How can we tell whether dark energy is composed by multiple fields?
  Dark energy is often assumed to be composed by a single scalar field. The
background cosmic expansion is not sufficient to determine whether this is true
or not. We study multi-field scalar-tensor models with a general dark matter
source and write the observable modified gravity parameters (effective
gravitational constant and anisotropic stress) in the form of a ratio of
polynomials in the Fourier wavenumber k of order 2N, where N is the number of
scalar fields. By comparing these observables to real data it is in principle
possible to determine the number of dark energy scalar fields coupled to
gravity. We also show that there are no realistic non-trivial cases in which
the order of the polynomials is reduced.
