Structure Formation in the Early Universe
  The evolution of the perturbations in the energy density and the particle
number density in a flat Friedmann-Lemaitre-Robertson-Walker universe in the
radiation-dominated era and in the epoch after decoupling of matter and
radiation is studied. For large-scale perturbations the outcome is in
accordance with treatments in the literature. For small-scale perturbations the
differences are conspicuous. Firstly, in the radiation-dominated era
small-scale perturbations grew proportional to the square root of time.
Secondly, perturbations in the Cold Dark Matter particle number density were,
due to gravitation, coupled to perturbations in the total energy density. This
implies that structure formation could have begun successfully only after
decoupling of matter and radiation. Finally, after decoupling density
perturbations evolved diabatically, i.e., they exchanged heat with their
environment. This heat exchange may have enhanced the growth rate of their mass
sufficiently to explain structure formation in the early universe, a phenomenon
which cannot be understood from adiabatic density perturbations.
