Q-balls in Non-Minimally Coupled Palatini Inflation and their
  Implications for Cosmology
  We demonstrate the existence of Q-balls in non-minimally coupled inflation
models with a complex inflaton in the Palatini formulation of gravity. We show
that there exist Q-ball solutions which are compatible with inflation and we
derive a window in the inflaton mass squared for which this is the case. In
particular, we confirm the existence of Q-ball solutions with $\phi \sim
10^{17}-10^{18}$ GeV, consistent with the range of field values following the
end of slow-roll Palatini inflation. We study the Q-balls and their properties
both numerically and in an analytical approximation. The existence of such
Q-balls suggests that the complex inflaton condensate can fragment into
Q-balls, and that there may be an analogous process for the case of a real
inflaton with fragmentation to neutral oscillons. We discuss the possible
post-inflationary cosmology following the formation of Q-balls, including an
early Q-ball matter domination (eMD) period and the effects of this on the
reheating dynamics of the model, gravitational wave signatures which may be
detectable in future experiments, and the possibility that Q-balls could lead
to the formation of primordial black holes (PBHs). In particular, we show that
Palatini Q-balls with field strengths typical of inflaton condensate
fragmentation can directly form black holes with masses around 500 kg or more
when the self-coupling is $\lambda = 0.1$, resulting in very low (less than 100
GeV) reheating temperatures from black hole decay, with smaller black hole
masses and larger reheating temperatures possible for smaller values of
$\lambda$. Q-ball dark matter from non-minimally coupled Palatini inflation may
also be a direction for future work.
