The observable 21cm signal from reionization may be perturbative
  We develop an effective perturbation theory (and, equivalently, a bias
expansion) for the inhomogeneous 21cm radiation field from reionization. Using
large-scale simulations of cosmological reionization, we find that this
expansion describes the modes in the simulated 21cm signal over much of the
wavenumber range probed by upcoming 21cm arrays. This result provides an
understanding of the potential signal shapes that compliments the nonlinear
numerical modeling that has been the focus of most previous work. We find that
the observable signal often can be described with 2-3 bias coefficients that
can be interpreted in terms of the source biases, the average neutral fraction,
the characteristic size of ionized regions, and the patchiness of reionization.
The 21cm signal serves as a wacky example of a bias expansion in cosmology,
with our approach synthesizing key results.
