**Lasha Berezhiani**, University of Pennsylvania

*Subluminal Vainshtein Screening in Massive Gravity*

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.

**Paolo Creminelli**, International Centre for Theoretical Physics

*Galileon dualities and superluminality*

**Cedric Deffayet**, Laboratoire Astroparticule et Cosmologie (APC)

*Galileon p-forms*

I will discuss generalizations and no-go theorems for generalizations to p-forms of Galileon actions

**Sergei Dubovsky**, New York University

*Asymptotic fragility, superluminality and pi*

**Fawad Hassan**, Stockholm University

*Some recent results in Bimetric Theory*

This talk will summarize some recent results in bimetric theory, including the existence of the square-root matrix, possible connection to partial masslessness and conformal gravity, the structure of constraints and finally, the cosmological implications of the theory.

**Timothy Hollowood**, Swansea University

* Causality constraints and the lightcone*

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.

**Keisuke Izumi**, National Taiwan University

*Causal structures in Massive gravity and Gauss-Bonnet gravity*

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.

**Antonio Padilla**, University of Nottingham

*The Cosmological Constant Problem (and its sequester)*

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.

**Massimo Porrati**, New York University

*Multi-Field Born-Infeld Lagrangians, Nilpotent Fields, N=2 *

*Supersymmetry and Cubic Polynomials*

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.

**Raquel Ribeiro**, Queen Mary University of London

*Lessons from QuantumLand*

Theories with large kinetic interactions have very relevant phenomenological applications in cosmology, in particular in the context of cosmic acceleration. Their Effective Field Theory (EFT) description relies on the so-called Vainshtein effect being operative. When incorporated at the quantum level, this mechanism ensures the validity of the theory in a non-trivial way. I will discuss how to estimate the regime of validity of such EFTs on the basis of computing the quantum corrections to the classical theory. I will point out how some lessons learned from the study of quantum effects in these theories might help us to tackle the significance of superluminalities and revisit what properties a healthy EFT should have.

**Andrew Tolley**, Case Western Reserve University

*Superluminalities in Galileon theories*

**Andrew Waldron**, University of California, Davis

*Quantum Gravity and Causal Structures*