On Cauchy surfaces in conformally stationary spacetimes

In this paper, we investigate static magnetic brane solutions in (n+1)-dimensional quartic quasi-topological gravity coupled to power-law Maxwell nonlinear electrodynamics. We derive the corresponding solutions and analyze their geometric and physical properties. We observe that the solutions are horizonless and free of curvature singularities in the physically accessible region, while the spacetime generally exhibits a conical singularity with a nonzero angular deficit at the lower radial endpoint. We demonstrate that the endpoint is independent of the higher-curvature couplings lambda, mu, and c, whereas the near-endpoint geometry is mainly controlled by the magnetic parameter q and the nonlinear parameter s. At large radial distances, we find that the higher-curvature couplings govern the asymptotic behavior of the metric. We also establish that finite asymptotic solutions require 1/2 < s <= 2 and analyze the dependence of the angular deficit on q, s, and the endpoint parameter rho_+. Using the counterterm method, we calculate the conserved mass density and show that the static magnetic brane carries no net electric charge. Finally, we verify that the power-law Maxwell theory recovers linear Maxwell electrodynamics at s=1 and Einstein gravity when lambda=mu=c=0.

gr-qc↗

arXiv2026

A note on decoherence in quantum cosmology by thermal perturbations: an open quantum systems approach

We study, within a toy model, how a thermal bath of cosmological perturbations can decohere the minisuperspace degrees of freedom of the universe, treating the perturbations as an environment in the framework of open quantum systems. Instead of tracing out the perturbations from a semiclassical ansatz for the total wave function, as in earlier approaches, we incorporate their backreaction on the background through an effective second-order coupling and derive a master equation. The model describes a spatially flat semiclassical universe during slow-roll inflation with a dust clock field, while scalar cosmological perturbations in a phenomenologically assumed thermal state constitute the environment. The master equation is non-Markovian, since the bath correlation functions do not decay in time. Within this framework, we find that, for a representative set of parameter estimates, the characteristic decoherence timescale corresponding to the thermal contribution can be shorter than the Hubble timescale. In addition, we address the question of whether the pointer-state structure induced by the decoherence in such a framework could provide a criterion for the choice of semiclassical states in quantum cosmology.

gr-qc↗

arXiv2026

Post-Newtonian secular dynamics of hierarchical triples II. Canonical derivation of the 1PN\,$\times$\,quadrupole cross terms and of the quadrupole-squared term

We derive the secular Hamiltonian of a hierarchical triple from the 1PN ADM three-body Hamiltonian by a two-stage Lie-Hori-Deprit transformation carried out entirely in canonical variables, and use it to settle the relativistic cross terms left open in Paper I. A bookkeeping rule enumerates all contributions to each order in the 1PN parameter $δ$ and the hierarchy parameter $\varepsilon$. The 1PN monopole interaction reduces to a renormalization of the inner mass by its binding energy, and the order $δ\varepsilon^{3/2}$, at which Lim and Rodriguez (LR) found a libration term, is empty. The electric and magnetic 1PN $\times$ quadrupole cross terms agree exactly with the effective-field-theory result of Kuntz, Serra, and Trincherini; for the magnetic term this is the first independent derivation. We give the next-to-leading order 1PN $\times$ quadrupole term in closed form; it depends on the outer argument of pericenter $ω_2$. At quadrupole-squared order we obtain the complete secular Hamiltonian, which generates the secular equations of Will and of Conway and Will; its $2ω_2$ harmonic is a gauge term, removable even on the commensurability between the ZLK period and the nodal precession, by a generator given in closed form via Tremaine's fictitious time. The EIH and ADM descriptions coincide at 1PN order; the formulations differ only in the definition of the elements, for which we give the osculating-contact map. In secular integrations of LR's triples, the eccentricity enhancement with the octupole is controlled by the $ω_2$-dependent part of the next-to-leading order term and is two to three times that of Paper I, while LR's accelerated mergers remain absent.

gr-qc↗