12. July 2018

BREMEN-OLDENBURG RELATIVITY SEMINAR
University of Oldenburg

Abstract: Alternative theories of gravity are well motivated due to various shortcomings of GR from a theoretical standpoint, despite its empirical success. Using Lovelock’s theorem, which demonstrates the uniqueness of GR, one can classify alternative theories according to which of the assumptions in the proof of Lovelock’s theorem they break.
One of the assumptions we know to be false already. That is the assumption that there is only the gravitational field, and it is described by a metric. There is matter, and not just a metric, in the Universe. A simple appeal to quantum corrections, and the fact that the Equivalence Principles do not seem to have any basis in symmetry, leads one to expect that all the Equivalence Principles should be broken at some level.
 Due to the strength of solar system constraints on such violations, if any appreciable violation occurs in nature then there must be some mechanism which suppresses them in the solar system. An example of such a mechanism for violations of the Strong Equivalence principle has been known since the early 90s. Perhaps surprisingly this same mechanism (and other, more recent, mechanisms) can be adapted to work for violations of the Weak Equivalence Principle.
Unfortunately, without the Weak Equivalence Principle, the space of possible theories grows enormously. There is, however, a redeeming feature. Violations of the Weak Equivalence Principle seem likely to generically have smoking gun phenomena. The motivation for this can be seen by comparing a toy model to a parametric oscillator.
A final interesting takeaway from studying Weak Equivalence Principle violations is that it demonstrates the ambiguity of the term “Alternative theory of gravity”. For example, in Scalar- Tensor theories without the Weak Equivalence Principle there is no such thing as a Jordan frame and there appears to be no general way to distinguish between the scalar field being of a “gravitational” nature or being a matter field.