacting Vector-like Dark Energy, the First and Second Cosmological   Coincidence Problems
  One of the puzzles of the dark energy problem is the (first) cosmological coincidence problem, namely, {\em why does our universe begin the accelerated expansion recently? why are we living in an epoch in which the dark energy density and the dust matter energy density are comparable?} On the other hand, cosmological observations hint that the equation-of-state parameter (EoS) of dark energy crossed the phantom divide $w_{de}=-1$ in the near past. Many dark energy models whose EoS can cross the phantom divide have been proposed. However, to our knowledge, these models with crossing the phantom divide only provide the possibility that $w_{de}$ can cross -1. They do not answer another question, namely, {\em why crossing the phantom divide occurs recently?} Since in many existing models whose EoS can cross the phantom divide, $w_{de}$ undulates around -1 randomly, {\em why are we living in an epoch $w_{de}<-1$?} This can be regarded as the second cosmological coincidence problem. In this work, the cosmological evolution of the vector-like dark energy interacting with background perfect fluid is investigated. We find that the first and second cosmological coincidence problems can be alleviated at the same time in this scenario. 
Hao Wei, Rong-Gen Cai