Vacuum Cherenkov radiation and bremsstrahlung from disformal couplings
  The simplest way to modify gravity is to extend the gravitational sector to
include an additional scalar degree of freedom. The most general metric that
can be built in such a theory includes disformal terms, so that standard model
fields move on a metric which is the sum of the space time metric and a tensor
constructed from first derivatives of the scalar. In such a theory
gravitational waves and photons can propagate at different speeds, and these
can in turn be different from the maximum speed limit for matter particles. In
this work we show that disformal couplings can cause charged particles to emit
Cherenkov radiation and bremsstrahlung apparently in vacuum, depending on the
background evolution of the scalar field. We discuss the implications of this
for observations of cosmic rays, and the constraints that arise for models of
dark energy with disformal couplings.
