Metric affine gravity with dynamical chronology protection

Moustafa Ismail; David Mattingly

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arXiv · 2604.08511

Metric affine gravity with dynamical chronology protection

Moustafa Ismail·David Mattingly

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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 Horava-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, the gravitational sector contains the two tensor modes of GR and a single scalar mode. We investigate the classical cosmological effects and scalar-mode dynamics of this purely geometric chronology-protecting extension of general relativity.

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Moustafa Ismail, David Mattingly. 2026-04-09. Metric affine gravity with dynamical chronology protection. https://arxiv.org/abs/2604.08511

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