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Massive Gravity is this year's must-have theory of modified gravity. The concept is simple - what if the graviton had a (very small) mass? However, building a consistent and viable theory from this idea has proved very difficult. It has now been achieved for the background-level cosmology, and can fit the accelerated expansion (with the usual fine-tuning problems, of course!) This paper takes the first steps towards the perturbation theory that needs to be developed if we are to test Massive Gravity with measurements of structure growth, etc.

There's been a growing awareness that the 'free' functions used to parameterise modified gravity aren't really as arbitrary as one might first think. This methods paper suggests how to use these ideas - although the forecast isn't all that encouraging. Still, the proof of the pudding is yet to come.

The chameleon mechanism suppresses the effects of a modified gravity theory in high-density environments. The authors of this paper suggest that this could lead to effects on stellar structure that would impact distance measures inferred from cepheid variables and red giant branch stars. Whilst it's not clear that the astrophysics involved is sufficiently well understood for such tests to be useful, I think there is some good thinking-outside-the-box here!

Celia visited us for several months last summer - this paper is the outcome of her work here. In EBI gravity there is an 'Eddington-dominated' epoch in the universe prior to radiation domination, which can avoid the need for a big bang singularity. However, it turns out that tensor perturbations in this early phase are unstable. It's particularly interesting that the instability only shows up at the perturbative level, whilst the background cosmology remains non-singular.

There are a number of `screening' mechanisms that are designed to suppress the effects of modified gravity below cluster scales. What are the characteristic radii at which this switch-off occurs in the different mechanisms?

The authors numerically showed that angular momentum of isolated galactic halos with mass less than 10^11 solar mass is systematically larger in the f(R) gravity model since they are less affected by shielding via chameleon mechanism.

Hints of something interesting or data artefact? Also: http://arxiv.org/pdf/1207.4781v1.pdf

Tim Clifton's epic review

Not read it yet though ;-)