Deformations of General Relativity, Geometrodynamics and reality conditions

Abstract

A remarkable aspect of 4-dimensional complexified General Relativity (GR) is that it can be non-trivially deformed: there exists an infinite-parameter set of modifications with the same degree of freedom count. It is trivial to impose reality conditions that lead to real theories with Euclidean or split signature, but the situation is more complicated and not yet fully understood in the Lorentzian case, which is the subject of this talk. I will first show that the choice of potentially consistent reality conditions is essentially unique and boils down to the reality of the underlying 3-metric at the canonical level, as in the case of GR. For simplicity, I will focus on a subset of modified theories that correspond to a natural extension of Ashtekar's Hamiltonian constraint, namely, a linear combination of EEE, EEB, EBB and BBB. Interestingly, the evolution equations for the 3-metric and its first time-derivative take the same form as in GR, but with an effective stress tensor source which cannot be expressed in terms of these two fields. Modified theories therefore appear as essentially "non-metric" in that they do not admit a closed geometrodynamics form. In particular, this obstructs the conservation of the reality conditions, because the effective source remains complex. Alternatively, if we insist on reality, we obtain extra reality conditions which then leave no room for degrees of freedom. I will finally argue that this should be a generic feature of the Lorenzian modified theories, in stark contrast to their Euclidean and split-signature counterparts.