Alternative Models of Gravity

The theory of General Relativity has been formulated almost a century ago and has been successfully tested… in weak field regime. Already at the galactic scale, an additional unseen ingredient is necessary in order to understand the structure of the galaxy. This may either tell us that indeed there is matter that we don’t see, that doesn’t interact or that the theory of General Relativity cracks already at this scale. On another hand, extreme regime of gravity may lead to singularity, e.g. in the center of a Black Hole. There, the theory of General Relativity (GR) is expected to break down. In fact, the singularity is the signal that we are in a regime out of the validity range of GR. Alternative models of gravity are supposed to be effective models of more fundamental theories such as String Theory, Loop Quantum Gravity, Quantum effects in GR, …

There are many of these alternative models and I’m interested in only a couple of them… In particular, those containing higher derivatives, such as Weyl Gravity or Chern Simons gravity and some other models that changes the matter/gravity coupling without additional degrees of freedom.