After numerous astronomical and experimental searches , the precise particle nature of dark matter is still unknown .
The standard Weakly Interacting Massive Particle ( WIMP ) dark matter , despite successfully explaining the large-scale features of the universe , has long-standing small-scale issues .
The spectral distortion in the Cosmic Microwave Background ( CMB ) caused by Silk damping in the pre-recombination era allows one to access information on a range of small scales 0.3 { Mpc } < k < 10 ^ { 4 } Mpc ^ { -1 } , whose dynamics can be precisely described using linear theory .
In this paper , we investigate the possibility of using the Silk damping induced CMB spectral distortion as a probe of the small-scale power .
We consider four suggested alternative dark matter candidates—Warm Dark Matter ( WDM ) , Late Forming Dark Matter ( LFDM ) , Ultra Light Axion ( ULA ) dark matter and Charged Decaying Dark Matter ( CHDM ) ; the matter power in all these models deviate significantly from the \Lambda CDM model at small scales .
We compute the spectral distortion of CMB for these alternative models and compare our results with the \Lambda CDM model .
We show that the main impact of alternative models is to alter the sub-horizon evolution of the Newtonian potential which affects the late-time behaviour of spectral distortion of CMB .
The y -parameter diminishes by a few percent as compared to the \Lambda CDM model for a range of parameters of these models : LFDM for formation redshift z _ { f } = 10 ^ { 5 } ( 7 % ) ; WDM for mass m _ { wdm } = 1 keV ( 2 % ) ; CHDM for decay redshift z _ { decay } = 10 ^ { 5 } ( 5 % ) ; ULA for mass m _ { a } = 10 ^ { -24 } eV ( 3 % ) . This effect from the pre-recombination era can be masked by orders of magnitude higher y -distortions generated by late-time sources , e.g .
the Epoch of Reionization and tSZ from the cluster of galaxies . We also briefly discuss the detectability of this deviation in light of the upcoming CMB experiment PIXIE , which might have the sensitivity to detect this signal from the pre-recombination phase .