The unique underground location of the Einstein Telescope (ET) is expected to substantially suppress seismic and gravity-gradient noise, providing a favorable environment for gravitational-wave detection at low frequencies compared with surface-based observatories. The baseline ET design aims to achieve broadband sensitivity through a xylophone configuration, in which separate cryogenic and room-temperature interferometers are optimised for low- and high-frequency gravitational-wave detection. However, achieving the target low-frequency sensitivity introduces substantial technical complexity caused by vertical suspension thermal noise, vibrations of the cryocooler, and detuned optical configuration. In this paper, we demonstrate an alternative, entirely room-temperature, fully resonant, single-interferometer architecture that addresses these challenges and achieves broadband sensitivity superior to that of the conventional ET xylophone configuration. Using mature fused-silica suspension technology, recent advances in optical coatings, and novel inertial isolation, we show that 4-m-long fused-silica fibres can meet the required suspension thermal-noise performance. We further demonstrate that vibrations, optical rigidity, and quantum back-action noise can be controlled through appropriate design of the interferometer, test mass suspensions, optical seismometers, and quantum states. less

Next-generation gravitational-wave detectors, such as LISA, the Einstein Telescope, and Cosmic Explorer, will require accurate and efficient models of long-lived, eccentric black-hole binary signals. In previous work, we reduced the cost of evolving nonspinning eccentric effective-one-body dynamics by recasting the equations of motion in terms of osculating orbital elements and applying near-identity averaging transformations to remove the fast orbital-timescale structure during the inspiral. However, the parametrization in terms of the eccentricity and relativistic anomaly becomes singular in the circular-orbit limit, excluding quasicircular binaries and the late stages of eccentric inspirals. Moreover, the cost of generating the waveform from the averaged dynamics limited the overall speed-up to a factor of $1.5-8$. In this work, we replace the eccentricity and relativistic anomaly by the Cartesian components of the eccentricity vector, in which the averaged equations are regular for all eccentricities $0 \leq e < 1$. We also decompose the waveform modes into radial harmonics whose coefficients are tabulated as functions of the averaged variables, so that the waveform is assembled directly on the output time grid at a fixed cost per sample. We validate the method against the full effective-one-body evolution across the parameter space of binaries observable by ground-based detectors, now including circular orbits. At second postadiabatic order, the waveforms reach median mismatches of $\sim 10^{-8}$ with a maximum of $1.1 \times 10^{-5}$, while achieving speed-ups of $3-100 \times$ on a single core or up to $300 \times$ on eight cores against the serial EOB waveform. Given the speed-ups demonstrated here, we plan to apply these techniques to state-of-the-art SEOBNR models in future work. less

Exceptional points (EPs) and eigenstate hysteresis, previously found in Kerr only with an auxiliary scalar field, arise intrinsically in the quasinormal-mode spectrum of Kerr-Newman black holes. Using a hyperboloidal formulation of the coupled gravito-electromagnetic Teukolsky equations and a Chebyshev pseudospectral method, we compute the $\ell=m=2$ spectrum and locate four EPs at up to $99.974\%$ of extremality, one from a new repulsion. The gaps close as a square root branch point, and quasistatic loops around one or more EPs exchange the coalescing modes, confirming hysteresis. less

I apply and extend the formalism of osculating orbits of Damour et al. (2004) and Lynch et al. (2026) to eccentric effective one body (EOB) models, in order to obtain an efficient yet accurate description of the inspiral-to-plunge dynamics of tidal, spinning compact binaries, including precessing ones by twisting the otherwise planar motion. The method is recast in terms of equinoctial orbital elements, to circumvent singularities associated with the zero-eccentricity limit. While the formalism is general and can be applied to any EOB model, I focus here on the TEOBResumS-Dalí family and demonstrate how this technique speeds up waveform generation without significantly compromising the accuracy of the model. Over $10^4$ binary neutron stars (BNSs) and as many binary black holes (BBHs) with initial eccentricities up to $0.6$ at $20$ Hz, the median speedup against an optimized version of the model is $\sim 2.6$ for BNSs and $\sim 1.5$ for BBHs, with mismatches against the full numerical solution always below $5 \times 10^{-7}$. The largest speedups, up to $\sim 7$ for individual systems, are obtained for light BBHs ($M \lesssim 8 M_\odot$) with low eccentricities $(e_0 \lesssim 0.1)$. In absolute terms, the median time needed to generate a BNS waveform is $\sim 0.5$ s, with $99.7\%$ of the systems below one second, and for long signals the model is up to $\sim 26$ times faster than a previous version. Applied to the calculation of initial conditions in the $e \rightarrow 0$ limit, the inclusion of radiation-reaction effects allows the model's input eccentricity to agree with the gauge-invariant waveform eccentricity down to initial values of $\sim 7 \times 10^{-8}$. Beyond time-domain applications, the same construction also gives frequency-domain EOB waveforms for eccentric compact binaries via a multi-harmonic stationary phase approximation. less

One of the major challenges of modern foundational physics is the combination of quantum physics and general relativity into a theory of quantum gravity. A severe obstacle are the required extremely high energy scales for direct tests of quantum gravity effects. In recent years, attention has shifted towards near-future experimental predictions of effective or phenomenological models, such as gravitationally induced entanglement or interactions of linearized gravity with non-relativistic measurement devices. These developments motivate us to study the coupling between the quantized metric perturbation and an anisotropic quantum rigid rotor. The intuitive motivation for our study is that a sufficiently heavy rotor consists of a very large number of component systems. Although quantum gravitational effects are completely negligible at the scale of a single atom or molecule, the effects can get amplified by the sheer number of constituents. The resulting effective model describes the gravitational analogue of the angulon quasiparticle excitations discussed in condensed matter physics, suggesting a pathway for future analogue experiments. As a first concrete test of our model, we derive the gravitational Lamb shift for the energy eigenstates of the rotor, which become dressed by a cloud of virtual gravitons. Although the effect is very small, it could provide a concrete test for whether gravity is truly quantum. less

A core mission of gravitational-wave astronomy is to test general relativity (GR) in the high-curvature, high-velocity regime. The field has already seen significant progress in this space, though a remaining challenge is how to combine data from a population of binary mergers. This can both boost the significance of potential GR violations, and develop a phenomenological picture of how deviations manifest in different mergers. We propose a framework to coherently combine gravitational-wave measurements to build a population-level test of physics beyond GR. We assume that deviations from GR are (i) strongest near the merger, (ii) vary smoothly with the total mass of the binary, and (iii) are most prominent in the $\ell=|m|=2$ mode. While these assumptions do not describe every possible deviation of GR, they provide us with a plausible starting point upon which a more general framework can be built. We apply our framework to 28 high signal-to-noise ratio binary black hole mergers from LIGO-Virgo-KAGRA's Fifth Gravitational-Wave Transient Catalog, finding no evidence for deviations from GR. Our framework allows one to show how deviations could vary as a function of total mass of the system. We argue that such a reconstruction can be used to convincingly distinguish between systematic waveform errors and genuine GR violations. less

We show that the photograph of a Schwarzschild black hole can be written in closed form. The image plane is embedded as a plane tangent to a sphere centred on the black hole, and the map between a point source and its image point, which we call the photograph function, is treated as the analytic object. Explicit expressions are obtained for sources on the celestial sphere at infinity and for sources in the equatorial plane, without solving for the intermediate null geodesics. The construction is exact, and it makes the multivalued branch structure, the relation to the photon sphere and to the shadow boundary, and the self-similar higher-order images directly readable. less

We investigate the stability of an axion dark energy model characterized by a periodic scalar-field potential motivated by pseudo Nambu Goldstone bosons. While such models are well suited to drive late-time cosmic acceleration and can exhibit rich dynamical features, including oscillatory behavior of the dark-energy equation-of-state parameter, their viability crucially depends on the existence of stable cosmological solutions. By reformulating the background evolution equations as an autonomous dynamical system, we identify the fixed points corresponding to radiation-dominated, matter-dominated, and dark energy dominated epochs. Using both analytical and numerical techniques, we analyze the stability properties of these fixed points within the observationally allowed region of parameter space. Our results show that the radiation- and matter-dominated phases emerge naturally as saddle points, ensuring a standard cosmological evolution, while the late-time dark energy dominated solution acts as a stable attractor. This guarantees the robustness of cosmic acceleration against variations in initial conditions. Overall, our study establishes the axion dark energy model as a dynamically consistent and observationally viable candidate for explaining the late-time accelerated expansion of the Universe. less

We present Pinndorama, a physics-informed neural network (PINN) code for solving the Hamiltonian constraint in the puncture formalism. The networks are trained solely on the governing equation and boundary conditions, without an analytical or perturbative guiding solution, with collocation points sampled in curvilinear coordinates. Training combines two optimization algorithms: an initial stage using adaptive moment estimation (Adam), followed by the self-scaled Broyden quasi-Newton method. We apply this approach to a spherically symmetric test problem with a known exact solution, axisymmetric binary punctures with unequal spins, and a fully three-dimensional GW150914-like configuration with unequal spins and orbital momenta. Comparisons with the NRPyElliptic solver yield relative $L^2$ errors of order $10^{-8}$ in axisymmetry and $10^{-5}$ in three dimensions. In addition, we reduce the hyperbolic-relaxation time in NRPyElliptic by a factor of approximately $4.6$ by training a parametric network that takes the puncture spin as input and provides a physics-informed initial guess to the solver. These results demonstrate that PINNs can provide accurate binary-puncture initial data and complement conventional elliptic solvers in numerical relativity. less

Black-hole perturbation theory remains remarkably accurate beyond its formal small-mass-ratio regime, but what limits this accuracy remains unclear. We develop a modular time-domain Teukolsky framework that isolates finite-mass-ratio corrections to the orbital dynamics while keeping the radiation at first perturbative order. Comparisons with numerical relativity show that the dominant source of discrepancy changes across the waveform. For nonspinning binaries, the available second-order dissipative flux removes $87\%$ of the inspiral rate discrepancy, bringing it to within $\sim1\%$ of numerical relativity. For precessing binaries, we identify a $ν^2\sin^2ι$ dependence in the inspiral time rescaling that is not explained by the leading orbit-averaged post-Newtonian terms, suggesting finite-mass-ratio corrections to precessional dynamics. Near merger, the same dissipative correction removes only $1$--$2\%$ of the discrepancy. We show instead that the merger offset is quantitatively accounted for by the difference between the initial Kerr background used in the perturbative evolution and the remnant black hole, with $96\%$ of the leading predicted frequency shift arising from the change in spin rather than mass. We release $1571$ time-domain waveforms for $348$ binaries as the Vishveshwara Catalog. less