  We investigate the effects of warm dark matter (WDM) on the cosmic 21-cm
signal. If dark matter exists as WDM instead of cold dark matter (CDM), its
non-negligible velocities can inhibit the formation of low-mass halos that
normally form first in CDM models, therefore delaying star-formation. The
absence of early sources delays the build-up of UV and X-ray backgrounds that
affect the 21-cm radiation signal produced by neutral hydrogen. With use of the
21CMFAST, code, we demonstrate that the pre-reionization 21-cm signal can be
changed significantly in WDM models with a free-streaming length equivalent to
that of a thermal relic with mass mx of up to ~10-20 keV. In such a WDM
cosmology, the 21-cm signal traces the growth of more massive halos, resulting
in a delay of the 21-cm absorption signature and followed by accelerated X-ray
heating. CDM models where astrophysical sources have a suppressed
photon-production efficiency can delay the 21-cm signal as well, although its
subsequent evolution is not as rapid as compared to WDM. This motivates using
the gradient of the global 21-cm signal to differentiate between some CDM and
WDM models. Finally, we show that the degeneracy between the astrophysics and
mx can be broken with the 21-cm power spectrum, as WDM models should have a
bias-induced excess of power on large scales. This boost in power should be
detectable with current interferometers for models with mx < 3 keV, while next
generation instruments will easily be able to measure this difference for all
relevant WDM models.
