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After numerous astronomical and experimental searches, the precise particlenature of dark matter is still unknown. The standard Weakly Interacting MassiveParticle(WIMP) dark matter, despite successfully explaining the large-scalefeatures of the universe, has long-standing small-scale issues. The spectraldistortion in the Cosmic Microwave Background(CMB) caused by Silk damping inthe pre-recombination era allows one to access information on a range of smallscales $0.3 \, {\rm Mpc} < k < 10^4 \, \rm Mpc^{-1}$, whose dynamics can beprecisely described using linear theory. In this paper, we investigate thepossibility of using the Silk damping induced CMB spectral distortion as aprobe of the small-scale power. We consider four suggested alternative darkmatter candidates---Warm Dark Matter (WDM), Late Forming Dark Matter, UltraLight Axion dark matter and Charged Decaying Dark Matter; the matter power inall these models deviate significantly from the $\Lambda$CDM model at smallscales. We compute the spectral distortion of CMB for these alternative modelsand compare our results with the $\Lambda$CDM model. We show that the mainimpact of alternative models is to alter the sub-horizon evolution of theNewtonian potential which affects the late-time behaviour of spectraldistortion of CMB. The $y$-parameter diminishes by a few percent as compared tothe $\Lambda$CDM model for a range of parameters of these models: LFDM forformation redshift $z_f = 10^5$ (7\%); WDM for mass $m_{\rm wdm} = 1 \, \rmkeV$ (2\%); CHDM for decay redshift $z_{\rm decay} = 10^5$ (5\%); ULA for mass$m_a = 10^{-24} \, \rm eV$ (3\%). We also briefly discuss the detectability ofthis deviation in light of the upcoming CMB experiment PIXIE, which might havethe sensitivity to detect this signal from the pre-recombination phase.

The effects of the small-scale behaviour of dark matter power spectrum on CMB spectral distortion