Braneworlds
In 1999, Lisa Randall and Raman Sundrum proposed a gravitational model where our fourdimensional universe was confined to a brane, which sat at an orbifold point of fivedimensional anti deSitter space. At the other end of the orbifold was another brane. This had the somewhat amazing effect that the Planck scale, the gravitational scale felt in our universe, was related to a fivedimensional scale by an exponential quantity, which allowed for the hierarchy problem to be "solved". Furthermore, it turned out that fourdimensional Einstein gravity was recovered on the brane. The last neat feature of this model was that it had the possibility that dark matter could reside on the other brane, which meant that interaction cross sections between normal matter and dark matter would be naturally small. You can see that this model had a number of things going for it!
Alas, it was a little too good to be true. It turns out that the basic model had a very light scalar field which mediated long range forces. To this end, we set about generalising upon the model, to try and understand if this constraint could be alleviated through the introduction of more branes. We considered a model of multiple branes floating in fivedimensional space, and investigated the gravitational behavior of the system. In particular, we looked at a low energy regime, in which a fourdimensional effective theory could be constructed. Our first paper addresses what the fourdimensional theory looks like.

Fourdimensional description of fivedimensional Nbrane models
Jolyon Bloomfield and Éanna É. Flanagan
Physical Review D 82 (2010), 124013
arXiv:1004.2477 (2010)
In our second paper, we analyse the physics of the fourdimensional effective theory, and place constraints on it, both theoretical and observational. Unfortunately, we found that our model couldn't get around any of the shortcomings of the original RandallSundrum model, and also wasn't helpful for dark matter sequestration. Regardless, it was a fun project to explore.

Gravitational Interactions in a General Multibrane Model
Jolyon Bloomfield and Éanna É. Flanagan
Physical Review D 84 (2011), 104016
arXiv:1009.4686 (2010)
I would like to come back to fivedimensional models one day, and ask the question of "How do you embed a black hole on a brane?" This was my original motivating question, but it turns out to be a rather difficult problem, and one that many people have attempted to tackle. Recently, it appears that people have succeeded in doing so numerically, but I would still like to contribute something to the problem.