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Kicking Chameleons: Early Universe Challenges for Chameleon Gravity

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Chameleon gravity is
a scalar-tensor theory that mimics general relativity in the Solar System. The
scalar degree of freedom is hidden in high-density environments because the
effective mass of the chameleon scalar depends on the trace of the
stress-energy tensor.  In the early Universe, when the trace of the
stress-energy tensor is nearly zero, the chameleon is very light and Hubble
friction prevents it from reaching its potential minimum.  Whenever a
particle species becomes non-relativistic, however, the trace of the
stress-energy tensor is temporarily nonzero, and the chameleon begins to
roll.  I will show that these "kicks" to the chameleon field
have catastrophic consequences for chameleon gravity.  The velocity imparted
to the chameleon is sufficiently large that the chameleon's mass changes
rapidly as it slides past its potential minimum.  This nonadiabatic
process shatters the chameleon field by generating extremely high-energy
perturbations, casting doubt on chameleon gravity's viability as an alternative
to general relativity.