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 m _ { \text { X } } of up to \sim 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 m _ { \text { X } } 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 m _ { \text { X } } \lesssim 3 \text { keV } , while next generation instruments will easily be able to measure this difference for all relevant WDM models .