Papers

We construct high-accuracy semi-analytic mode functions for scalar cosmological perturbations in mLQC-I within the hybrid approach to study ultraviolet regularity and gravitational particle creation. The evolution comprises an asymptotic pre-de Sitter branch, a pre-inflationary region containing the quantum bounce, and inflation. In the intermediate region, we construct the complete third-order uniform asymptotic solution and introduce adaptive defect correction. The third-order approximation achieves sub-percent complex-mode accuracy for representative higher-wavenumber modes; defect-corrected solutions agree with numerical evolution at the 0.00001% level over the tested low-wavenumber range. Retaining amplitudes and phases provides a controlled basis for future precision calculations of primordial power spectra and non-Gaussian correlations. We compare the de Sitter vacuum selected in the remote contracting branch with a fourth-order adiabatic state imposed at the bounce. Finite-wavenumber matching and the strict ultraviolet limit require different asymptotic treatments: extrapolating the leading connection to infinite wavenumber yields a spurious constant negative-frequency coefficient and an apparent ultraviolet divergence. Instead, the large-wavenumber de Sitter mode agrees with the fourth-order adiabatic expansion through the order required for energy-momentum tensor renormalization, and smooth exact evolution preserves this ultraviolet structure. Finite and intermediate modes can nevertheless undergo nonadiabatic Bogoliubov mixing and particle creation. Finite-mode particle production thus coexists with ultraviolet regularity. The ultraviolet argument also extends to the dressed-metric approach in its corresponding large-wavenumber regime, while quantitative mode functions must be recalculated using its effective mass.

Asymptotic Safety provides a promising framework for the quantization of Scalar-Tensor theories as non-perturbatively renormalizable quantum field theories. Over the past years, growing evidence has accumulated for the existence of non-trivial renormalization group fixed points in a variety of Scalar-Tensor models. In this work, we investigate the robustness of these fixed points under significant modifications of the computational setup. In particular, we employ a gauge condition that explicitly depends on the dynamical scalar field, going beyond a mere gauge-parameter deformation of the standard linear covariant gauges commonly used in the literature. Exploring different field parameterizations, we identify fixed points belonging to the Gaussian Matter class in four spacetime dimensions that are consistent with previous studies. The real part of the critical exponents associated with the scalar-field mass and the non-minimal scalar-curvature coupling turn out to have very small absolute value. We provide a comprehensive exploration of how the anomalous dimension of the scalar field and field parameterization impact those critical exponents showing that fixed-point candidates with two or four relevant directions emerge.

The island proposal guarantees a unitary Page curve for evaporating black holes, implying that operators deep behind the horizon must be fully reconstructable from asymptotic Hawking radiation. However, because the interior is causally disconnected and highly dynamical, standard entanglement wedge reconstruction methods, relying on global isometries or continuous modular flow, fail to provide this mapping. We resolve this by constructing a protocol for island reconstruction using the algebraic Petz map. By using modular theory, we reflect island operators into the accessible complement, transport them to the boundary, and map them into the radiation algebra. Furthermore, we demonstrate that this purely algebraic derivation naturally yields the structure of Papadodimas-Raju mirror operators. This framework provides a rigorous operational dictionary for the island formula, proving that information trapped inside a black hole can be decoded.

Primordial black holes, whose formation in the early Universe was first hypothesized by Ya.B.~Zel'dovich and I.D.~Novikov in 1966 (the text of this pioneering article is provided in Appendix II to this review), have attracted increasing attention in recent years. Modern astronomical data across the electromagnetic spectrum, together with observations of gravitational waves and cosmic neutrinos, have revealed a number of unique astrophysical phenomena that may be related to primordial black holes formed in the pre-stellar cosmological epoch. This review describes various models of primordial black hole formation, as well as methods for testing these models with astronomical and astrophysical observations. In particular, we discuss in detail the formation of primordial black holes and their clusters through the collapse of closed domain walls and through effects associated with extra spatial dimensions of the Universe.

In massive scalar-tensor gravity, a scalarized neutron star must shed its scalar hair as it collapses into a black hole. Using 3D simulations of a realistic rotating progenitor we show that dispersion stretches the emitted burst into a quasi-monochromatic signal in the kHz frequency range lasting $>10^4$ yr and amenable to continuous-wave methods. We find that detecting these signals in directed searches of supernova remnants without a central pulsar requires sensitivity depths of 233–762 $\rm{Hz^{-1/2}}$ with the A+-ET-CE detector network. We estimate up to $\sim3$ of these signals in transit at any observational epoch.

We construct the general theory of a spin-1 field propagating on an inflationary background consistent with scale invariance, rotations, Bunch-Davies conditions and microcausality. Our Boostless Proca theory generalises the usual massive Proca theory to a Lagrangian with 5 free parameters: 4 corresponding to parity-even terms and a single parity-odd term. We impose three degrees of freedom from the outset and find that microcausality, the statement that field operators vanish outside the lightcone, requires the kinetic terms to be gauge invariant. We couple this spin-1 state to the inflaton and study the signatures it imprints on cosmological correlators. We find no new signals for the bispectrum meaning that the known signals are the most general thereby providing a very clean model-independent probe of new physics in this leading form of non-Gaussianity. For the trispectrum and higher-point functions, the boostless Proca theory produces new shapes with all 5 parameters in principle measurable.

We investigate how site-dependent Newtonian noise configurations affect the scientific performance of the Einstein Telescope (ET), a third-generation gravitational-wave observatory. We compare the baseline triangular and 2L detector geometries, with arm lengths of 10km and 15km, respectively, under three site-dependent noise scenarios constructed from measurements in the Euregio Meuse-Rhine region, Lausitz, and Sardinia. The differences among the adopted sensitivities are concentrated mainly below 10Hz and thus have the greatest impact on science cases that rely on the early inspiral of light objects, or on massive systems whose characteristic frequencies lie in the low-frequency band. We quantify these effects for four ET science cases: (1) detection and parameter estimation of intermediate-mass black-hole binaries; (2) post-Newtonian tests of General Relativity based on the early inspiral; (3) black-hole ringdown spectroscopy with massive remnants; and (4) early-warning and sky-localization capabilities for binary neutron-star mergers. Using Bayesian parameter estimation, we find that improved low-frequency sensitivity can significantly enhance ET's scientific performance across all four cases. Within the configurations considered here, a 2L network in which at least one of the two L-shaped interferometers attains a noise level comparable to that assumed for the Sardinian site generally provides better performance than the triangular configurations. Conversely, strong degradation of the sensitivity below 10Hz can substantially reduce, and in some regimes offset, the scientific gains expected from increasing the arm length from 10km to 15km. These results highlight the importance of site selection and low-frequency noise mitigation, alongside network geometry, for fully realizing ET's scientific potential in the largely unexplored frequency band below 10Hz.

It has been argued that in quantum gravity the Hilbert space associated with any closed universe is trivially one-dimensional. To reconcile this argument with the richness of our own universe, it was suggested that a nontrivial Hilbert space will emerge once an “observer" is included. Such an “observer" has been modeled in different ways. There are three concrete classes of models: $\textbf{1)}$ The “observer" is modeled by a term in the full Hamiltonian of the system which is linear in a phase space variable; $\textbf{2)}$ The “observer" is modeled in the quantum circuit model of holography as a subsystem in the closed universe which is entangled with an external non-gravitational system; $\textbf{3)}$ The “observer" is argued to be described by the emergent Goldstone bosons when the diffeomorphisms are spontaneously broken. In fact, the third class was discovered by an attempt to physically realize the first class. In this paper, we will show that the second class can also be unified into the third class. Our result indicates that the emergence of the “observer" is a manifestation of the $\textit{Massive Islands}$ paradigm. It demonstrates that the graviton mass is a feature of the quantum state. One should think of the islands as closed universes and the Goldstone boson associated with the graviton mass as the “observer", which is exactly the case at late-times for evaporating black holes. An implication of this work is that the “observer" can “observe" global symmetries in quantum gravity.

Black-hole shadows and geometrothermodynamics (GTD) are defined in different spaces but depend on the same macroscopic black-hole parameters. We review their correspondence for Reissner–Nordström (RN), Kerr, and Kerr–AdS black holes. The three GTD metrics reveal complementary thermodynamic structures, including Davies instabilities, extremal and degenerate limits, and regular curvature zeros, which can be mapped to shadow configurations. Kerr–AdS extends this correspondence to asymmetric shadows and multidimensional equilibrium states. We formulate a local Jacobian-rank criterion for thermodynamic reconstruction from shadow observables and discuss shadow–microstructure (SM) diagrams as optical representations of GTD-classified equilibrium states. The application to Sagittarius A* illustrates the inverse problem: shadow observations constrain thermodynamic states and provide a basis for identifying additional metric-dependent observables that could distinguish among the GTD metric structures.

We introduce a flexible mixture model for the joint source-frame component-mass distribution of compact binaries, inferred simultaneously with cosmology, together with a framework that identifies structures in the reconstructed density, quantifying their posterior stability, local prominence, and cosmological relevance. Applied to GWTC-5.0, the workflow clarifies several familiar and emergent structures. The $\sim35~M_\odot$ feature is classified as a clear near-equal-mass peak despite an ambiguous interpretation in the primary-mass projection. A weaker unequal-mass concentration near $(21,12)~M_\odot$ is recovered frequently but with lower prominence. A tentative joint transition near $60~M_\odot$ appears at mildly unequal masses, with a more robust manifestation in the primary-mass projection. For cosmology, the two-dimensional mass model constraint on $H_0$ is $33\%$ narrower than a model that flexibly reconstructs only the primary mass, and only $\sim~10\%$ wider than analyses based on strongly parameterized one-dimensional spectral structure. At the feature level, we show that the $\sim10~M_\odot$ and $\sim35~M_\odot$ concentrations depend on $H_0$ mainly through a common scale which captures the entire $\sim~(1+z)$ cosmological rescaling of both masses; the intermediate unequal-mass feature instead retains residual dependence in the orthogonal mass-ratio direction. More generally, we argue that population prominence, formal inclusion in an astrophysical feature catalogue, and cosmological relevance need not coincide. These results suggest that a genuinely two-dimensional view may be required to fully exploit spectral-siren information. Our workflow provides a reproducible way to detect population structure and identify which aspects of its geometry are associated with spectral-siren constraints. The feature finder is directly applicable to other population reconstructions.

We establish a charge-regular formulation of separated Kerr–Newman perturbations and an explicit map from master solutions to physical radiative fields. We derive regular angular and radial first-order systems and their second-order reductions, with finite-charge deformations of the spin $\pm1$ and $\pm2$ Teukolsky equations. The field and boundary normalizations recover the standard Kerr conventions while accounting for the nonuniform zero-charge limit at infinity. For physical Einstein–Maxwell perturbations with the usual peeling behavior, we obtain explicit outgoing gravitational and electromagnetic amplitudes and energy fluxes at nonzero real frequency. The resulting finite-charge radiation-composition ratios depend only on the regular angular functions, independently of the radial excitation.

We study the low-frequency scattering of gravitational and pseudoscalar waves off a non-rotating black hole in dynamical Chern-Simons (dCS) gravity, where the axial (odd-parity) gravitational sector is irreducibly coupled to the dynamical pseudoscalar while the polar sector remains general relativistic. Working to first order in the dimensionless coupling zeta = (ell_CS/M)^4 and using matched asymptotic expansions, we obtain an exact closed form for the graviton-to-pseudoscalar conversion probability, d_2 = 3(pi^2-9)^2/8, and a high-precision reconstruction, in a proven transcendental basis, of the graviton greybody (absorption) asymmetry, kappa_2 = (21/80) pi^4 + (27/8) pi^2 - 89833/1536, for the quadrupole (ell=2) channel. Both reduce to combinations of pi^4, pi^2, and rational numbers, with no ln2 or odd-zeta terms in the reconstructed basis; the transmission-phase sector, by contrast, contains an analytically derived zeta(3)-9/8 static integral, while its overall matching prefactor is identified numerically. The leading reflection-phase asymmetry vanishes, so that no polarization rotation is imprinted on the reflected wave at leading low-frequency order, while a subleading O(omegazeta) asymmetry remains in the transmitted phase. The normalized flux asymmetries are numerically consistent with a 1/(Momega)^2 falloff in the geometric-optics limit, where the leading remaining flux effect is conversion loss. The matched-asymptotic results are checked against direct two-channel integration, while the one-dimensional integral determining kappa_2 is evaluated to high precision within an analytically controlled transcendental basis. The results provide analytic targets linking dCS parity violation in absorption and mode conversion to the axial/polar quasinormal-mode splitting.

Quasi-periodic eruptions (QPEs) are repeating X-ray transients from galactic nuclei potentially caused by a stellar-mass orbiter repeatedly colliding with the accretion disk of a supermassive black hole (SMBH) in an extreme mass-ratio inspiral (EMRI). In this picture, the eruption arrival times encode the orbit and its relativistic precessions, while the quiescent spectral energy distribution (SED) probes the same disk and SMBH, providing two independent constraints on the same orbit-disk system. We present a joint spectral-timing framework that takes advantage of this by simultaneously fitting an analytic QPE timing model built on Kerr geodesic frequencies and a warped disk geometry, coupled to a self-irradiated disk SED model sharing the same SMBH mass, spin, and disk parameters. We apply it to the QPE sources GSN 069 and eRO-QPE2, showing that the QPE timings and quiescent spectra can be reproduced simultaneously in each, providing circumstantial support for the EMRI model and warped disks in these systems. However, given the sparse timing data in both sources, neither admits a unique solution. In eRO-QPE2, we identify six physical modes which reproduce the timings and SED comparably well, motivating denser observing campaigns to identify one unambiguously. Our results demonstrate both the promise of QPE spectral-timing as a precision probe of SMBHs, their accretion disks, and orbiting companions, and the aliasing subtleties of sparsely sampled QPE timings.

We establish an almost-splitting theorem for spacetimes under timelike sectional curvature bounds in the spirit of the celebrated Riemannian result due to Cheeger and Colding. More concretely, we show that if a globally hyperbolic spacetime with timelike sectional curvature bounded below by a nonpositive constant close to zero contains a long maximizing timelike segment, then causal diamonds along that segment are close in the Lorentzian Gromov–Hausdorff sense of Minguzzi–Suhr to those in a product metric spacetime. Our result can be interpreted as a quantitative analogue of the Lorentzian splitting theorem for spacetimes with nonnegative timelike sectional curvature by Beem–Ehrlich–Markvorsen–Galloway.

We compute the on-shell closed, generalized Komar charges associated to an arbitrary Killing vector of the two 10-dimensional $\mathcal{N}=2$ supergravity theories which are the low-energy effective field theories of the type~IIA and IIB superstrings. We study different rewritings of this charge in terms of Page charges and forms with special properties that are used in the derivation of thermodynamical relations in black $p$-brane spacetimes. The, we show that, when the Killing vector is the supersymmetric Killing vector built as a bilinear of a Killing spinor, the momentum maps of the different fields of the theory are

Composition gravity ($g$) modes have been proposed as a probe of dark matter in neutron stars, on the basis of one-fluid models in which the dark matter is in chemical equilibrium with the neutrons. We computed in full general relativity the $g$- and $f$-modes of stars in which fermionic dark matter couples to the nucleons only through gravity, and inferred the dark mass fraction from NICER and GW170817 data with a prior that comprises $63$ nucleonic equations of state (EOSs) from Skyrme and relativistic mean-field functionals, among them the models of Dutra et al. and of the compilation of Sun, Bhattiprolu and Lattimer. The data do not prefer dark matter and bound its mass fraction at $0.12 - 0.26$ ($95\%$) for particle masses of $0.5$–$2$~GeV. Coupled only through gravity, the dark matter reaches the buoyancy that restores a $g$-mode only through the gravitational field: a dark core holding $10\%$ of the mass raises the fundamental $g$-mode by $10-15\%$ at fixed EOS, several times less than in the one-fluid picture. The larger effect is indirect: a star with a dark core needs a stiffer nucleonic EOS, and since the $g$-mode is set by the density dependence of the symmetry energy, its frequency reflects the EOS that the star requires rather than the dark matter it contains; at fixed radius and symmetry energy a core holding $10\%$ of the mass is equivalent to a change of the slope $L$ of $9-12$~MeV. The $f$-mode, which follows the mean density, behaves differently: a dark core breaks the relation between the $f$-mode and the tidal deformability by about one percent, far outside the posterior spread of nucleonic stars.

We develop a geometric characterization of electromagnetic memory in terms of the optical data of charged-particle congruences. We derive the worldline deviation of charged particles near null infinity for both massless and massive particles and construct the corresponding optical data. We show that the asymptotic shear of the charged-particle congruence provides an equivalent encoding of electromagnetic memory and its associated large gauge transformation. This establishes a geometric interpretation of electromagnetic memory and large gauge transformations in terms of the asymptotic evolution of charged-particle congruences.

We study horizon-brightened acceleration radiation for a neutral scalar field in separable Kerr-like rotating metrics with nonlinear-electrodynamic mass profiles. The principal branch has $M=0$ and $\mu=3$, with finite but direction-dependent central curvature limits. An analytic extremal charge–spin parametrization fixes the black hole domain. At a simple outer horizon, the scalar equation reduces to local conformal quantum mechanics with corotating energy $\epsh=\omega-m\OmH$. For an isolated outgoing channel we derive the first finite-gap correction in proper time, including the radial Frobenius coefficient, angular variation, geodesic terms, and switching slope. This replaces a coordinate-dependent trajectory-phase multiplier by a protocol-defined detailed-balance affinity. Direct integration of the outgoing scalar equation and an equatorial infalling trajectory validates the expansion for an explicitly specified finite gate; a solvable logarithmic-mode limit separates switching from geometric contributions. We also analyze forced equatorial circular atoms and show that co-rotating and counter-rotating ultra-relativistic motion yields a mode-dependent orbital response rather than the infalling Hawking affinity. We identify the shrinking near-extremal radial scale and quantify the effect of an unselected second radial branch, without asserting a uniform extremal limit. Random atomic injection gives a selected-mode master equation and a nonnegative population entropy-production identity. A direct first-law integrability test shows why horizon-area bookkeeping remains conditional for this off-shell metric. Independently converged Chebyshev scalar quasinormal frequencies validate the WKB results and probe the exterior geometry beyond the leading horizon data. The calculation distinguishes branch-resolved horizon thermality, finite detector protocol, and exterior scalar dynamics.

We provide a rigorous representation of timelike geodesics in the maximally extended Kerr spacetime, using projection diagrams introduced by Chruściel, Ölz, and Szybka. In contrast to standard conformal diagrams, projection diagrams allow for plotting general causal curves (not necessarily confined to the two-dimensional surface associated with the diagram), preserving their causal characteristics. Our analysis utilizes two systems of horizon-penetrating coordinates (both ingoing and outgoing) to ensure a regular description of geodesics crossing the event and Cauchy horizons. On the other hand, at most four causal blocks can be joined smoothly by a Kruskal-Szekeres-type construction proposed for projection diagrams. Consequently, depicting a trajectory spanning several causal blocks may require more than one diagram, as the conformal factor either vanishes or diverges at the block boundaries.

Part I: Semiclassical Gravity Efficiently Solves $\mathsf{NP}$-Complete Problems Assuming the gravitational field is classical and that it couples to quantum fields via the semiclassical Einstein field equations (EFEs), we show that the weak-field dynamics of a massive and non-relativistic qubit can in principle be used to solve an $\mathsf{NP}$-complete problem in polynomial time. We attribute this vast computational power to the non-linear qubit dynamics afforded by the semiclassical EFEs. Consequently, the above two assumptions entail a violation of the Physical Extended Church–Turing Thesis, which we regard as evidence for the quantization of gravity. Part II: Spacetime Quasicrystals We generalize self-similar quasicrystals, such as the Penrose tilings, from Euclidean space to Minkowski spacetime. We construct the first examples of Lorentzian quasicrystals and identify their key novel features. We then explore possible connections to fundamental physics. First, we propose a speculative scenario in which our $(3+1)$D universe is embedded in a particularly symmetric $(9+1)$D torus $\mathbb{T}^{9,1}$, previously identified as the most symmetric toroidal compactification of the superstring. We suggest that this construction may shed light on the seesaw relation $M_{\rm Pl}M_{\rm vac} \approx M_{\rm EW}^{2}$ between the Planck, vacuum-energy, and electroweak scales. Second, we replace the random causal sets of causal set theory with ordered, highly symmetric quasi-crystalline causal sets. We find suggestive evidence that our quasicrystals outperform Poisson random sets in the number-volume correspondence in both $(1+1)$D and $(3+1)$D, where such behaviour has been conjectured to be impossible. Finally, extending hyperuniformity from space to spacetime, we show that our $(1+1)$D quasicrystal is the first demonstrated example of a spacetime-hyperuniform structure.

We study linear perturbations of the Maldacena-Milekhin-Popov (MMP) wormhole in the parity sector that pairs axial metric perturbations with polar gauge perturbations. It contains the axial gravitational mode and the electric field along the magnetic field lines. The lowest Landau level (LLL) of the charged fermions responds through its charge. The gauge components $a_\tau$, $a_\rho$ form a two-dimensional gauge field on the worldsheet of each field line, and the fermions respond with the exact Schwinger current. Lifted over the sphere, it gives the electric field the MMP screening mass $m_S^2=\alpha q^2$, which is not small and survives in the mouth. In the throat the perturbation tilts the worldsheets, and the Casimir stress tensor acquires mixed components. Force balance then requires a current across the field lines, and charge conservation fixes an accompanying polarisation current, the counterpart of the Hall current of the polar sector. These currents close the Bianchi identities at first order in the fermionic backreaction $\alpha$. The system reduces to three coupled master equations, given in closed form and matched to the fermion-free mouth. Consistency alone does not select the fermionic constitutive relation, but locality, covariance and the index theorem do. At $\alpha=0$ the throat reduces to Poschl-Teller problems with masses $l(l-1)$ and $(l+1)(l+2)$ at $m_S^2=0$. At finite $m_S^2$ one mode is screened, one tends to the mass $l^2+l+2$, and one neutral charge-density wave stays exactly massless. The $O(\alpha)$ shifts of the massive normal frequencies are real and negative. The two parity sectors are isospectral only without fermions. The neutral channel sees an attractive region in each mouth, and a numerical solution coupled to the gravitational channel in the exact mouth potential finds no bound state. The sector has no growing mode for $l\ge2$ at this order.

There exist two different paradigms for records in isolated quantum systems: decoherent histories (which determines which past events leave formal records) and quantum Darwinism (which determines which current events get redundantly recorded). Neither one implies the other, and connections between the two are scarcely investigated. We unify quantum Darwinism and decoherent histories based on the insight that only redundant records about past events matter. The resulting framework of past redundancy encompasses quantum Darwinism when applied to events at the current time, and it connects redundancy to decoherence of histories and the probability sum rule via the Bhattacharyya coefficient (or fidelity). Importantly, various examples show that past redundancy is a more widespread phenomenon than the fine-tuned models necessary for quantum Darwinism suggest. We thus arrive at a richer picture in which redundancy can be caused by mechanisms different from quantum Darwinism (such as macroevents or cascade structures), which emphasizes that redundancy is necessary but not sufficient for objectivity, and which, overall, allows to realistically investigate the question: how generic is redundancy in nature?

Third-order dispersion turns the soliton of the nonlinear Schrödinger equation, which underlies optical analogues of event horizons in fibers, into a weakly radiating solitary wave. We compute three of its properties without assuming a thermal emission law. First, the linearized (Bogoliubov-de Gennes) problem must be posed about the stationary state of the dispersive equation: linearizing about the integrable profile produces a complex quartet that is converged in grid and box, grows as the square root of the dispersion strength and is spurious. About the stationary state, on a periodic grid, the zero-mode sector consists of two Jordan blocks of size two and the spectrum is real at the operating point, including negative-energy box modes of the resonant branch. Second, we measure the steady Cherenkov emission of adiabatically prepared solitons from the flux of the radiation behind them, over eight decades. The radiation is phase-matched in the frame of the drifting soliton, and the first-order rate falls short of the measured one by factors of 29 to 45. Expressed through a scaled dispersion coefficient of the local soliton, the measured rate describes launched solitons of three amplitudes; with norm and momentum balance, and no further fitted parameter, it reproduces their recoil and the slow decay of their loss rate; at weak dispersion the loss of a launched soliton is the shedding of its launch mismatch. Third, a census of the asymptotic channels shows that long-wavelength upstream emission requires the chemical potential of the soliton, lowered by the comoving-frame Doppler shift of a drifting soliton, to lie inside the analysis band, which bounds the soliton amplitude. In a kinematic model of the flow the surface gravity has a closed form; the thermal relation and the entanglement built on it are imposed inputs of that model. The construction applies to Raman-free Kerr media.

We prove global rigidity of smooth asymptotically flat stationary vacuum black holes with sufficiently small ADM angular momentum, under global regularity, bifurcation, and maximal-hypersurface assumptions, with a threshold depending on fixed geometric bounds. This answers the question of Alexakis, Ionescu, and Klainerman concerning replacement of their smallness assumption on the stationary field at the bifurcation sphere by smallness of the angular momentum at infinity. The main step establishes Kerr rigidity near infinity for arbitrary angular momentum. We transform the vacuum metric into an Einstein–Maxwell family and compare null focusing at its limiting end with the area–charge inequality on compact maximal hypersurfaces, constructed by continuation through the family. Equality determines both the transformed metric and its stationary field, from which the inverse transformation recovers Kerr. Two Kerr identities then give the horizon estimate required for the global conclusion.

We present a low entry barrier numerical code LazyTB that solves the Einstein equations in the ADM-formulation for a general spherically symmetric problem with an arbitrary number of fluids with user defined equations of state and optional interactions between the fluids mediated by drag-vectors. We present the ADM equations in terms of carefully chosen dimensionless variables that avoid some numerical instabilities by analytic cancellation of potentially diverging terms. We demonstrate the code by applying it to a number of examples including the standard LTB model, a model with radiation and dark matter, a polytropic dark energy model with a non-vanishing sound speed and a model with coupled interacting dark matter and dark energy. We also implement a suite of diagnostic tools, including ray-tracing algorithms, that allow extracting physical observables such as luminosity distances for observers at arbitrary locations from the simulations. The code is built to be easy to extend also beyond the late-universe applications considered in the examples of this paper. LazyTB is available at https://github.com/ollijvaisanen/LazyTB.

Whether gravitational caloric theory admits spherical black-hole solutions beyond Schwarzschild remains open. A well-defined black-hole solution requires both a regular horizon and a nonsingular exterior extending to infinity. To investigate this question, we cast the vacuum equations for a purely radial caloric field in a general static spherically symmetric metric into a three-dimensional autonomous system, which facilitates the global classification of solutions. We show that the regular asymptotic geometry can be Minkowski or conical, with the latter requiring a specific relation between the model parameters. Within this parameter slice, Schwarzschild is the only asymptotically flat black hole, whereas non-Schwarzschild black holes with conical infinity exist. These black holes can remain arbitrarily close to Schwarzschild from the near-horizon region to the conventional weak-field region (e.g., Solar System scales), yet retain an order-unity solid-angle deficit or excess at infinity. The asymptotic solid angle depends on a single model parameter and is independent of the caloric field amplitude. At fixed locally inferred mass, the caloric amplitude sets the conical transition scale, with weaker fields shifting the transition outward. We derive the local metric corrections and use the resulting periapsis shift to formulate an orbital test of the transition scale.

The binary black hole merger GW250114, the loudest gravitational-wave signal detected to date, offers an opportunity to perform a precise test of Einstein's general relativity (GR). Its exceptionally high signal-to-noise ratio enables accurate measurements of the quasi-normal modes (QNMs) and properties of the remnant black hole. In this work, we present a ringdown analysis of GW250114 using the RingCWB algorithm. We measure the dominant $(2,2,0)$ QNM along with its first overtone and constrain the fractional deviations in frequency and damping time from their GR-predicted values, finding $\delta f_{220} = 0.004_{-0.023}^{+0.022}$ and $\delta\tau_{220} = 0.010_{-0.087}^{+0.096}$ , both consistent with zero within the quoted uncertainties. These results represent stringent constraints on deviations from GR. The analysis also places tight constraints on the remnant spin, consistent with a binary black hole formed through stellar evolution.

We investigate semiclassical backreaction of a quantized scalar field in four-dimensional anti-de Sitter (AdS) and de Sitter (dS) spacetimes from two complementary perspectives. First, we determine self-consistent field parameters for which the prescribed geometry remains an exact solution with the bare cosmological constant fixed to its classical geometric value. Second, we retain the renormalized vacuum stress–energy tensor on the prescribed background and study its contribution to the effective cosmological constant. In AdS, the self-consistent solutions depend on the admissible boundary conditions, while the corresponding dS solutions are obtained within the Bunch–Davies state, revealing sensitivity to global and state-dependent information. For massless configurations, the effective cosmological constant takes the universal form $\Lambda_{\rm eff}=A/\ell^{2}+B/\ell^{4}$, allowing comparison of classical and quantum contributions and identification of the characteristic curvature scales at which their relative importance changes. We then test whether the classical relation between AdS and dS extends to the quantum level through the formal continuation $\ell\rightarrow i\ell$. Although the classical curvature term is mapped correctly, the quantum contribution is not: for minimal coupling the AdS and dS coefficients are in the ratio $29/61$, while for conformal coupling they have equal magnitude and opposite sign. These discrepancies show that semiclassical backreaction is not determined by local curvature alone, but retains information about the quantum state, boundary conditions, and global structure. The results clarify the limitations of analytic continuation as a map between the semiclassical quantum theories of AdS and dS.

Boson stars are among the best-studied exotic compact objects and constitute one of the few black-hole mimickers for which both the formation mechanism and dynamical behavior can be investigated from first principles. Constructing boson star solutions is generally more challenging than constructing neutron star solutions, often requiring either extremely accurate initial guesses for the oscillation frequency or the use of computationally intensive relaxation methods to solve the field equations. In the strong-coupling limit, massive boson stars can be described effectively as perfect- fluid configurations, enabling their construction using methods analogous to those developed for neutron stars. In this paper, we relax the strong-coupling assumption and derive a phenomenological anisotropic-fluid model for spherically symmetric massive boson stars that remains applicable over a broad range of model parameters. We provide ready-to-use fits for both the anisotropy profile and the effective equation of state in terms of the central scalar-field amplitude and the ratio of the self-interaction coupling constant to the squared scalar-field mass. To validate the model, we compute the masses and radii of boson stars using a standard neutron-star construction approach and compare the results against full boson-star calculations. We find excellent agreement between the two methods, while the new approach reduces the computational cost by a factor of about 10. Our phenomenological fluid framework offers an efficient and accurate method for constructing spherically symmetric massive boson stars and computing their observable properties. We provide a ready-to-use Python implementation of the Tolman-Oppenheimer-Volkoff solver, allowing users to construct boson star configurations within the proposed framework.

Numerical relativity has a profound impact on gravitational theory and gravitational wave astronomy. In this work, we present a systematic study of the head-on collisions of multipule black hole systems within the framework of numerical relativity using the Baumgarte-Shapiro-Shibata-Nakamura (BSSN) formulation. The numerical simulations of the dynamical evolution of gravitational fields are computed using the AMSS-NCKU code. We report numerical results for black hole collisions in several symmetric and non-symmetric configurations, in which the black holes are initially at rest and are described by Bowen-York type initial data. Our results suggest that black hole systems with perfectly symmetric configurations (e.g., black holes located at the vertices of a regular polygon or a regular polyhedron) impose strong constraints on the emitted gravitational waves, and no gravitational kick arises in these symmetric systems. For regular polygonal configurations, the radiation is dominated by the plus polarization of the $l=2$, $m=0$ mode. For regular polyhedral configurations, all the quadrupole modes with $l=2$ nearly vanish, and gravitational wave amplitudes are strongly suppressed to the order of $10^{-4}$ or below, which is in accordance with an analysis based on the quadrupole approximation. Gravitational radiations from $m \ne 0$ modes become non-negligible only when non-symmetric configurations are encountered. Keywords: Numerical Relativity, Black Holes, Gravitational Waves, N-Body Systems