Abstract:
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 < 104 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 ΛCDM model at small scales. We compute the spectral distortion of CMB for these
alternative models and compare our results with the ΛCDM 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 ΛCDM model for a range of parameters of these
models: LFDM for formation redshift zf = 105
(7%); WDM for mass mwdm = 1 keV (2%);
CHDM for decay redshift zdecay = 105
(5%); ULA for mass ma = 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.
