Solving the Coincidence Problem: Tracking Oscillating Energy
  Recent cosmological observations strongly suggest that the universe is dominated by an unknown form of energy with negative pressure. Why is this dark energy density of order the critical density today? We propose that the dark energy has periodically dominated in the past so that its preponderance today is natural. We illustrate this paradigm with a model potential and show that its predictions are consistent with all observations. 
Scott Dodelson (1 and 2), Manoj Kaplinghat (2), and Ewan Stewart (1   and 3) ((1) NASA/Fermilab Astrophysics Center, (2) Uni...