Speaker
Description
We discuss the role of consistency with causality and quantum mechanics in determining the properties of gravity. We begin by constructing theories of interacting massless spin 2 particles -- gravitons. One theory involves coupling the graviton with the lowest number of derivatives to matter, the other theories involve coupling the graviton with higher derivatives to matter. The first theory requires an infinite tower of terms for consistency, which is known to lead to general relativity. The other theories only require a finite number of terms for consistency, which appear as an entire new class of theories of massless spin 2. We recap the causal consistency of general relativity and show that in general this fails for the new class of theories. This appears to be a deep reason for minimally coupling the graviton with the lowest number of derivatives. Then, as a causal modification of general relativity, we discuss the so-called $F(\mathcal{R})$ theories, which have interesting applications to cosmology including inflation and dark energy. We show that, unlike general relativity, these theories do not possess the requisite counter-terms to be consistent quantum effective field theories. Together this helps to remove some of the central assumptions made in deriving general relativity.
