White Noise from Dark Matter: 21 cm Observations of Early Baryon Collapse

In concordance cosmology, dark matter density perturbations generated by inflation lead to nonlinear, virialized minihalos, into which baryons collapse at redshift $z \sim 20$. We survey here novel baryon evolution produced by a modification of the power spectrum from white noise density perturbations at scales below $k \sim 10 h {Mpc}^{-1}$ (the smallest scales currently measured with the Lyman-$\alpha$ forest). Exotic dark matter dynamics, such as would arise from scalar dark matter with a late phase transition (similar to an axion, but with lower mass), create such an amplification of small scale power. The dark matter produced in such a phase transition collapses into minihalos, with a size given by the dark matter mass within the horizon at the phase transition. If the mass of the initial minihalos is larger than $\sim 10^{-3} M_\odot$, the modified power spectrum is found to cause widespread baryon collapse earlier than standard $\Lambda$CDM, leading to earlier gas heating. It also results in higher spin temperature of the baryons in the 21 cm line relative to $\Lambda$CDM at redshifts $z > 20$ if the mass of the minihalo is larger than $1 M_\odot$. It is estimated that experiments probing 21 cm radiation at high redshift will contribute a significant constraint on dark matter models of this type for initial minihalos larger than $\sim 10 M_\odot$. Early experiments reaching to $z\approx 15$ will constrain minihalos down to $\sim 10^3 M_\odot$.