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Stefano Liberati

Publications and source records attributed to Stefano Liberati.

At least 19 recordsLinked to original sources
arXiv2026

Testing the Kerr hypothesis with orbital frequencies: lessons from spacelike singularities and black bounces

The inner horizon of stationary black holes is subject to classical and semiclassical instabilities, suggesting that the Kerr interior should be modified. Quantum backreaction may drive the curvature to the Planck scale, producing a spacelike singularity, or halt its growth at lower curvatures, leading to a regular black bounce. We investigate the phenomenology of these two possibilities using, respectively, a Gürses--Gürsey geometry with a spacelike singularity and the rotating Simpson--Visser black bounce. Both geometries reproduce the light ring and ISCO frequencies of a Kerr black hole with the same ADM mass and angular momentum, by calibration in the first case and identically in the second. We find that the null and timelike convergence conditions are violated outside the horizon, up to a finite radius beyond the light ring in the Gürses--Gürsey geometry and everywhere in the rotating Simpson--Visser spacetime. Moreover, their fundamental scalar quasinormal modes deviate from those of Kerr, most markedly in the damping, which is enhanced in the Gürses--Gürsey geometry and reduced in the rotating Simpson--Visser one. Thus, agreement with Kerr in a limited set of observables is not sufficient to establish the Kerr nature of a compact object, motivating complementary observational tests.

gr-qc↗
arXiv2026

Analog regular black holes and black hole mimickers for surface-gravity waves in fluids

Recent advances in the observation of black-hole candidates have renewed interest in probing their near-horizon structure and in searching for departures from the standard singular solutions of general relativity. In this context, significant effort has been devoted to regular black holes and to horizonless black-hole mimickers, motivated primarily by quantum-gravitational effects. Depending on the value of the regularization parameter relative to the object mass, typical spherically symmetric solutions can describe either of these two scenarios. Regular black-hole configurations generically feature an outer and an inner horizon surrounding a maximally symmetric core; the inner horizon in turn triggers mass inflation and semiclassical instabilities. The horizonless branch of the same solutions, by contrast, supports stable inner light rings when sufficiently compact, yet is itself subject to instabilities associated with long-lived quasinormal modes. Here we investigate how to emulate these spacetimes in an analogue-gravity platform based on surface-gravity waves in a shallow-water basin, with the aim of reproducing these instabilities experimentally. We begin by identifying the flow profiles and boundary conditions required to replicate the relevant effective geometries. In particular, we show that the inner-core metrics can be simulated with a non-rotating central-drainage configuration, and we propose a graded-drainage profile to connect them to an asymptotically flat exterior region. We then assess the experimental feasibility of studying the instabilities mentioned above with current technology. Our conclusion is that, while the required setup is realizable in principle, alternative media, such as Bose-Einstein condensates, may offer a more practical route to faithfully capturing the targeted physical features.

gr-qc↗
arXiv2026

Analog regular black holes and black hole mimickers in a Bose-Einstein condensate: from the core to the light rings

We show how static, spherically symmetric regular black holes and black-hole mimickers with radius-dependent mass function $m(r)$ can be emulated in a quasi-two-dimensional Bose--Einstein condensate, with the aim of probing inner-horizon and inner-light-ring instabilities in the laboratory. Matching the acoustic and gravitational line elements on an equatorial disc fixes the ratio of radial flow velocity to sound speed and reproduces both horizon locations and surface gravities exactly. The analogue geometry nevertheless contains a residual radially varying sound-speed factor in $g_{tt}$, which shifts the light-ring positions and reduces the range of the regularization parameter $\ell/M$ for which a light-ring pair exists. We derive the interaction profile that removes this shift. Combining the metric correspondence with the continuity and Bernoulli equations, we then obtain closed-form expressions for the external potential and interaction strength realizing the full target spacetime, with $M$ and $\ell$ directly tunable through parameters of the potential. Finally, we translate the construction into experimental requirements (quasi-two-dimensional confinement, healing-length resolution, three-body lifetime, and stationary-flow lifetime) and present three fiducial designs achievable with present-day $^{39}\mathrm{K}$ platforms.

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

Cosmology with a Non-minimally Coupled Dark Matter Fluid II. Cosmological Perturbations

We extend our study of a cosmological scenario in which dark matter is non-minimally coupled to gravity at the fluid level. In previous work, we showed that this interaction can drive an early phase of accelerated expansion, addressing the horizon and flatness problems, and can also lead to a cosmological bounce in the presence of negative spatial curvature. Here we analyse the evolution of linear perturbations in this framework. We derive the equations governing scalar, vector and tensor perturbations, and obtain analytic solutions in the relevant cosmological regimes. We find that perturbations generated during the accelerated expansion phase produce a strongly blue scalar power spectrum and are therefore incompatible with observations. By contrast, if primordial fluctuations are generated during the contracting phase of a bouncing solution, the model yields an approximately scale-invariant scalar power spectrum while keeping the tensor-to-scalar ratio compatible with current bounds. Although our treatment relies on simplifying approximations that should be refined in future work, these results indicate that an observationally viable primordial spectrum can be obtained only with a bouncing solution, and hence with a non-singular early-time evolution.

gr-qc↗
arXiv2026

Phenomenology of dynamical trapped regions

Regular black holes (RBHs) and horizonless black hole mimickers (BHMs) are often studied as stationary alternatives to classical black holes, although their interiors may be unstable or undergoing relaxation. We investigate the phenomenological consequences of this evolution by numerically evolving linear scalar perturbations on prescribed, time-dependent Hayward-like geometries. We consider BHM--RBH--BHM histories in which an initially horizonless object temporarily develops a trapped region before returning to a horizonless configuration, through either a single bounce or a sequence of repeated bounces. Using horizon-penetrating Painlevé--Gullstrand coordinates, we follow the perturbations through the formation and disappearance of trapping horizons and compute the resulting waveforms and scalar-field energy. For histories with identical initial and final geometries, we find that changing the duration of the intermediate regular black hole phase produces differences in the amplitude and phase of the late-time signal, including its echoes. Longer trapped phases also yield greater energy amplification, consistent with the blueshift of outgoing modes near the inner trapping horizon. These results show that the late-time response of a dynamically relaxing BHM need not be determined by its final configuration alone: it can retain a memory of the dynamical history of its interior.

gr-qc↗
arXiv2026

Emergent scalar field dynamics in a cosmological spacetime from GFT quantum gravity

We derive an effective scalar field theory for matter in group field theory condensate cosmology, starting from the fundamental quantum-gravity dynamics in a fully relational framework and encompassing both early- and late-universe regimes. The collective hydrodynamics of the underlying quantum geometry allows us to reconstruct both the homogeneous cosmological dynamics of matter and geometry and an inhomogeneous local field-theory description. Localization in space and time is defined relationally with respect to a material reference frame. At the homogeneous level, we obtain a modified scalar field theory on the emergent FLRW spacetime selected by the condensate. It recovers the standard dynamics of a massless scalar field in the late-time general-relativistic regime while retaining quantum-gravity corrections near the cosmological bounce. At the perturbative level, scalar inhomogeneities obey an effective wave equation that carries signatures of the underlying quantum-gravity microstructure. In the early-universe regime, this equation exhibits a modified dispersion relation with both dispersive and dissipative contributions. These corrections provide a concrete avenue for identifying phenomenological signatures of quantum gravity directly from a fundamental quantum-gravity framework.

gr-qc↗
arXiv2026

Semiclassical Black Hole-White Hole transitions: an analytical treatment

Recent numerical studies of semiclassical gravity suggest that, in spherically symmetric black holes with both outer and inner horizons, the semiclassical instability of the inner horizon can drive the complete evaporation of the trapped region on timescales shorter than the standard Hawking evaporation time. Independent simulations further indicate that the disappearance of the trapped region is followed by the formation of an anti-trapped region, i.e.~a dynamical white hole. In this work, we develop an analytic treatment of quantum effects in trapped and anti-trapped regions, showing how these numerical results can be understood within simplified two-dimensional models. We consider collapse models describing the formation of charged and regular black holes and compute the renormalized stress-energy tensor of the $|\textit{in}\rangle$ vacuum state. We show that, within this framework, the emergence of an anti-trapped region is a generic consequence of the amplification of negative energy fluxes propagating along the outgoing direction inside the initial trapped region. This provides an analytic explanation for the black-hole-to-white-hole transition observed in numerical simulations. Our analysis further suggests that the fluxes generated by the subsequent anti-trapped region, now propagating along the ingoing direction, may trigger the formation of a new trapped region. This raises the possibility of a cascade of black-to-white-hole transitions, potentially ending in a horizon-free, bouncing spacetime without invoking additional quantum-gravitational dynamics. Although establishing the complete evolution requires a self-consistent treatment of semiclassical backreaction, our framework identifies which features of the mechanism are universal and which depend on the geometry, laying the groundwork for a systematic investigation of semiclassical black-hole-to-white-hole transitions.

gr-qc↗
arXiv2026

A parity selection rule for regular black holes

Regular black hole metrics are usually studied kinematically, but a finite-curvature static core does not guarantee that the underlying theory can consistently evolve generic matter through a regular center. We derive a necessary local consistency condition within the most general class of action-based, identically conserved, second-order gravitational field equations in spherical symmetry. Regularity requires that the two functions defining the theory have opposite parities under reversal of the signed radial coordinate, together with additional center-regularity and nondegeneracy conditions. In the integrable sector, this criterion is equivalent to requiring the generalized Misner--Sharp--Hernandez mass to be odd across the center, to vanish cubically there, and to contain no point-mass contribution. For theories reconstructed from static one-parameter vacuum families, the condition becomes covariance under simultaneous reversal of radius and mass. The theories associated with the Hayward and Dymnikova geometries satisfy this selection rule. In contrast, the Bardeen theory does not, demonstrating that curvature regularity of a static solution is insufficient for dynamical consistency with generic matter. We also characterize an infinite class of admissible theories containing Hayward-like black holes with de Sitter cores. The selection rule provides a necessary condition for theories intended to describe regular collapse, but does not by itself establish well-posedness or guarantee a nonsingular endpoint.

gr-qc↗
arXiv2026

Bridging Superfluid and Nonminimally Coupled BEC Dark Matter through RAQUAL

Motivated by their common condensed-matter inspiration and their shared aim of reconciling MOND-like phenomenology on galactic scales with particle dark matter on larger scales, we investigate the relation between Superfluid Dark Matter (SFDM) and Bose--Einstein Condensate Dark Matter (BECDM). Since SFDM is formulated in the Newtonian regime whereas BECDM is fully relativistic, we first show that the MONDian formulation of SFDM arises as the Newtonian, low-acceleration limit of a Relativistic AQUAdratic Lagrangian (RAQUAL) theory in the Einstein frame. We then transform its covariant interaction sector to the Jordan frame and compare it with BECDM. The phonon--baryon interaction of SFDM maps onto the BECDM derivative coupling to the Einstein tensor, supplemented by a small non-minimal coupling to the Ricci scalar. The interaction sectors are therefore equivalent up to a linear perturbation of the Einstein--Hilbert term. Their kinetic sectors, however, remain inequivalent: the standard quadratic kinetic term of BECDM cannot be mapped onto the non-analytic kinetic term required by SFDM. The two models are consequently related but not dynamically equivalent. This mapping provides a covariant interpretation of the SFDM interaction and clarifies which theoretical properties can be transferred between the two frameworks.

gr-qc↗
arXiv2026

Off-shell equivalence in quantum field theory and gravity

Field redefinitions connect many formulations of the same physics, but the standard equivalence theorem is an on-shell result and cannot be used to decide when two quantum descriptions are equivalent off shell. This paper develops an operational criterion for that problem in terms of the Vilkovisky--DeWitt effective action. The central idea is that equivalence should be tested on scalar observables built by pairing configuration-space tensors with admissible probes. This makes the criterion sensitive to the observable class under consideration and separates the usual on-shell notion of equivalence from the stronger off-shell notions needed in gravity, cosmology and non-equilibrium quantum field theory. Metric $f(R)$ gravity and a scalar field theory example serve as prototypes, showing how the formal criterion distinguishes local, branchwise and genuinely global equivalence. In particular, we show that metric $f(R)$ gravity and its auxiliary-field reformulation yield the same quantum theory when the auxiliary constraint is enforced in the path integral. This is distinct from quantizing the corresponding scalar--tensor action with the metric and scalar treated as independent integration variables, which defines a different quantum theory. Apparent quantum inequivalences can then be traced to comparisons between different quantum objects, rather than to a failure of actual equivalences. This leads to general and precise notions of local and global equivalence under both field redefinitions and auxiliary-variable extensions.

hep-th↗
arXiv2026

Semiclassical regularity of compact trapped regions: From dynamical horizons to inner extremality

In eternal black-hole spacetimes, inner horizons are Cauchy horizons and are generically unstable. For non-extremal inner horizons, this includes both the classical mass-inflation instability and a semiclassical instability associated with divergences in the renormalized stress-energy tensor (RSET). Inner-extremal geometries, for which the inner-horizon surface gravity vanishes, evade classical mass inflation, but in stationary settings still suffer from singular behavior of the RSET. In this work, we show that the dynamical case is qualitatively different. Considering spacetimes describing the formation and evaporation of a compact trapped region in finite time, and working in the $s$-wave Polyakov approximation, we compute the expectation value of the stress-energy tensor in the in-vacuum state. Given that in this case the inner horizon is not a Cauchy horizon, the RSET remains finite everywhere. For generic non-extremal inner horizons, however, the RSET grows exponentially in time at the inner horizon, with a divergence emerging only in the asymptotic limit of an ever-lasting trapped region. For inner-extremal geometries this exponential growth is replaced by a considerably milder power-law growth. Such spacetimes may therefore be considered natural candidates for classically and semiclassically meta-stable black-hole interiors.

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↗
arXiv2026

Null geodesic defocusing in dynamical black-hole-to-white-hole transitions

We investigate the defocusing of null geodesics in dynamical, non-singular black-hole-to-white-hole transitions. Working at the level of spacetime kinematics, and without assuming any specific gravitational field equations, we show that the contraction and disappearance of a trapped region, as well as the subsequent formation and expansion of an anti-trapped region, necessarily require a violation of the null convergence condition. This conclusion follows directly from the behaviour of the null expansions across the trapping and anti-trapping horizons, and is therefore independent of the microscopic mechanism responsible for singularity resolution. We then illustrate this general argument by constructing a class of explicit bouncing geometries in generalised Painlevé-Gullstrand coordinates, obtained by promoting static regular black holes with de Sitter cores to time-dependent black-hole-to-white-hole transition models. For a Bardeen-type mass function, we show that the required violation of the null convergence condition is localised within the intermediate dynamical phase in which the trapped region evaporates and the anti-trapped region forms. Finally, we argue that the limiting case of an instantaneous black-hole-to-white-hole transition would require an unbounded violation of the null convergence condition, signalling a breakdown of the effective continuum metric description, and the need to appeal to a full quantum-gravitational description.

gr-qc↗
arXiv2026

Hawking radiation with dispersion: reconciling the Bogoliubov and tunneling approaches

We investigate Hawking-like particle production in analogue gravity systems with superluminal modified dispersion relations. For a broad class of even, convex, and polynomially bounded dispersion relations, we show that the relevant outgoing modes are governed by an effective horizon induced by dispersive propagation. Extending the near-horizon S-matrix method beyond the purely sonic regime, we compute the Bogoliubov coefficients and demonstrate that, in the low-energy and adiabatic limits, they agree with the tunneling result obtained from the approximant ray. In both cases, the emission spectrum is controlled by an effective surface gravity associated to the effective horizon, leading to controlled deviations from exact thermality. Our results establish an analytical connection between the Bogoliubov and tunneling descriptions in dispersive settings and clarify the conditions under which Hawking radiation remains robust against ultraviolet modifications, with implications extending beyond analogue gravity.

gr-qc↗
arXiv2026

Taming the Aretakis instability: extremal black holes with multi-degenerate horizons

Stationary black hole geometries with non-degenerate Cauchy horizons are classically unstable due to mass inflation. At extremality, mass inflation is absent, but a different dynamical instability arises: the Aretakis instability. In this work, we investigate the properties of degenerate horizons and their associated Aretakis instabilities. By studying examples with increasingly higher-order horizon degeneracy, we show that the Aretakis instability weakens as the degree of degeneracy grows. Motivated by these results, we propose a new black hole geometry characterized by an infinitely degenerate horizon, which we argue is stable under Aretakis-type perturbations and may therefore provide a concrete realization of a "graveyard" end state for these objects.

gr-qc↗
arXiv2026

A Covariant Phase Space Approach to Einstein-AEther Gravity

Black hole thermodynamics in Lorentz-violating gravity is subtle because different excitations propagate at different speeds and hence identify different causal horizons. We revisit Einstein--AEther gravity using the covariant phase space formalism with boundaries and derive a consistent first law for stationary black holes. For a mode of propagation speed $c_s$, we introduce a disformal frame in which the corresponding causal horizon is a Killing horizon, so that the standard Wald-type derivation can be carried out. The result is then mapped back to the original frame, where it mantains the same structure. The associated horizon charge contains, besides the usual Komar term, an irreducible entropic AEther contribution that can be interpreted as heat due to the AEther flux across the horizon; accordingly, the total entropy splits into a gravitational part and an AEther part. We further develop an extended-thermodynamics framework in which the couplings of the theory are allowed to vary, obtaining generalized Smarr relations. Finally, we analyze the probe-mode limit $c_s \to +\infty$, clarifying its connection to universal-horizon thermodynamics and resolving the apparent tension in the literature between approaches that (i) fix the entropy to be proportional to the area and infer a corresponding temperature, and (ii) impose the Hawking temperature associated with modes peeling from the universal horizon and infer the entropy. Once the independent AEther contribution is properly taken into account, the two prescriptions are reconciled.

gr-qc↗
arXiv2026

Freezing lakes as analogue models of $Λ$CDM cosmology and beyond

We extend previous conduction-based analogies between ice growth in a lake and cosmological expansion by incorporating buoyancy-driven heat transport. Reformulating the Stefan problem with both conductive and convective fluxes yields an evolution equation for the ice thickness $s(t)$ that is structurally analogous to the Friedmann equations for the cosmological scale factor $a(t)$. Beyond reproducing radiation-, matter-, and curvature-like behaviors, we introduce a reduced description of convection in which the vertically integrated heat flux reaching the moving ice-water interface is modeled as a power-law function of the instantaneous liquid-layer thickness, generating two additional effective contributions. The first is a constant term, directly analogous to a cosmological constant, arising from the persistence of buoyancy-driven transport under geometric confinement. The second is a $s^{-1}$ contribution originating from the coupling between the moving ice boundary and the convective boundary layer. This term reflects the specific reduced flux-height Ansatz adopted, rather than a universal physical prediction. When expressed in Friedmann-like cosmological form, this term entails a fluid with negative energy density and equation-of-state parameter $w=-2/3$. In cosmology this term may be an effective one associated to a network of domain walls made of exotic energy/matter, but it might also arise from an energy exchange between cosmological components. Overall, the results should be interpreted as a structural analogy between evolution equations, showing how nonlinear transport mechanisms in a classical moving-boundary problem can reproduce the hierarchy of scaling terms familiar from cosmology within a reduced and analytically tractable framework.

gr-qc↗