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The small-scale clustering power spectrum and relativistic decays

We present constraints on decaying-particle models in which an enhanced relativistic density allows an $Ω=1$ Cold Dark Matter universe to be reconciled with acceptable values for the Hubble constant. Such models may contain extra small-scale power, which can have important consequences for enhanced object formation at high redshifts. Small-scale galaxy clustering and abundances of high-redshift damped Lyman-$α$ absorption clouds give a preferred range for the mass of any such decaying particle of 2 to 30 keV and a lifetime of 0.5 to 100 years for models with a high Hubble constant ($h>0.75$). A lower Hubble constant, $h \simeq 0.5$, weakens the constraint to $0.5< m < 30$ keV, $0.2 < τ< 500$ years. In permitted versions of the model, reionization occurs at redshifts $\sim 10-200$, and this feature may be of importance in understanding degree-scale CMB anisotropies.