Ionization history of the Universe as a test for Super Heavy Dark Matter   particles
  In this paper we discuss the possible distortions of the ionization history of the universe caused by an injection of non-thermal energy due to decays of hypothetical Super Heavy Dark Matter (SHDM) particles. These particles are usually considered as a possible source of Ultra High Energy Cosmic Rays (UHECRs) in the framework of the Top-Down model. Estimates of fraction of energy of decays converted to the UV range show that, for suitable parameters of SHDM particles, the significant distortions of power spectra of CMB anisotropy appear. Comparison with the observed power spectrum allows to restrict some properties of the SHDM particles. These decays can also increase of about 5 -- 10 times the degree of ionization of hydrogen at redshifts $z\sim$ 10 -- 50 that essentially accelerates the formation of molecules $H_2$ and first stars during "dark ages". 
A.G. Doroshkevich and P.D. Naselsky