Small scale isocurvature perturbation of WIMP dark matter

The smallest size of the cosmic structures is known to be limited in the standard dark matter models with weakly interacting massive particles. However this constraint can be relaxed in the low reheating temperature scenarios. During the transition from the early matter domination to the radiation domination, the small scale isocurvature perturbation of WIMP is created and become large. These are not erased during the kinetic decoupling and can form the small size structures such as ultra compact mini halos. Combining with the WIMP properties, the small objects can show their existence with visible signatures like gamma ray, cosmic ray or neutrinos.