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The cosmic 21-cm background frequency spectrum related to the spin-fliptransition of neutral Hydrogen present during and before the era ofreionization is rich in features associated with physical processes that governtransitions between the two spin states. The intervening electrons inforeground galaxy clusters inversely Compton scatter the 21-cm backgroundspectrum and modify it just as the cosmic microwave background (CMB) spectrumis modified by inverse-Compton scattering. Towards typical galaxy clusters atlow redshifts, the resulting modification is a few tenths milli-Kelvincorrection to the few tens milli-Kelvin temperature of 21-cm signal relative tothat of the cosmic microwave background black body spectrum. The modificationsare mostly associated with sharp changes in the cosmic 21-cm backgroundspectrum such as due to the onset of a Lyman-$\alpha$ radiation field orheating of neutral gas. Though low frequency radio interferometers that are nowplanned for 21-cm anisotropy measurements are insensitive to the mean 21-cmspectrum, differential observations of galaxy clusters with theseinterferometers can be utilized to indirectly establish global features in the21-cm frequency spectrum. We discuss the feasibility to detect the spectrummodified by clusters and find that for upcoming interferometers, while adetection towards an individual cluster is challenging, one can average signalsover a number of clusters, selected based on the strength of theSunyave-Zel'dovich effect at high radio frequencies involving CMB scatteringalone, to establish the mean 21-cm spectrum.Comment: 6 pages, 3 figures, PRD in press; expanded and title changed from v1.  Final version in pres

Modifications to the cosmic 21-cm background frequency spectrum by scattering via electrons in galaxy clusters