Constraints on small-scale primordial density fluctuation from cosmic microwave background through dark matter annihilation

Understand

The cosmic microwave background (CMB) observation by the Planck satellite precisely determines primordial curvature fluctuations on larger scales than $\mathcal O(1)\,\mathrm{Mpc}$, while the small-scale curvature fluctuation is still less constrained.

• The constraint on small-scale fluctuations is highly improved if we assume the standard thermal relic dark matter scenario.
• When small-scale fluctuations are large enough, dense regions collapse to form small halos even in a redshift $z\gtrsim10^3$, which is called ultracompact minihalos.
• These minihalos enhance the annihilation of the dark matter and it is constrained by observations such as extragalactic gamma rays and the CMB.