Imperial Collage of London
With now Nobel-prize-winning observational evidence for the accelerated expansion of the Universe, we are still in search of a thorough theoretical understanding of its cause. Modified gravity theories such as DGP, galileon and chameleon theories are strong candidates in this enterprise. These models introduce a new scalar degree of freedom in order to modify GR on large scales and generate late-time acceleration. However, this degree of freedom will generically give rise to a new fifth force. In order to comply with tight fifth force constraints on solar system scales, such theories therefore need to come equipped with a screening mechanism that shields the scalar and allows us to recover GR on these scales. I will discuss the connection of these theories with Lovelock gravity and induced metrics as arising in higher dimensional setups. In particular I will show how this allows an exhaustive classification of galileon and chameleon theories, generating new types of such screened theories in the process.