We analyse the evolution of light Q-balls in a cosmological background, and
find a number of interesting features. For Q-balls formed with a size
comparable to the Hubble radius, we demonstrate that there is no charge
radiation, and that the Q-ball maintains a constant physical radius. Large
expansion rates cause charge migration to the surface of the Q-ball,
corresponding to a non-homogeneous internal rotation frequency. We argue that
this is an important phenomenon as it leads to a large surface charge and
possible fragmentation of the Q-ball. We also explore the deviation of the
Q-ball profile function from the static case. By introducing a parameter
ϵ, which is the ratio of the Hubble parameter to the frequency of
oscillation of the Q-ball field, and using solutions to an analytically
approximated equation for the profile function, we determine the dependence of
the new features on the expansion rate. This allows us to gain an understanding
of when they should be considered and when they can be neglected, thereby
placing restrictions on the existence of homogeneous Q-balls in expanding
backgrounds.Comment: 5 pages, 4 figure