Two weeks ago at AIMS we were visited by Danielle Wills from the University of Durham and Tomi Koivisto from the Institute for Theoretical Astrophysics in Oslo. After powering through 110km of the Cape Argus, they came in on Monday to give us a two part talk on disformal gravity and D-branes.
Tomi kicked off the proceedings with a short outline of the phenomenology of disformal gravity. He started by explaining that the basic idea was to have the gravitational and matter sectors have different metrics. Tomi explained that there exist theoretical constraints on the relationship between the two metrics — they must be related by some linear combination of a conformal and disformal transformation. The latter is a transformation where the metrics are related by a scalar function of a scalar multiplied by the product of two spacetime first derivatives of said scalar field.
After showing us that existing modified gravity models could be put into the form of particular disformal theories, Tomi explained how disformal gravity models can have screening. This is needed to ensure there can be large scale cosmological effects of the modified gravity theory without violating solar system constraints etc.
Then, after summarizing some possible tests of disformal theories, Tomi handed over to Danielle to explain the string theory motivation for such models.
After leading leading slowly through an outline of the mathematics, Danielle summarized the state of the work to date and the possibilities for going further.