Charting the Parameter Space of the Global 21-cm Signal
  The early star-forming Universe is still poorly constrained, with the
properties of high-redshift stars, the first heating sources, and reionization
highly uncertain. This leaves observers planning 21-cm experiments with little
theoretical guidance. In this work we explore the possible range of
high-redshift parameters including the star formation efficiency and the
minimal mass of star-forming halos; the efficiency, spectral energy
distribution, and redshift evolution of the first X-ray sources; and the
history of reionization. These parameters are only weakly constrained by
available observations, mainly the optical depth to the cosmic microwave
background. We use realistic semi-numerical simulations to produce the global
21-cm signal over the redshift range $z = 6-40$ for each of 193 different
combinations of the astrophysical parameters spanning the allowed range. We
show that the expected signal fills a large parameter space, but with a fixed
general shape for the global 21-cm curve. Even with our wide selection of
models we still find clear correlations between the key features of the global
21-cm signal and underlying astrophysical properties of the high redshift
Universe, namely the Ly$\alpha$ intensity, the X-ray heating rate, and the
production rate of ionizing photons. These correlations can be used to directly
link future measurements of the global 21-cm signal to astrophysical quantities
in a mostly model-independent way. We identify additional correlations that can
be used as consistency checks.
