arXiv subjects

Moustafa Ismail

Publications and source records attributed to Moustafa Ismail.

3 recordsLinked to original sources

arXiv2026

Metric--affine gravity with dynamical chronology protection

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, $Λ=0$, 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.

gr-qc↗

arXiv2026

Asymptotic AEther Misalignment and Universal-Horizon Mechanics

Universal horizons trap even arbitrarily fast signals, providing a test of black-hole thermodynamics in theories with a preferred time direction. In Einstein--aether theory, this direction is defined by a dynamical field, the aether, that can remain non-aligned relative to stationary time at infinity. In particular, for asymptotically anti-de Sitter black holes, it was so far impossible to recover a first law of black-hole mechanics when such asymptotic tilt of the aether is present. Here, we resolve this issue for an exact family of static, spherically symmetric solutions. Using the covariant phase-space formalism, we identify the asymptotic tilt and the decaying part of the aether field as a boundary source and its conjugate response. Allowing both to vary independently obstructs the definition of an energy that generates time translations. Fixing either quantity, or imposing a relation between them, restores such an energy, while externally varying the boundary data introduces work into the first law. The energies obtained by fixing the source or its response are related by a Legendre transformation. At the universal horizon, heat includes both the metric area contribution and an irreducible aether energy flux. The resulting balance separates work at infinity from horizon heat and explains why the physical temperature cannot generally be inferred from the coefficient of an area variation in the black-hole mass.

gr-qc↗

arXiv2026

Einstein-aether Elliptic Charges and the First Law of Asymptotically AdS Black Holes

We investigate the thermodynamic role of asymptotic aether alignment for universal horizons in Einstein-aether theory. In the static, spherically symmetric, asymptotically AdS sector with $c_{14}=0$, the known first law for universal horizons contains an additional term whenever the aether is misaligned with the timelike Killing vector at infinity. While this term has recently been interpreted in Hořava--Lifshitz gravity as the contribution of an elliptic charge associated with khronon reparameterizations, no corresponding explanation was available in Einstein-aether theory. We show that, in the same sector, Einstein-aether theory possesses a previously unidentified symmetry of the reduced action, generated by infinitesimal transformations of the form $δu^a=f a^a$, where $a^a$ is the aether acceleration and $f$ obeys an elliptic constraint. We derive the associated current and charge, and show that the aligned limit is naturally interpreted as the ensemble in which this aether-charge contribution vanishes. This provides the Einstein-aether counterpart of the elliptic-charge mechanism in Hořava--Lifshitz gravity and clarifies the thermodynamic significance of asymptotic aether alignment.

gr-qc↗