Talks

Cuscuton field theory is an extension of general relativity that does not introduce additional propagating degrees of freedom, or violate relativistic causality. We construct a general geometric description of the cuscuton field theory by introducing curvature corrections to both the volume (potential) and the surface (kinetic) terms in the original cuscuton action. Our assumptions involve a stack of spacelike branes, separated by 4-dimensional bulks. We conjecture that the cuscuton, initially a discrete field, becomes continuous in the limit, there are many such transitions. From this we derive an effective action for the cuscuton theory and show that at the quadratic level our theory propagates only the two tensorial degrees of freedom.

I will first introduce screened modified gravity theories and then discuss the chameleon mechanism. Light scalars can be produced from the sun and detected on earth. I will discuss the production of chameleons, including novel production channels, and discuss potential detection in helioscopes.

The Horndeski program is motivated by arguing that scalar-tensor modifications to gravity should have two properties: effective interactions that are at most second-order in time derivatives and only a single scalar. I will argue against both of these criteria. First I argue why the low-energy limit of known well-behaved theories can have more than two-derivative field equations. Second I argue why the scalar-tensor interactions most likely to be found competing with gravity at very low energies typically are those with two derivatives, at least when semiclassical methods are justified, and this suggests exploring multiple-scalar models.

I extend EFT of inflation/dark energy to arbitrary background with timelike scalar profile. In this framework a set of consistency relations among EFT coefficients ensures the spatial diffeo invariance. Some applications to will also be discussed.

We study the weak-gravity regime of higher-order scalar-tensor theories that are degenerate in the unitary gauge. In a certain subset of theories analogous to Lorentz-violating scalar-tensor theories, we show that the Vainshtein mechanism due to nonlinear derivative interactions does not work. For this family of theories we determine all the PPN parameters in terms of the EFT of dark energy parameters and discuss the experimental bounds.

Since the discovery of the accelerated expansion of the universe, significant progress has been made to develop modified gravity theories as alternatives to dark energy and these have been developed into tests of General Relativity itself via cosmological observations. These models share common properties such as screening mechanisms they use to evade the stringent Solar System tests. In this talk, I will review recent status of observational tests of screened modified gravity models and discuss the prospect of cosmological tests of gravity from ongoing surveys such as Euclid.