Astrophysics

I will review the notorious cosmological constant problem, sometimes described as the worst fine tuning problem in Physics. I will explain the true nature of the problem, which is one of radiative instability against any change in the effective description. I will recall Weinberg’s venerable no-go theorem that prohibits certain attempts to “solve” this problem before going on to explain a new mechanism that circumvents Weinberg. This is the vacuum energy sequester, a global modification of GR that results in the cancellation of large vacuum energy contributions from a protected matter sector (taken to include the Standard Model) at each and every order in the perturbative loop expansion. Cosmological consequences are a Universe which has finite space-time volume, will ultimately crunch, and for which dark energy can only be a transient. Furthermore, using a linear scalar potential within the sequestering set-up, I will show that dark energy today can be intimately related to the trigger that brings about cosmological collapse in the not too distant future, at the same time providing a possible solution to the “Why Now?” problem.

We show how nonlinearly realized N=2 supersymmetry gives rise, in the low-energy limit, to an N=1 Born-Infeld U(1) Lagrangian. We then extend the construction to many N=2 vector multiplets. We show how the classification of inequivalent nilpotency constraints arising in the low-energy limit is connected to the theory of cubic polynomials and curves. We comment on causality of signal propagation in these systems.

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.