Testing modified gravity with globular clusters: the case of NGC 2419
  The dynamics of globular clusters has been studied in great detail in the
context of general relativity as well as with modifications of gravity that
strongly depart from the standard paradigm such as MOND. However, at present
there are no studies that aim to test the impact that less extreme
modifications of gravity (e.g. models constructed as alternatives to dark
energy) have on the behaviour of globular clusters. This Letter presents fits
to the velocity dispersion profile of the cluster NGC 2419 under the symmetron
modified gravity model. The data shows an increase in the velocity dispersion
towards the centre of the cluster which could be difficult to explain within
general relativity. By finding the best fitting solution associated with the
symmetron model, we show that this tension does not exist in modified gravity.
However, the best fitting parameters give a model that is inconsistent with the
dynamics of the Solar System. Exploration of different screening mechanisms
should give us the chance to understand if it is possible to maintain the
appealing properties of the symmetron model when it comes to globular clusters
and at the same time recover the Solar System dynamics properly.
