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Events like inflation or phase transitions can produce large densityperturbations on very small scales in the early Universe. Probes of smallscales are therefore useful for e.g. discriminating between inflationarymodels. Until recently, the only such constraint came from non-observation ofprimordial black holes (PBHs), associated with the largest perturbations.Moderate-amplitude perturbations can collapse shortly after matter-radiationequality to form ultracompact minihalos (UCMHs) of dark matter, in far greaterabundance than PBHs. If dark matter self-annihilates, UCMHs become excellenttargets for indirect detection. Here we discuss the gamma-ray fluxes expectedfrom UCMHs, the prospects of observing them with gamma-ray telescopes, andlimits upon the primordial power spectrum derived from their non-observation bythe Fermi Large Area Space Telescope.

Constraints on small-scale cosmological perturbations from gamma-ray searches for dark matter