Cosmology

It is an attractive idea that effective theories admitting a consistent UV completion require quanta to propagate sub-luminally in non-trivial backgrounds. However, there is a counter example to this proposition in the form of QED in a curved geometry, a theory that is certainly causal. Nevertheless, Drummond and Hathrell showed that there is always at least one choice of polarization for which low frequency photons propagate super-luminally. Conventional arguments involving dispersion relations would then normally imply that the high frequency phase velocity would also exceed c yielding a contradiction with the UV completion. We show how the contradiction is avoided by a mechanism that relies on the subtle behaviour of the lightcones in the geometry and that, in the end, super-luminal low frequency propagation is perfectly consistent with causality. In particular, time machines cannot be constructed using the effect. The lesson is that causality constraints in low energy effective theories need to be treated with some caution.

I will discuss the Vainshtein mechanism in massive gravity. I will show that the spherically symmetric backgrounds that were believed to have superluminal sound speed are in fact unstable. Instead, there is a new class of phenomenologically relevant solutions with stable and subluminal perturbations.

In General Relativity, gravitons propagate to null directions, because of its well-organized structures. Modifying the gravity theory slightly, meanwhile, the beautiful structure is broken and gravitons can easily propagate superluminaly. Here, applying the characteristic method, which is the well-established powerful way to analyze causal structures, the causal structures in Massive gravity and Gauss-Bonnet gravity are analyzed. We discuss the superluminality, acausality and black holes.