  If long-range attractive forces exist and are stronger than gravity then
cosmic halo formation can begin in the radiation-dominated era. We study a
simple realization of this effect in a system where dark matter fermions have
Yukawa interactions mediated by scalar particles, analogous to the Higgs boson
in the standard model. We develop a self-consistent description of the system
including exact background dynamics of the scalar field, and precise modelling
of the fermion density fluctuations. For the latter, we provide accurate
approximations for the linear growth as well as quantitative modelling of the
nonlinear evolution using N-body simulations. We find that halo formation
occurs exponentially fast and on scales substantially larger than simple
estimates predict. The final fate of these halos remains uncertain, but could
be annihilation, dark stars, primordial black holes, or even the existence of
galaxy-sized halos at matter-radiation equality. More generally, our results
demonstrate the importance of mapping scalar-mediated interactions onto
structure formation outcomes and constraints for beyond the standard model
theories.
