A Separate Universe Approach to Quintessence Perturbations
  There is some observational evidence that the dark energy may not be smooth on large scales. This makes it worth while to try and get as simple and as intuitive a picture of how dark energy perturbations behave so as to be able to better constrain possible models of dark energy and the generation of large scale perturbations. The separate Universe method provides an easy way to evaluate cosmological perturbations, as all that is required is an understanding of the background behavior. Here, this method is used to show how the size of the dark energy perturbations, preferred by observations, is larger than would be expected, and so some mechanism may be required to amplify them. 
Christopher Gordon