Catastrophic Consequences of Kicking the Chameleon
  The physics of the "dark energy" that drives the current cosmological
acceleration remains mysterious, and the dark sector may involve new light
dynamical fields. If these light scalars couple to matter, a screening
mechanism must prevent them from mediating an unacceptably strong fifth force
locally. Here we consider a concrete example: the chameleon mechanism. We show
that the same coupling between the chameleon field and matter employed by the
screening mechanism also has catastrophic consequences for the chameleon during
the Universe's first minutes. The chameleon couples to the trace of the
stress-energy tensor, which is temporarily non-zero in a radiation-dominated
universe whenever a particle species becomes non-relativistic. These "kicks"
impart a significant velocity to the chameleon field, causing its effective
mass to vary non-adiabatically and resulting in the copious production of
quantum fluctuations. Dissipative effects strongly modify the background
evolution of the chameleon field, invalidating all previous classical
treatments of chameleon cosmology. Moreover, the resulting fluctuations have
extremely high characteristic energies, which casts serious doubt on the
validity of the effective theory. Our results demonstrate that quantum particle
production can profoundly affect scalar-tensor gravity, a possibility not
previously considered. Working in this new context, we also develop the theory
and numerics of particle production in the regime of strong dissipation.
