f(R) gravity

The term f(R) gravity refers to a particular type of theory of gravity that is a modification of the current well-accepted model, general relativity (GR). f(R) gravity theories include some of the modifications to GR that are currently under investigation: modifications to explain current cosmology data (particularly the data motivating dark energy), and as a step toward combining GR with quantum mechanics for a Theory of Everything (TOE). The gist of f(R) gravity and the motivation for its name is that for some GR equations, it plugs in some selected function of R (R being the Ricci scalar) in an equation term where "normal" GR has just R itself. Some consequences can be worked out based on just such a substitution, that the term replacing the Ricci scalar is dependent on the Ricci scalar but independent of other factors. By this definition of f(R) gravity, some otherwise-explored alternatives can be classified as f(R) gravity theories, but they typically continue to go by their already-established names. Another class of recent gravity theories is termed scalar-tensor gravity (and I don't know whether these two classes of theory are mutually exclusive).

As our ability to observe black holes continues to improve, opportunities grow to eliminate or confirm these theories, providing motivation to further-develop them. Observation improvements include the development of the Event Horizon Telescope (EHT), gravitational-wave detectors, refined techniques for detecting black hole characteristics through observation of galaxies, and continuing improvement of telescopes and instruments.