  Scalar modifications of gravity have an impact on the growth of structure.
Baryon and Cold Dark Matter (CDM) perturbations grow anomalously for scales
within the Compton wavelength of the scalar field. In the late time Universe
when reionisation occurs, the spectrum of the 21cm brightness temperature is
thus affected. We study this effect for chameleon-f(R) models, dilatons and
symmetrons. Although the f(R) models are more tightly constrained by solar
system bounds, and effects on dilaton models are negligible, we find that
symmetrons where the phase transition occurs before z_* ~ 12 will be detectable
for a scalar field range as low as 5 kpc. For all these models, the detection
prospects of modified gravity effects are higher when considering modes
parallel to the line of sight where very small scales can be probed. The study
of the 21 cm spectrum thus offers a complementary approach to testing modified
gravity with large scale structure surveys. Short scales, which would be highly
non-linear in the very late time Universe when structure forms and where
modified gravity effects are screened, appear in the linear spectrum of 21 cm
physics, hence deviating from General Relativity in a maximal way.
