Shedding light on the small-scale crisis with CMB spectral distortions

The small-scale crisis, discrepancies between observations and N-body simulations, may imply suppressed matter fluctuations on subgalactic distance scales. Such a suppression could be caused by some early-universe mechanism (e.g., broken scale invariance during inflation), leading to a modification of the primordial power spectrum at the onset of the radiation-domination era. Alternatively, it may be due to nontrivial dark-matter properties (e.g., new dark-matter interactions or warm dark matter) that affect the matter power spectrum at late times, during radiation domination, after the perturbations reenter the horizon. We show that early- and late-time suppression mechanisms can be distinguished by measurement of the mu distortion to the frequency spectrum of the cosmic microwave background. This is because the mu distortion is suppressed, if the power suppression is primordial, relative to the value expected from the dissipation of standard nearly scale-invariant fluctuations. We emphasize that the standard prediction of the mu distortion remains unchanged in late-time scenarios even if the dark-matter effects occur before or during the era (redshifts 5 x 10(4) less than or similar to z less than or similar to 2 x 106) at which mu distortions are generated.