Metric--affine gravity with dynamical chronology protection

Abstract

Modified theories of gravity often introduce geometric structure beyond general relativity to address unresolved problems in the gravitational sector without invoking ad hoc matter fields. Mimetic gravity, for example, generates an effective cosmological dark sector by isolating the conformal mode of the metric, while Hořava--Lifshitz gravity attains power-counting renormalizability by endowing spacetime with a preferred dynamical foliation. Although chronology protection was not the original motivation for either theory, both can realize it classically through stable causality. This suggests that chronology protection itself may be elevated from a derived property to a guiding principle for constructing modified gravitational theories, especially if its implementation at the quantum-gravitational level leaves infrared imprints in the effective action. Motivated by this possibility, we introduce a toy metric--affine gravity model whose chronology-protecting sector is purely geometric. The model breaks projective invariance and selects a timelike, closed non-metricity one-form. In the globally exact sector, this one-form defines a time function, realizing stable causality through affine geometry. On the matter-free, spatially flat FLRW branch with non-compact slices, $\Lambda =0$, and positive chronology-sector density, the local quadratic gravitational spectrum contains the two tensor modes of GR and a single scalar mode. The scalar has positive kinetic energy but a gradient instability, $c_s^2=-2$, requiring stabilization before a viable cosmological application can be established. We investigate the classical cosmological effects and scalar-mode dynamics of this purely geometric chronology-protecting extension of general relativity.

arXiv comment22 pages, phenomenological analysis modified compared to v1, matches accepted version