Observational Constraints on Undulant Cosmologies
  In an undulant universe, cosmic expansion is characterized by alternating periods of acceleration and deceleration. We examine cosmologies in which the dark-energy equation of state varies periodically with the number of e-foldings of the scale factor of the universe, and use observations to constrain the frequency of oscillation. We find a tension between a forceful response to the cosmic coincidence problem and the standard treatment of structure formation. 
Gabriela Barenboim, Olga Mena Requejo, Chris Quigg