Vacuum Cherenkov radiation and bremsstrahlung from disformal couplings

Understand

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.