Papers

We establish a bottom-up model for studying the `would-be' de-Sitter/interface CFT (dS/ICFT) correspondence on the gravity side, using the Coleman-De Luccia instanton. After absorbing the region where the inflaton changes dramatically by a brane, the spacetime consists of two de Sitter spacetimes with different radii joined by a brane, which is expected to be dual to an interface CFT provided that the dS/CFT conjecture is correct. Our goal is to analyze existing prescriptions for the holographic computation of pseudo-entropy, i.e. the extremal surface in such a spacetime and argue that some of them would cause parodox with the results expected the universal properties of interface CFT and compare our results with that of Anti de Sitter/interface CFT correspondence (AdS/ICFT) after analytic continuation. We also read off the $g$-function from the entanglement entropy associated with a subregion perpendicular to the interface.

In this work we study gravitational perturbations of black holes carrying primary scalar hair in shift-symmetric beyond-Horndeski gravity. By focusing on the axial sector, we study the geometric structure of the perturbation equations and perform time-domain evolutions both in the physical and in the effective metric descriptions, in order to quantify differences in the response characteristics of solutions with different hair. In the frequency domain, we then explore the QNM spectrum in the physical-metric formulation, thus verifying and extending existing approximate calculations derived in the effective-metric approach. We also compute the corresponding greybody factors and absorption cross-sections. Finally, we use localized deformations of the effective potential as a diagnostic of the relative stability of quasinormal frequencies and greybody-factor observables.

It is well known that the Einstein equations are tensor equations for a Lorentzian metric. Hence, choosing a suitable gauge condition is crucial for solving them. As a powerful gauge condition, maximal foliations have played a pivotal role in two groundbreaking works in general relativity: the global stability of Minkowski spacetime and the bounded L^2 curvature conjecture. Nevertheless, if the initial hypersurface fails to be maximal, the usual elliptic approach for constructing maximal foliations encounters fundamental difficulties when solving the Einstein equations. The purpose of the paper is to provide a construction of maximal foliations around any initial asymptotically flat Cauchy surface satisfying vacuum Einstein constraint equations, under a suitable smallness condition on the mean curvature.

We investigate whether a Hayward regular black hole can be observationally distinguished from a Schwarzschild black hole through strong gravitational lensing when effective field theory (EFT) corrections to photon propagation are taken into account. We derive the modified photon propagation law induced by non-minimal couplings between the electromagnetic field and spacetime curvature, and analyze the resulting photon trajectories in the Hayward spacetime. Using the strong deflection limit, we derive the corrections to the photon sphere and the logarithmically divergent part of the deflection angle. Although the EFT corrections are parametrically small, their effects can be enhanced near the critical propagation region, where the deflection angle exhibits a logarithmic divergence. We evaluate the strong-deflection observables for representative values of the Hayward parameter and compare them with the Schwarzschild case. We find that EFT corrections to photon propagation can leave characteristic imprints on strong-lensing observables. Furthermore, the contribution of the EFT corrections becomes more pronounced as the Hayward parameter $g^3$ approaches its critical value, indicating that curvature-dependent corrections to photon propagation can become particularly relevant in the near-critical regime. These results suggest that strong gravitational lensing may provide a means of distinguishing Hayward regular black holes from Schwarzschild black holes.

Thermodynamic significance of near-horizon symmetries remains an important open question in black hole physics, particularly in the context of black hole evaporation and information recovery. In this work, we investigate the role of horizon-adapted Bondi-van der Burg-Metzner-Sachs (BMS)-like supertranslations in the thermodynamic description of a dynamical Schwarzschild black hole. Working in a near-horizon Rindler coordinate system, we construct a class of diffeomorphisms that preserve the horizon structure and promote the associated supertranslation parameter to a Goldstone-like mode arising from the breaking of horizon symmetry. By expanding the Einstein-Hilbert action around the background geometry, we obtain the effective action for the Goldstone mode and identify the corresponding conserved horizon charge from the surface contribution of the action. The relevant horizon is defined at the future outer trapping horizon, while the surface gravity is computed using the Kodama-vector construction appropriate for dynamical spacetimes. We show that the horizon supertranslation mode contributes non-trivially to the surface gravity and modifies the thermodynamic description of the black hole beyond the stationary limit. Using the associated Noether charge, we derive the entropy of the horizon-BMS transformed geometry and find that the Bekenstein-Hawking area law is recovered at leading order, while subleading corrections depend explicitly on the supertranslation sector and the dynamical evolution of the black hole.

With the rapid development of modern atomic clock technology, their unprecedented precision elevates them from timekeeping tools to gravitational potential sensors, thereby fostering the highly interdisciplinary field of Relativistic Geodesy. Given the potential for high-precision clock networks to detect periodic gravitational variations, it is imperative to assess their capability to invert solid Earth tide parameters via space-to-ground links in the presence of complex observational noise. To this end, we incorporate Earth's gravitational potential, direct lunisolar tidal potentials, and solid Earth tide effects into a high-precision relativistic framework for space-to-ground clock comparisons. By employing a three-link Doppler cancellation configuration to isolate the target signal, we perform numerical simulations for an inclined geosynchronous orbit satellite to analyze the effects of clock instability and colored precise orbit determination errors on parameter extraction. Our findings reveal that while high orbital altitudes cause severe collinearity between individual Love numbers, an effective parameter combining the $h_2$ and $k_2$ Love numbers successfully converges to a stable estimate within a 30-day continuous observation window. Furthermore, sensitivity analysis demonstrates that extraction accuracy is currently limited by clock stability rather than radial precise orbit determination errors.

We study the weak coupling of two identical two-level atoms to a four-dimensional massless conformally coupled scalar field and compare three settings with the same local temperature but different physical origins: comoving atoms in the Bunch–Davies vacuum of de Sitter spacetime, transversely separated uniformly accelerated atoms in the Minkowski vacuum, and static atoms in a Minkowski thermal bath. We first derive the two-atom cross spectra in the three settings and then obtain the single-atom local spectra uniformly from the zero-separation limit. When $a=H$ and $\beta=2\pi/H$, the complete local Wightman kernels and local spectra are identical in all three cases. At finite separation, a genuine thermal bath produces a $\sinc(\Omega L)$ spatial factor, whereas de Sitter spacetime and the transversely accelerated vacuum produce a hyperbolic geometric factor. In the long-distance limit, the cross correlations in de Sitter spacetime and the transversely accelerated vacuum decay as $L^{-2}$, whereas those in the Minkowski thermal bath decay only as $L^{-1}$. The de Sitter and transversely accelerated vacuum cross spectra coincide when $a=H$ and the separations are instantaneously matched. For a fixed experimental setup, the physical separation between the two comoving atoms in de Sitter space evolves with cosmic expansion, whereas the pulled-back cross correlations in the uniformly accelerated and thermal Minkowski configurations are stationary.

Non-relativistic objective collapse theories have been remarkably successful in addressing the conceptual problems of standard quantum mechanics. Despite substantial efforts, the project of extending them to the relativistic domain remains burdened by significant conceptual objections and technical challenges, often taken to cast doubt on the viability of the program as a whole. On the conceptual side, relativistic collapse theories have been claimed to face challenges involving tension between instantaneous collapse and relativity, frame-dependence of property values and probabilities, the possibility of superluminal signaling and the failure of narratability. On the technical side, persistent infinities, difficulties in constructing fully covariant frameworks and the apparent need for non-standard fields have hindered the development of workable models. In this paper, we offer a systematic rebuttal of the conceptual objections and provide a structured account of the remaining technical challenges. We conclude that relativistic collapse theories do provide a promising route toward a fully relativistic quantum framework that overcomes the conceptual limitations of standard quantum theory.

Three-dimensional Einstein gravity and Chern-Simons-Higgs matter both admit Chern-Simons descriptions, but their holonomies encode different physics. We prove that charge conjugation reverses the vortex winding $n\to-n$ while leaving the metric and the gravitational connections pointwise unchanged, so every closed gravitational holonomy character, the BTZ charges, the Bañados data and the leading geometric mixed-state hierarchy are exactly even in $n$. On a fixed marked Euclidean replica filling the same change of variables gives $Z_q[n;\mathcal J_+]=Z_q[-n;\mathcal J_+]$, so the uncharged Rényi hierarchy, the Faulkner-Lewkowycz-Maldacena one-loop correction, the renormalized generalized entropy and the quantum extremal surfaces coincide for the two orientations. The complementary sector is fixed exactly: with an Aharonov-Bohm angle $\mu$ conjugate to the entangling-region charge, $Z_q[n;\mu]=Z_q[-n;-\mu]$, so the symmetry-resolved spectrum obeys $\mathcal Z_q(\mathcal Q;n)=\mathcal Z_q(-\mathcal Q;-n)$ and every odd charged cumulant reverses with the winding. The orientation is therefore stored entirely in the charge asymmetry of the entanglement spectrum, and is read at topological order by a linked charged matter line. The pair is a bulk logical qubit on which the neutral gravitational-replica algebra acts as a multiple of the identity. Exact BTZ cross sections show that the purification plateau and the entanglement wedge disconnection transition are orientation blind, the latter closing with the cutoff-independent jump $(c/3)\log(1+\sqrt2)$. Numerical Bogomolny solutions verify odd flux and even energy and spin; used as linearized charge-matching data they give the even asymptotic shifts $\delta\widehat M=16\pi Gv^2|n|$ and $|\delta\widehat j|=16\pi Gv^2n^2/(mL)$ and, conditionally, a finite continuous winding envelope.

This paper studies homogeneous \Godel-type geometries in a gravitational model where local Lorentz symmetry is spontaneously broken by the vacuum expectation value of an antisymmetric Kalb-Ramond two-form. We consider the Einstein-Hilbert action supplemented by two independent nonminimal curvature couplings: a Ricci-tensor coupling of the form $B_{a}{}^{c}B_{bc}R^{ab}$ and a Riemann-tensor coupling of the form $B^{ab}B^{cd}R_{abcd}$. In the homogeneous vacuum sector, where the Kalb-Ramond field strength and the first derivatives of the symmetry-breaking potential vanish, the field equations reduce to a closed algebraic system in the tetrad frame of the \Godel-type metric. We derive the curvature tensors, the reduced metric equations, and the Kalb-Ramond consistency equations for several inequivalent orientations of the vacuum two-form. Particular attention is given to the pseudo-magnetic background $B_{12}=b$, because it preserves the symmetry of the rotation plane and yields a transparent deformation of the usual bumblebee result. For this sector the Kalb-Ramond equation fixes $\frac{m^2}{\omega^2}=\frac{2(\xi_1+3\xi_2)}{\xi_1+2\xi_2}$, where $\xi_1$ and $\xi_2$ are the Ricci and Riemann couplings. The Ricci coupling alone reproduces the noncausal \Godel value $m^2=2\omega^2$, while the Riemann coupling shifts the chronology bound and allows the critical causal value $m^2=4\omega^2$ for $\xi_2=-\xi_1$. We also show that the Riemann coupling can move the critical radius for closed timelike curves, but ordinary positive-energy matter severely restricts completely causal solutions. Finally, we discuss perfect fluid, scalar field, and electromagnetic sources.

The pursuit of a quantum theory of gravity, aiming to unify general relativity and quantum mechanics, remains one of the most enduring challenges in physics. Because of the extreme energy scales associated with the Planck regime, direct experimental evidence for quantum gravity remains elusive. However, recent proposals suggest that quantum entanglement between two massive particles may provide a pathway to probe the quantum nature of gravity. In this study, we examine the interferometer geometries proposed in these works, with particular attention to the commonly used approximation that neglects phase contributions from the vertical segments of the particle trajectories. Our analysis shows that this approximation can lead to incorrect predictions and, in certain parameter regimes, to null results where entanglement would otherwise be expected. We derive exact solutions that incorporate the full particle trajectories and demonstrate that the vertical arms can significantly affect the accumulated phase. Crucially, we identify configurations in which the induced entanglement vanishes entirely, a feature missed by simplified treatments. These findings show that accounting for the full interferometer geometry is not merely a refinement, but is essential for accurately assessing gravity-induced entanglement.

Recent work on metaparticle black-hole thermodynamics suggested the existence of a finite evaporation endpoint characterized by a minimal horizon area, a maximal Hawking temperature, and a stable cold modular remnant. However, the corresponding spacetime geometry remained unknown. In this work we construct an effective geometric realization of the modular remnant within the framework of Born geometry, modular spacetime, and metaparticle dynamics. Starting from a doubled phase-space description in which spacetime coordinates are supplemented by dual coordinates, we derive a Born-doubled Schwarzschild geometry governed by a Born-invariant radial distance. The associated modular uncertainty relation prevents arbitrary localization at the phase-space origin, rendering the classical Schwarzschild singularity physically inaccessible and replacing it with a finite modular core. We then incorporate the metaparticle duality constraint, which induces an effective conformal deformation of the Born-geometric background and generates a nontrivial effective stress-energy tensor. The resulting interior region develops a finite anisotropic source characterized by a tension-dominated radial sector. Localized violations of the radial Null Energy Condition and the Strong Energy Condition arise naturally near the modular core, providing a mechanism through which the focusing assumptions underlying the Hawking–Penrose singularity theorems are evaded. These results establish an effective geometric counterpart of the modular remnant inferred from metaparticle black-hole thermodynamics and suggest that the Schwarzschild singularity is replaced by a finite Born-geometric phase-space structure whose ultraviolet behavior is governed by dual, non-geometric degrees of freedom.

We show how amplitudes from the scattering of gravitational waves off compact objects in worldline effective field theory can be efficiently computed to arbitrary order in Newton's constant $G$. Our approach solves an effective wave equation for the partial-wave amplitude in which Wilson coefficients (tidal Love numbers) enter through boundary conditions at short distances. We then perform the sum over partial waves to obtain the momentum-space Compton amplitude in terms of elliptic polylogarithms. We reproduce recent results through $\mathcal{O}(G^4)$ and obtain new predictions up to $\mathcal{O}(G^7)$, with Love numbers first contributing at $\mathcal{O}(G^5)$. We find the first ultraviolet divergence in a classical gravitational amplitude at $\mathcal{O}(G^7)$, showing that a pure point-particle description is not consistent in general relativity. Matching to black hole perturbation theory, we show that static Love numbers vanish on-shell and predict subleading non-zero Schwarzschild black hole Love numbers.

We draw on 't Hooft's seminal formulation of the holographic principle to analyse the methodological and conceptual role of information in quantum gravity. We argue that, in 't Hooft's work and in later developments, information functions as a substantive guiding principle. We distinguish three aspects of this role: First, holographic bounds on the amount of information that can be stored in a region function as theory selection criteria that constrain viable quantum gravity theories. Second, holography functions as a principle of theoretical equivalence: the bulk and boundary theories must describe the same physical content, even though 't Hooft privileges a more fundamental, lower-dimensional, and potentially deterministic boundary description. Third, the distribution and encoding of information connect 't Hooft's proposals to contemporary work on bulk reconstruction, holographic quantum error correction, and ER=EPR, where emergent spacetime structure is tied to patterns of entanglement and redundancy. On the basis of these three roles, we argue that purely epistemic or Shannon-style conceptions of information are inadequate in this context: we instead outline a distinction between what we call maximal and intermediate conceptions that aim to capture the methodological and interpretative roles of information in holographic quantum gravity. We therefore suggest that, in the context of holography and quantum gravity, a more systematic philosophical treatment of the role of information as an interpretation-guiding principle would be desirable.

We develop a nonperturbative, group-theoretic quantization of the effective guiding center theory of a charged particle drifting in a magnetic field in two spatial dimensions, and compare the resulting quantum theory with a corresponding coarse graining of the underlying microscopic theory. In the classical effective theory, the small gyro motion is not resolved, while the motion of the center of the gyro orbit remains observable. The reduced phase space is the physical space itself, so quantization leads to noncommuting spatial coordinates, and the effective theory loses access to the metric structure of physical space, retaining only its area structure. By formulating a prescription to match between the microscopic and effective quantum theories, we find that the predictions of the quantized effective theory are generally consistent with those of the microscopic theory. However, for closed isomagnetic contours the effective theory predicts a quantization of “radius”, a spatial discreteness absent from the microscopic theory. This illustrates that quantization of an effective theory may yield spurious nonperturbative predictions, even if that quantum theory shows no internal signs of breakdown.

Extreme and intermediate mass-ratio inspirals in a dense dark-matter distribution have the effects of the dark matter imprinted on the orbital dynamics of and the emitted gravitational waves from these systems. Prior work has shown that space-based gravitational-wave detectors can measure the dark-matter-induced effects on the gravitational waves, which would give evidence for the presence of dark matter around the massive black hole. In this earlier work, the dark matter has been assumed to have only gravitational interactions (namely, no dark-matter self-annihilation or interactions between dark matter and ordinary baryonic or leptonic matter). In this article, we investigate the gravitational-wave effects of introducing such interactions of dark matter with itself or with ordinary matter in binaries with a neutron-star secondary. We consider broad classes of dark-matter models that change the distribution of dark matter (spikes or annihilation plateaus) and which permit accretion onto the secondary, as well as an increasing or static mass of the neutron star during the inspiral (in addition to the purely gravitational effect of dynamical friction). We find distinctive gravitational-wave signatures of these interactions and of self-annihilation, which in some of the scenarios could be sufficiently large for space-based detectors to distinguish them using gravitational-wave observations of these systems.

We present results of a systematic validation of the \lalsuite{} timing model for continuous gravitational waves against \pint{}, a modern high-accuracy pulsar-timing package. An accurate timing model is essential for tracking the signal phase, and hence for detecting and accurately characterizing continuous gravitational waves. In order to quantify the impact of timing inaccuracies, we derive and validate the leading-order relation $\mu \approx (2\pi f)^2\stddtau^2$, where $\mu$ is the fractional loss of signal power, $f$ is the signal frequency, and $\stddtau^2$ is the variance of the timing errors. We then compare the solar-system and binary components of the \lalsuite{} timing model against the corresponding models in \pint{}. With the original \lalsuite{} Einstein-delay implementation, the total disagreement is dominated by that component and has $\stddtau\simeq\SI{2.3}{\micro\second}$ (corresponding to $\mu\simeq\SI{0.02}{\percent}$ at $f=\SI{1000}{\hertz}$). With the newer Einstein-delay implementation, the total disagreement (over one year) drops to $\stddtau\lesssim\SI{31}{\nano\second}$ (or $\mu\lesssim\num{4e-8}$ at $f=\SI{1000}{\hertz}$) and is dominated by the observatory contribution to the \Romer{} delay, owing to the approximate Earth-rotation model used by \lalsuite{}. We additionally test binary delays using orbital parameters from \num{474} catalogued binary pulsars and verify the self-consistency of the \lalsuite{} source-time derivatives. Finally, we derive and discuss the \lalsuite{} Shapiro delay for signals passing through the solar interior, a case only relevant to gravitational waves.

We study overlaps of late-time states in semiclassical de Sitter quantum gravity. In a family of deformations of de Sitter JT gravity, we compute the no-boundary wavefunction to one-loop and derive the ultralocal late-time measure. The one-universe contribution to the norm is exactly eight times the sphere amplitude, so the pairing used by the Lorentzian measure differs from that implicit in Euclidean gravity. After summing over disconnected final universes, the no-boundary norm exponentiates the one-universe result, giving $\langle \!\langle \text{HH}|\text{HH}\rangle\!\rangle \approx \exp(4Z_{\rm sphere})$ rather than the sum over closed geometries, $Z_{\rm closed}\approx \exp(Z_{\rm sphere})$. We also review an analogous mismatch in Einstein gravity with positive cosmological constant, where the late-time no-boundary norm vanishes at one-loop, differing from the Euclidean sphere amplitude. We discuss the implications for cutting and gluing in perturbative gravity, and features that arise when extending this construction to sums over topologies.

Motivated by inflation and Liouville theory, we consider random $d$-dimensional geometries characterized by an overall scale factor $ds^2 = e^{2 \zeta} dx^2 $ given in terms of a random Gaussian field $\zeta(x)$ with logarithmic correlations. We discuss aspects of the renormalization of the volume element $e^{d \zeta }$, connecting well-known Liouville theory formulas (KPZ) and inflationary ones. By starting from a fixed physical cutoff and coarse-graining operators to a fixed fiducial cutoff, defined via the flat metric $dx^2$, we provide a direct physical derivation of the KPZ scaling relation. We point out that the same renormalization problem arises in stochastic inflation, where it is modeled by Brownian motion of the inflaton field. We show that the breakdown of the renormalization of the Liouville volume when the fluctuation amplitude exceeds a critical value corresponds to the transition to eternal inflation. In $d = 2$, this matches the familiar $c_m = 1$ barrier in Liouville gravity. We also discuss connections to mathematical probabilistic approaches to random surfaces.

Calibrated torsion-balance spectra constrain deterministic and stochastic deviations from standard Newtonian gravity. Using one-sided spectra, we derive a calibrated angle-equivalent noise budget and finite-time matched-filter/Cramér-Rao bounds for known torque templates. For stochastic models, subtracting the calibrated standard noise budget (thermal, Newtonian, environmental, imprecision, backaction) from the observed angle spectrum yields a residual spectrum, convertible to an equivalent residual torque spectrum via calibrated torsional susceptibility. This bounds additional stationary stochastic torque noise. This frequency-resolved bound compresses to a single torque-diffusion coefficient, $D_\tau$, only in the Markovian white-noise limit; non-Markovian or colored models require the full residual spectrum. Page–Geilker branch discrimination and Fedida-Kent mixture-equivalence tests address distinct physical questions, but upon projection onto torque templates, both reduce to the same statistical matched-filter discrimination problem. For a room-temperature Cavendish benchmark, resonant thermal angle ASD is $1.36\times10^{-4}\,\mathrm{rad}/\sqrt{\mathrm{Hz}}$, while measurement-added SQL is $3.25\times10^{-12}\,\mathrm{rad}/\sqrt{\mathrm{Hz}}$. For the Yan et al. search, the reported $0.3\,\mu\mathrm{rad}/\sqrt{\mathrm{Hz}}$ sensitivity at $2.5\,\mathrm{mHz}$ yields a conservative bound $D_\tau \lesssim 2.4\times10^{-23}\,\mathrm{N^2\,m^2\,s}$, assuming white torque noise. These formulations provide an interface linking calibrated torsion-balance data, deterministic tests, and stochastic semiclassical-gravity searches, without asserting a direct test of the full relativistic semiclassical Einstein equation.

We investigate the problem of confinement of massive ghost, which violates the unitarity of the physical S-matrix, in quadratic gravity on the basis of a manifestly covariant and local canonical operator formalism. First, we reconsider the manifestly covariant quantization of quadratic gravity in the de Donder gauge (the harmonic gauge) from the viewpoint of dipole fields. Next, we also reconsider a possible mechanism of confinement of the massive ghost and derive an effective Lagrangian for asymptotic fields of a BRST quartet where the asymptotic field corresponding to the massive ghost is found to obey a dipole field equation as in the Froissart model, which is a characteristic feature in our formalism. To understand quantum aspects of our theory, we perform a manifestly covariant quantization of the effective Lagrangian in two different ways based on the three-dimensional Fourier transform and the four-dimensional Fourier transform, and derive the same result that the quantum Fock space is spanned by multipole states.

The purpose of the paper is to place Whiting's classical growing mode stability argument, extended to real frequencies by Shlapentokh-Rothman for the scalar wave equation and by Andersson, Ma, Paganini and Whiting in general, in the framework of classical PDE theory. The key steps are: a description of the dual or adjoint modes, a singular phase space pairing argument which is technically executed via the Fourier transform, followed by a standard unique continuation result. One part of our description of the dual modes connects them directly to the standard mode solutions which are smooth over the event horizon. In the zero spin case, there is a geometric interpretation of this: quasinormal modes (for non-zero real frequencies) which are smooth across the future event horizon can be extended as distributional QNM solutions by 0 through the past event horizon. We also describe the behavior of mode solutions at the bifurcate sphere.

Measurements of Newton's gravitational constant remain limited by environmental and apparatus-dependent systematics whose treatment is often less explicit than that of instrumental noise. Among these, atmospheric density fluctuations generate unshieldable gravity gradients that couple directly to torsion-balance observables as atmospheric Newtonian noise. Here, we develop a GUM-consistent framework for propagating this contribution through the torque estimator into the uncertainty budget of torsion-balance measurements. We derive closed-form spatial transfer functions for two benchmark geometries: a two-mass dumbbell, which provides an upper-coupling reference, and a perfectly symmetric cross, which serves as an idealized rejection limit and exposes the trade-off between suppressing low-order environmental coupling and preserving signal response. We also separate stationary correlated inputs from non-stationary baseline drift, using the Ornstein–Uhlenbeck process only as a benchmark for the former. Atmospheric pressure benchmarks indicate that the resulting background contribution is below present reference uncertainty levels, but can become relevant as systematic floors approach the part-per-million regime. This framework provides a practical route for incorporating site-specific environmental gravity gradients into future torsion-balance uncertainty budgets.

This paper re-examines the Dark Matter (DM) problem through the lens of physical principles. We first develop a functionalist account and taxonomy of principles. We propose to define principles in terms of their intended functions or purposes. Their argumentative force boils down to context-sensitive and often tentative reasons for believing them adequate for those purposes. In research contexts in which novel theories or models are sought which push the boundaries of knowledge into new domains, reliance on well-motivated principles as principal building blocks is a historically widespread, natural, and methodologically prudent research strategy—what we'll dub "principled inquiry". In a second step, we apply this methodological machinery to the DM problem. Through detailed analyses of four mainstream DM approaches—supersymmetric WIMPs, axions, sterile neutrinos, and primordial black holes—we substantiate four claims. (C1) The DM paradigm is formed by the framework of possibilities, circumscribed by a shared cluster of regulative and constitutive principles of gravitational and non-gravitational physics. (C2) Besides those common principles, the four DM models also depart from other principles of orthodox physics in distinct ways. (C3) They do so to different degrees—displaying different forms and extents of "minimal mutilation" with respect to some of the principles of established non-gravitational physics. (C4) The guidelines of principled inquiry clarify the sense in which the DM problem constitutes an ever-more disconcerting crisis: worries are growing that the principles on which DM researchers have routinely and rationally relied may have run out of heuristic steam. Pressure is mounting to "leave no stone unturned" (Bertone and Tait 2018): scientists ought to increasingly explore ideas that stray more radically from the cautious explorations of principled inquiry.

We prove mode stability for the Klein-Gordon equation on Kerr-de Sitter black hole spacetimes in the full subextremal range and for the range $m_{\rm KG}^2\in[0,\Lambda\sqrt{2}]$ of squared scalar field masses that includes, in particular, the minimally coupled case $m_{\rm KG}=0$ and the conformally coupled case $m_{\rm KG}^2=\frac{2}{3}\Lambda$ (i.e., the Teukolsky equation for spin $0$). In combination with recent work by Petersen-Vasy, this proves, unconditionally, that sufficiently regular solutions of the scalar wave equation decay exponentially fast to constants, and in fact to zero for nonzero $m_{\rm KG}$.

We investigate the cosmological evolution of a universe with an initially large negative vacuum energy, corresponding to an anti-de Sitter (AdS) space-time. Such a universe inevitably contracts and collapses into a singularity. To prevent this collapse, we propose a dynamical mechanism of vacuum energy compensation based on a scalar field non-minimally coupled to the curvature scalar. Numerical and analytical calculations demonstrate that the suggested mechanism completely compensates the gravitational impact of the vacuum energy, successfully driving the curvature to zero. As a result, the universe avoids the singularity and evolves into a power-law expansion, typical for a radiation-dominated cosmology.

Generalized horizon entropies are widely used as theoretical modifications of the Bekenstein-Hawking area law, but through the Wald construction they may also encode modifications of the underlying gravitational dynamics. We reconstruct metric $f(R)$ gravity from prescribed entropy-area relations and show that the procedure is intrinsically branch dependent through the required area-curvature map. On the maximally symmetric branch, where $A=48\pi/R$ exactly, the reconstruction reduces to a single quadrature and can be performed non-perturbatively. We obtain closed-form Lagrangians for several generalized entropies and show that an entropy term $a~S_{BH}^{q}$ generates a curvature term proportional to $R^{2-q}$. In particular, Kaniadakis entropy produces a $1/R$ correction, while logarithmic entropy generates an $R^2\ln R$ term. We further derive a branch-independent criterion, $\partial_{R}^{2} f=(ds/dR)d(S/s)/ds$, relating Dolgov-Kawasaki stability directly to the entropy functional, together with $m_{\rm sc}^2=S'(s)/(3f_{RR})$ on the maximally symmetric branch. Comparison with the fixed-mass Schwarzschild-de Sitter branch reveals different reconstructed Lagrangians and reversed stability properties. Finally, the weak-isolated-horizon boost charge reproduces the original generalized entropy. These results establish a direct non-perturbative link between generalized horizon thermodynamics and modified gravitational dynamics.

Several pulsar timing arrays have reported evidence for a nanohertz gravitational-wave background. This background is thought to arise from the superposition of signals from a population of inspiraling supermassive black hole binaries. A key test of this interpretation is the search for individually resolvable binaries whose signals emerge above this background. Here we investigate this possibility by addressing two related questions: (1) what are the most probable properties of the first resolvable binary; and (2) how can pulsar timing observations be optimized to maximize its prospects for electromagnetic identification? By simulating populations of supermassive black hole binaries constrained by evidence for the gravitational-wave background and studying the loudest binary in each simulation, we infer the expected properties of the first individually resolvable systems. If MeerKAT continues its current observing strategy for a total of 9 years, the probability of detecting an individual binary is $\approx10-27\%$. Improving the timing precision by a factor of two changes this probability to $\approx11-28\%$, whereas increasing the cadence by a factor of four gives $\approx12-30\%$. Combining both improvements yields a detection probability of $\approx12-33\%$. Furthermore, implementing a high-cadence observing campaign preferentially resolves higher-frequency systems ($\gtrsim\unit[10]{nHz}$), whose shorter orbital periods make them considerably easier to identify through electromagnetic observations than lower-frequency binaries that repeat infrequently. We discuss prospects of electromagnetic follow-up of gravitational-wave resolved binaries with current high-cadence all-sky optical surveys.

Separating Galactic dust emission from cosmic infrared background (CIB) anisotropies is challenging because both the emission components have similar spectral properties. The primary objective of this work is to develop a component-separation algorithm to extract the Galactic dust emission from CIB anisotropies dominated sky regions using Wavelet Phase Harmonics statistics (WPH). We apply our algorithm to the Planck $353\,\mathrm{GHz}$ frequency band over three sky regions spanning a range of neutral hydrogen ($\text{HI}$) column densities at high Galactic latitudes. In all three regions, we successfully recover the dust signal without significant leakage between the dust and CIB anisotropy components. We use the low signal-to-noise ($\mathrm{S/N}$) region to validate the component-separation algorithm and compare its performance with the standard template-fit approach that uses the linear dust-$\text{HI}$ correlation relation. For intermediate and high $\mathrm{S/N}$ regions, we separate the dust signal map from the contamination while preserving the statistical correlation with $\text{HI}$ column density map. Dust maps are used to analyze the relationship between dust and $\text{HI}$ emissions. We characterize the spatial variations in dust emission relative to the hydrogen column density as a function of the angular scale.

We study a localized spin-$1/2$ detector falling into a Schwarzschild–MOG black hole. The detector interacts with a neutral massless scalar field through a charge-preserving two-level transition, while its translational wave packet obeys the MOG-charged Dirac equation. Near the outer horizon, the separated radial system reduces to an inverse-square equation with index $\Theta_{\rm D}=E_{\rm H}/(2\hbar\kappa_\alpha)$. The gauge-invariant horizon energy satisfies $E_{\rm H}=m_\psi U_{\rm H}>0$ for every future-directed crossing trajectory. We quantize the scalar field in a globally normalized Boulware scattering basis and retain both radial-flux branches of a mode that is outgoing at infinity. This treatment does not identify a local outgoing ansatz with a complete mode. A finite radial gate $\chi_p(x)=(x/L_\chi)^p e^{-x/L_\chi}$ gives closed-form excitation and absorption probability densities that include the ingoing contribution and scattering-phase interference. Within the controlled near-horizon approximation, the full detailed-balance ratio factorizes into a branch-resolved outgoing ratio and a two-branch factor. Only the outgoing ratio approaches $\exp(-2\pi\nu/\kappa_\alpha)$ when the near-horizon, adiabatic, high-gap, and branch-isolation conditions all hold. The leading switching correction is controlled by $(\nu/\kappa_\alpha)/(S_\pm L_\chi)$, not by $(S_\pm L_\chi)^{-1}$ alone. In a weak-MOG expansion, the local correction separates into surface-gravity, trajectory-prefactor, and finite-gate terms. The full response also contains a contribution from global scattering. This separation shows which terms follow from the local horizon geometry and which depend on the detector protocol and scalar propagation outside the horizon region.