We study the stability of $\textit{d}$-dimensional ($\textit{d}$ = 3, 4, 5) de Sitter and Minkowski spacetimes within the framework of semiclassical gravity sourced by a strongly coupled quantum field with a gravity dual. Our stability results are derived from a careful analysis of the $\textit{d}$-dimensional Lichnerowicz equation with mass-squared $\textit{m}$ and of semiclassical equations involving the dimensionless parameter $\textit{γ}$ . For $\textit{d}$ = 3, we find that Minkowski spacetime is always unstable against perturbations, whereas de Sitter spacetime becomes stable when a dimensionless parameter $\textit{γ}$ exceeds a critical value. In $\textit{d}$ = 4, both de Sitter and Minkowski spacetimes become unstable when the parameter $\textit{γ}$ exceeds its critical value. In contrast, in $\textit{d}$ = 5, de Sitter and Minkowski spacetimes remain stable for almost all values of the parameter $\textit{γ}$ , except for a regime in which higher-curvature corrections become comparable to the Einstein tensor.

Japan

Akihiro Ishibashi

Kengo Maeda

Takashi Okamura

Instability thresholds for de Sitter and Minkowski spacetimes in holographic semiclassical gravity

In this paper, we would like to investigate whether a generalized Einsteinian cubic gravity, in which three possible cubic interactions $$\mathcal{P}$$ , $$\mathcal{C}$$ , and $$\mathcal{C}'$$ are treated on an equal footing, admits a de Sitter solution as its stable cosmological solution. As a result, we are able to confirm the existence of the corresponding de Sitter solution for this gravity by solving analytically its field equations. Remarkably, only the cubic interaction $$\mathcal{P}$$ gives rise to the existence of the de Sitter solution. Then, we convert the field equations into the corresponding dynamical system for a stability analysis purpose. A fixed point of this dynamical system is found and shown to be equivalent to the obtained de Sitter solution. However, the perturbed dynamical system turns out to be incomplete, leaving undetermined information of the stability of the fixed point (or equivalently the de Sitter solution). Fortunately, we show that this loophole can be cured once the well-known Starobinsky term $$R^2$$ is introduced into the action of the generalized Einsteinian cubic gravity, despite the fact that it contributes nothing to the value of the de Sitter solution.

Vietnam

Tuan Do

Effect of $$R^2$$ on the stability of de Sitter solution of the generalized Einsteinian cubic gravity

We construct a manifestly covariant and coordinate-free plane-wave representation of scalar two-point functions in $\textit{d}$-dimensional anti-de Sitter spacetime. The construction is based on a new class of holomorphic plane waves defined globally on the universal covering of the AdS via chiral cones in the complex null cone. Imposing AdS invariance, locality, positive definiteness and a spectral condition, we obtain integral representations as superpositions over relative homology cycles, reproducing the standard maximally analytic solutions in terms of Legendre functions of the second kind. In Poincaré coordinates, the two-point functions diagonalize into a Källén-Lehmann superposition of ($\textit{d}$ – 1)-dimensional Minkowski correlators where the weight is a product of Bessel functions. This diagonalization clarifies the relation between Euclidean and Lorentzian AdS quantum field theory and allows Wick rotation of Euclidean Feynman diagrams to Lorentzian integrals supported on a single Poincaré patch while preserving full AdS covariance.

Italy

Ugo Moschella

Two point functions and quantum fields in the anti-de Sitter universe

In this article, we test the validity of the weak cosmic censorship conjecture (wCCC) in the background of a quantum Kerr–de Sitter (qKdS) black hole, incorporating exact backreaction from quantum matter fields. Using a test particle approach, we analyze whether an extremal qKdS black hole can be over-extremized to expose a naked singularity. Our results indicate that the black hole remains stable against horizon-destroying processes induced by infalling matter. Quantum corrections enhance the horizon’s robustness rather than destabilize it, offering further evidence that wCCC remains preserved in the presence of quantum effects.

India

Ankit Anand

Ankur Dey

Kerr–de Sitter black holes: quantum aspects and cosmic censorship conjecture

We investigate the superradiant instability of Ayón-Beato–García–de Sitter (ABG-dS) black holes under charged scalar field perturbations, employing both time-domain evolutions and frequency-domain computations. We demonstrate that the instability manifests exclusively for the spherically symmetric mode with $$\ell =0$$ for both massless and massive scalar field perturbations. In contrast, asymptotically flat ABG black holes remain stable in the massless limit, a finding that highlights the crucial role of the cosmological horizon in providing a confining boundary. We further analyze the dependence of the instability growth rate on the cosmological constant $$\Lambda $$ , the black hole charge $\textit{Q}$, the scalar field charge $\textit{q}$, and the scalar field mass $$\mu $$ . We find that the growth rate peaks at intermediate values of $$\Lambda $$ and $\textit{q}$, and increases monotonically with $\textit{Q}$. Conversely, an increase in the scalar field mass $$\mu $$ suppresses the instability, eventually quenching it when $$\mu $$ is sufficiently large. Compared with Reissner–Nordström–de Sitter black holes, ABG-dS black holes exhibit distinct instability characteristics, which arise from the modified electrostatic potential induced by nonlinear electrodynamics.

China

Yizhi Zhan

Hengyu Xu

Haowei Chen

Shao-Jun Zhang

Charged superradiant instability of spherically symmetric regular black holes in de Sitter spacetime: time- and frequency-domain analysis

We construct charges for four-dimensional spacetimes with a non-vanishing cosmological constant, including charges that are not conserved because of a leaky boundary and charges associated with corner terms in the symplectic current. The construction leads to manifestly finite charges for any choice of boundary conditions, and reveals new charges in partial Bondi gauge for an enlarged class of field configurations which are not fully on-shell, including a Weyl charge for conformal boundary conditions. In addition, we generalize the variational principle for AdS gravity with Dirichlet boundary conditions to a wedge of spacetime where the conformal boundary includes a corner. The boundary data is completely general, with no conditions restricting time dependence or the determinant of the metric. The Ward identities associated with boundary diffeomorphisms are shown to reproduce the evolution equations for the mass and angular momentum of fully on-shell field configurations.

United States

Robert McNees

Canada

France

Céline Zwikel

(Anti)-de Sitter with leaky boundaries and corners

We construct the graviton propagator in de Sitter space in a one-parameter family of non-covariant gauges. This family generalizes the simple gauge in which most graviton loop computations in de Sitter space have been performed. The resulting propagator has a relatively simple form and will facilitate checks of the gauge dependence of one-loop computations and proposed observables.

Czechia

Dražen Glavan

Graviton propagator in de Sitter space in a simple one-parameter gauge

Effective field theories (EFTs) provide a powerful framework to parametrise unknown aspects of possible ultraviolet (UV) physics. For scalar fields in de Sitter space, however, new emergent phenomena can arise when the cut-off scale of the theory lies below the horizon scale $\textit{H}$, as seen in the stochastic formalism of inflation. In this work, we study EFTs that, at leading order, reproduce the standard quartic theory in de Sitter, but with a variable cut-off identified with the mass of an integrated-out hidden sector. We perform the complete analytic computation for the tree- and loop-level matching between the effective $\textit{λϕ}$ theory and two possible UV realisations. We find that when the cut-off is much larger than the horizon, the theory admits a unitary description, up to exponentially suppressed corrections. In contrast, when the cut-off is lowered below $\textit{H}$, the system evolves into a mixed state and diffusive effects emerge. Nevertheless, at leading order, the EFT remains local and reproduces the same effective quartic coefficient as in the unitary regime. Furthermore, for the EFT matching at the loop-level, the effective quartic coupling changes sign and becomes negative as the cut-off decreases, in agreement with the result obtained from the stochastic formalism. In general, for cosmological EFTs, our findings highlight the role of non-unitary effects and illustrate their regimes of validity, within and beyond perturbation theory.

United Kingdom

Sebastián Céspedes

Zhehan Qin

Dong-Gang Wang

λ$\textit{ϕ}$ 4 as an effective theory in de Sitter

The one-loop gravitational path integral around Euclidean de Sitter space $\textit{S}$ has a complex phase that casts doubt on a state counting interpretation. Recently, it was proposed to cancel this phase by including an observer. We explore this proposal in the case where the observer is a charged black hole in equilibrium with the de Sitter horizon. We compute the phase of the one-loop determinant within a two-dimensional dilaton gravity reduction, using both numerical and analytical methods. Our results interpolate between previous studies of a probe geodesic observer and the Nariai solution. We also revisit the prescription for going from the Euclidean path integral to the state-counting partition function, finding a positive sign in the final density of states.

Yiming Chen

Douglas Stanford

Haifeng Tang

Zhenbin Yang

On the phase of the de Sitter density of states

Elliptic de Sitter (dS) spacetime dS/$\textit{ℤ}$ is a non-time-orientable spacetime obtained by imposing an antipodal identification to global dS. Unlike QFT on global dS, whose vacuum state can be prepared by a no-boundary Euclidean path integral, the Euclidean elliptic dS does not define a wavefunction in the usual sense. We propose instead that the path integral on the Euclidean elliptic dS defines a no-boundary density matrix. As an explicit example, we study the free Dirac fermion CFT in two-dimensional elliptic dS and analytically compute the von Neumann and the Rényi entropies of this density matrix. The calculation reduces to correlation functions of vertex operators on non-orientable surfaces. As a by-product, we compute the time evolution of entanglement entropy following a crosscap quench in free Dirac fermion CFT. We also comment on a striking feature of free QFT in elliptic dS: its global Hilbert space is one-dimensional, wheres the Hilbert space associated to each observer is a nontrivial Fock space.

Raphaël Dulac

Zixia Wei

No boundary density matrix in elliptic de Sitter dS/$\textit{ℤ}$ 2