  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.
