Spontaneous lorentz breaking and massive gravity.
Theories with an extra spin-two field coupled to gravity admit a massive phase with broken Lorentz symmetry. While the equivalence principle is respected, the Newtonian potentials are in general modified, but they may be protected by a scale symmetry of the coupling term. The gravitational waves phenomenology is quite rich: two gravitons, one massive and one massless, oscillate and propagate with distinct velocities, different from the speed of light. A time of flight difference between gravitons and photons from a common source would provide a clear signal of this theory.
Berezhiani
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Dipartimento di Fisica, Università di L'Aquila and INFN-LNGS, I-67010 L'Aquila, Italy.
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