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Lorentz-Violating Massive Gravity in Curved Space

In models of modified gravity, extra degrees of freedom usually appear. They must be removed from the spectrum because they may indicate the presence of instabilities and because otherwise the model might not agree with observation. In the present letter, we will discuss a model that modifies General Relativity through the addition of a Lorentz-violating potential-like term for the metric. No additional propagating modes and no classical instabilities are present. The model departs from GR only in the extreme infrared region, and the cosmological evolution contains a bounce when the size of the Universe is of the same order of the added deformation.