CDM and Baryons as Distinct Fluids in a Linear Approximation for the
  Growth of Structure
  A single fluid approximation which treats perturbations in baryons and dark
matter as equal has sometimes been used to calculate the growth of linear
matter density perturbations in the Universe. We demonstrate that properly
accounting for the separate growth of baryon and dark matter fluctuations can
change some predictions of structure formation in the linear domain in a way
that can alter conclusions about the consistency between predictions and
observations for $\Lambda$CDM models versus modified gravity scenarios . Our
results may also be useful for 21cm tomography constraints on alternative
cosmological models for the formation of large scale structure.
