3D simulations with boosted primordial power spectra and ultracompact minihalos

Abstract: We perform three-dimensional simulations of structure formation in the early Universe, when boosting the primordial power spectrum on approximately kpc scales. We demonstrate that our simulations are capable of producing power-law profiles close to the steep $\rho\propto r^{-9/4}$ halo profiles that are commonly assumed to be a good approximation to ultracompact minihalos (UCMHs). However, we show that for more realistic initial conditions in which halos are neither perfectly symmetric nor isolated, the steep power-law profile is disrupted and we find that the Navarro-Frenk-White profile is a better fit to most halos. In the presence of background fluctuations even extreme, nearly spherical initial conditions do not remain exceptional. Nonetheless, boosting the amplitude of initial fluctuations causes all structures to form earlier and thus at larger densities. With sufficiently large amplitude of fluctuations we find that values for the concentration of typical halos in our simulations can become very large. However, despite the signal coming from dark matter annihilation inside the cores of these halos being enhanced, it is still orders-of-magnitude smaller compared to the usually assumed UCMH profile. The upper bound on the primordial power spectrum from the non-observation of UCMHs should therefore be re-evaluated.