Cerenkov radiation emitted by charged particles in a gravitational field

Charged particles propagating in an external gravitational field with negat ive Ricci tensor components can emit radiation by the Cerenkov process. The external gravitational field acts like an effective refractive index for l ight. Such a negative Ricci tensor arises in the core of cosmic strings owi ng to the negative pressure along the string axis. Cosmic ray protons movin g along the core of cosmic strings emit Cerenkov radiation with a character istic spectrum which can be used as a signal for the detection of cosmic st rings. Another example of a metric with negative Ricci curvature is the gra vitational field of galactic dark matter. We derive the expressions for the gravitational Cerenkov radiation of charged particles for these metrics.