Papers

We apply the Poincaré-Lindstedt method to the perturbed Binet equation describing conservative dynamics of the perturbed Kepler problem. We show that a simple treatment of divergent terms directly determines the frequency shift of the radial motion and hence the periastron advance. We provide explicit expressions for polynomial, logarithmic and inverse power perturbations, extend the procedure to higher perturbative orders and to problems involving several perturbation parameters. Applying this method to different physical effects relevant to compact binaries, we recover known results. We also obtain new contributions to the periastron advance, including the leading-order effects of arbitrary mass- and current-type tidal multipoles and mass-type spin-induced multipoles, the NNLO current-type tidal quadrupole contribution, electromagnetic electric-dipole contributions to NNLO and the eccentric corrections to the electric charge. Finally, we illustrate how the method can be directly applied to the problem of particle motion in Reissner-Nordström, de Sitter-Schwarzschild metrics and in a Yukawa potential.

Classical soft theorems govern the non-analytic structure of electromagnetic and gravitational radiation in the low-frequency regime, with the gravitational memory effect providing the canonical realization of the classical soft graviton theorem. While the soft behavior of radiation from hyperbolic scattering is well understood, its realization in gravitationally bound systems remains less explored. In this work, we study the soft structure of gravitational waveforms generated by compact binaries, focusing on the scattering of emitted radiation off the static background curvature within the multipolar post-Newtonian framework. We isolate the infrared (IR) logarithms associated with hereditary effects, including the tail-of-memory, and their higher generalizations with n-tail insertions. The resulting amplitudes exhibit a Laurent expansion in frequency space consistent with the Sahoo-Sen logarithmic soft theorems, thereby establishing a direct correspondence between gravitational memory effects, their higher-order tail corrections, and the soft expansion. We also comment on ultraviolet (UV) logarithms arising from short-distance sensitivity of the multipole expansion and their associated renormalization-group running.

We investigate 3d de Sitter axion wormhole which contribute to the no-boundary density matrix. We identify "cosmological necklace" solutions: an infinite series of Euclidean saddles corresponding with repeated bounces. This results in an unbound gravitational entropy, and a divergent path integral. To remedy this, we study the gravitational path integral using a (mostly) Lorentzian lapse contour. Within a minisuperspace steepest-descent analysis, we find that a single necklace dominates, leading to a finite entropy. A crucial element is to take into account an $(a\to -a)$ redundancy in the FLRW path integral, where $a$ is the scale factor. Surprisingly, the dominant solution is not purely Euclidean. Its entropy turns out to be independent of the axion flux, and equals the empty de Sitter entropy. We also study higher-dimensional necklaces, sourced by either an axion flux or by Yang-Mills instantons, and argue for qualitatively similar results: the single necklace solution dominates along a Lorentzian lapse contour.

Extreme-mass-ratio inspirals (EMRIs), systems in which a stellar-mass compact object spirals into a supermassive black hole ($M_1$), are prime targets for the space-based gravitational-wave interferometers and are acutely sensitive to their astrophysical environment. We quantify the joint imprint of a dark matter (DM) spike and an accretion disk on the waveforms of eccentric, equatorial Kerr EMRIs, integrating four modular effects—DM dynamical friction, DM self-gravity, Newtonian accretion-disk torques, and disk self-gravity—into a fifth post-Newtonian (5PN) augmented analytic kludge (AAK) waveform model, and assess their measurability via a Fisher-matrix analysis. The DM dynamical-friction dephasing peaks at $M_1\sim 10^6\,M_\odot$, reaching $\sim 10^2$–$10^3\,\mathrm{rad}$; there the DM spike slope is measurable to sub-percent precision and both DM channels are unambiguously detectable, while omitting DM biases the intrinsic parameters significantly. At $M_1\gtrsim 10^8\,M_\odot$ the DM and disk self-gravity channels dominate their dissipative counterparts. The disk parameters remain degenerate across the explored parameter space, since the supersonic regime dominates the inspiral and the subsonic-to-supersonic transition only mildly weakens this degeneracy without lifting it; a combined DM$+$disk analysis further reveals a cross-sector degeneracy between the DM slope and the disk parameters. This physical degeneracy, reflected in the near-singular Fisher matrix, implies that independently measuring the disk parameters requires a relativistic torque model or an electromagnetic counterpart.

A first-order phase transition (PT) alters a compact star on two scales: globally, via growth of a quark-matter core, and microscopically, via collision of quark bubbles, driving a simultaneous gravitational-wave (GW) emission in the kHz and MHz bands. Using a constrained ensemble of model-agnostic equations of state, we follow the accretion-driven evolution of proto-neutron stars through the PT and compute the resulting multi-band GW signal. The correlations found between the kHz emission and the MHz burst could help constrain nuclear matter and the physics of the PT. A delayed neutrino burst should accompany the signal, with the delay set by the details of the PT, thus providing a prime multi-messenger source.

Primordial black holes (PBHs) form from rare fluctuations in the high-density tail of the primordial distribution, making their abundance highly sensitive to primordial non-Gaussianity. Heavy fields during inflation can source such non-Gaussianity, producing characteristic signatures widely explored in cosmological collider physics, yet its impact on PBH formation remains largely unexamined. We investigate this question within quasi-single-field inflation (QSFI), the minimal realization that couples a heavy field to the inflaton. Unlike most PBH studies that consider only the bispectrum, the QSFI can generate a sizable trispectrum alongside the bispectrum, allowing us to investigate their combined effects on PBH formation. In our leading-order calculation, heavy-field non-Gaussianities can amplify the high-density tail and substantially enhance PBH production. Within the parameter scan, the largest enhancement occurs at the smallest heavy-field mass and the largest sound-speed ratio considered. The trispectrum contribution can be comparable to that of the bispectrum and may even dominate the total non-Gaussian correction. Our results establish PBH abundance as a novel observational channel for cosmological collider physics, demonstrating how heavy fields during inflation can significantly shape PBH formation.

The LUX-ZEPLIN (LZ) collaboration recently reported a singular anomalous nuclear recoil event at $248 \pm 23 \, \rm keV$, accompanied by a strict null result for unexplained excesses in their primary $5.4-50 \, \rm keV$ search window. The macroscopic momentum transfer required to generate this event is kinematically inaccessible to standard halo cold dark matter (CDM), naturally motivating models featuring light, relativistic relics. In this work, we investigate whether a contemporary flux of Hawking-boosted dark matter (DM) emitted by evaporating Primordial Black Holes (PBHs) in the $10^{11}-10^{13} \, \rm g $ mass range can source the anomaly. By parameterizing the nuclear scattering with heavy non-relativistic effective field theory (NREFT) operators and endothermic inelastic mass transitions, we formulate a strict kinematic exclusion. We demonstrate that the inherent thermal nature of the Hawking emission, coupled with the relativistic kinematics of the incident flux, inevitably overproduces low-energy recoils, aggressively violating the LZ background bounds. Utilizing this failure, we map the LZ low-energy null results into novel, highly stringent upper limits on the PBH abundance fraction, excluding $f_{\rm PBH} \gtrsim 10^{-6}$ for $M_{\rm PBH} \sim 2 \times 10^{11} \, \rm g $ at $ 90 \% $ confidence level.

We present a new Plebanski formulation of General Relativity. The key difference is that we work with a gauge vector field derived from the Lanczos potential of the Weyl tensor. We show that, on shell, the gauge vector field encodes all nontrivial gravitational degrees of freedom and, in a suitable Lorentz gauge, Bianchi identities ensure that this gauge vector field satisfies a source-free wave equation for Ricci-flat spacetimes. This description suggests new ways of interpreting the true geometrical degrees of freedom with applications in both classical and quantum gravity.

This investigation aims to test the validity of general relativity (GR) on kiloparsec scales via galaxy-scale strong gravitational lensing (SGL) combined with multiple cosmological probes. To circumvent the circularity problem induced by the presumption of a cosmological model based on GR, we apply the distance sum rule in the Friedmann-Lemaitre-Robertson-Walker (FLRW) metric to constrain the post-Newtonian parameter gamma PPN and spatial curvature Omega k independently of any cosmological model. The study introduces two methodological improvements: (i) parameterizing the dark-energy evolution factor with second-order Chebyshev orthogonal polynomials, which ensures uniformly high convergence across the full redshift baseline while retaining Omega k as an explicit free parameter; (ii) extending the original two-probe configuration (SGL + Type Ia supernovae) to a four-probe joint analysis that includes baryon acoustic oscillations and cosmic chronometers. Our results show that gamma PPN = 1.119 (+0.072/-0.072) and Omega k = 0.101 (+0.077/-0.076) at 68% confidence. The gamma PPN value is consistent with the GR prediction of unity at 1.7 sigma, while Omega k favors a mildly open universe, with flatness consistent at 2 sigma. Taking model-independent constraints as a benchmark, we quantify the systematic shift in gamma PPN induced by eight representative dark energy models, and find that all offsets are negligible, well below the baseline 1 sigma uncertainty, with no significant bias at current precision. These results confirm that conventional strong-lensing gravity tests introduce no measurable systematic bias, validating our model-independent framework.

There are strong motivations to modify gravity in the ultraviolet as well as the infrared. Such modifications are usually pursued independently from one another. Using the predictive power of asymptotically safe quantum gravity, we can constrain the effective-field-theory coefficients of scalar-tensor theories and thereby connect ultraviolet and infrared modifications of gravity. We focus on two non-minimal couplings, which are naturally generated in asymptotic safety and belong to the Horndeski Lagrangian only if the couplings satisfy a specific ratio. A priori, one would expect to find a negative answer to the question in our title. Surprisingly, we find that despite the constraints from asymptotic safety, this ratio can be achieved for a specific value of the cosmological constant. We interpret this as a non-trivial hint that asymptotically safe scalar-tensor theories could be free of extra propagating degrees of freedom in the infrared. We further find that the same result holds in an effective asymptotic safety scenario, where asymptotic safety is not a fundamental theory and quantum scale symmetry, the symmetry underlying asymptotic safety, only holds over an intermediate range of scales. Finally, we report novel non-trivial indications of approximate radiative stability in the aforementioned Horndeski sector for a range of coupling values, independently of any particular UV completion.

We investigate the cosmological implications of a power-law $f(Q, B)$ gravity model, where $Q$ denotes the non-metricity scalar and $B$ is the associated boundary term. We employ the Noether symmetry approach to identify the admissible functional form of the gravitational Lagrangian and to obtain the corresponding conserved quantities and exact cosmological solutions. Using the exact solution, we find that for this model parameter condition $\alpha+\beta>\frac{3}{2}$, the model exhibits the accelerating phase of the Universe. Furthermore, we have also discussed the model with recent observational data using a Bayesian Markov Chain Monte Carlo analysis. We consider different combinations of Cosmic Chronometers (CC), Pantheon+ & SH0ES, and DESI DR2 observations to constrain the model parameters and the Hubble constant $H_0$. The inferred value of $H_0$ depends on the adopted dataset combination, ranging from a higher value for the CC+Pantheon+ combination to a lower value for CC+DESI DR2, with the latter yielding $H_0=67.5^{+1.4}_{-1.6}\,\mathrm{km\,s^{-1}\, Mpc^{-1}}$, consistent with the Planck 2018 CMB-preferred value within the standard $\Lambda$CDM framework. The inferred \(H_0\) is strongly dataset dependent, spanning the range between the early-Universe and locally calibrated determinations. We further examine the posterior covariance and correlation matrices to identify parameter degeneracies and assess the robustness of the MCMC constraints. The background evolution also exhibits late-time accelerated expansion and remains compatible with the observational constraints. These results demonstrate that the $f(Q, B)$ model provides a viable alternative description of late-time cosmology.

We study large-overtone quasi-normal modes of a massive scalar field in a four-dimensional Schwarzschild de-Sitter black hole in the presence of a wall placed at the static sphere. Dirichlet boundary condition is imposed at the wall. At leading order the quasi-normal modes with large overtones scale as $n$ for $n\gg 1$ and are inversely proportional to the bouncing time. We analyze correction terms up to $n^{-1}$ order in the asymptotic expansion.

We present a first background-level observational test of a thermogravity theory in which the Einstein equations are clipped to a trace-free version, the cosmological constant $\Lambda$ does not gravitate, and controlled violations of energy conservation obstruct the usual reinstatement of $\Lambda$ as an integration constant. We therefore set $\Lambda=0$ and ask whether late-time acceleration can instead be generated solely by energy non-conservation, with no extra background parameter relative to flat $\Lambda\mathrm{CDM}$. We contrast a minimal universal implementation with a model in which only CDM partakes in non-conservation. The universal model is strongly disfavored when combining Supernovae and BAO distances, with the inclusion of DESI BAO worsening the fit by $\Delta\chi^2\simeq 51$ relative to $\Lambda\mathrm{CDM}$, because matter creation ties the intermediate-redshift normalization of $H(z)$ too rigidly to the present acceleration. This conclusion, however, should be interpreted with caution, since universal non-conservation would modify the observational dictionary itself. By contrast, restricting non-conservation to CDM leaves baryonic and photon observables unaffected at the background level and therefore allows a self-contained analysis. The resulting $\xi\mathrm{CDM}$ model, which can be thought of as a one parameter extension of $\Lambda\mathrm{CDM}$, provides an improved fit relative to $\Lambda\mathrm{CDM}$ for the background dataset combinations considered, with improvements reaching $\Delta\chi^2=-4.85$ and a maximum Bayesian preference of $\ln\mathcal{B}=2.77$. However, when the BAO ruler is calibrated using BBN or CMB information, the reduced effective early-time CDM density increases the sound horizon and drives $H_0$ towards lower values, thereby increasing rather than alleviating the tension with the distance-ladder calibration.

Modeling the full inspiral-merger-ringdown signal requires combining analytically controlled inspiral physics with the nonlinear strong-field information supplied by numerical relativity. We represent the numerical relativity contribution beyond an analytic inspiral waveform as residual amplitude and phase corrections. Retaining the leading-order frequency-domain amplitude and the $3.5$ Post-Newtonian TaylorF2 phase, we use a Kolmogorov-Arnold network to learn these residual corrections from SXS waveforms. After training, the learned corrections are stored as explicit spline functions, so waveform evaluation no longer requires the network itself. On $75$ simulations excluded from training and model selection, the model achieves a median flat-noise mismatch of $2.7\times10^{-5}$. Our results demonstrate a machine learning driven waveform modeling strategy in which numerical relativity augments, rather than replaces, analytically known waveform structure.

Electrically charged regular black holes in general relativity coupled to nonlinear electrodynamics are known to require a double-valued Lagrangian density $L(F)$, with a branch point at the radius where the electromagnetic invariant attains its extremum. We extend the construction of such black holes to an arbitrary number of spacetime dimensions $D\geq4$ and obtain a new family of solutions that unifies the Bardeen and Hayward black holes in a single analytic form, with a single nonlinearity parameter $\gamma$. We show that the Maxwellian weak-field limit is recovered if and only if $\gamma=D-3$, so that the Bardeen and Hayward black holes, which lack this limit in four dimensions, become Maxwellian in five and six dimensions, respectively. The matter sector is written in the dual $P$ framework as a Hamiltonian density $H(P)$ that contains only fixed coupling constants, so that the mass and the charge of the black hole appear as integration constants of a fixed theory; the regular solutions form a one-parameter subfamily in which the mass is tied to the charge. We locate the branch point of $\L(F)$ in closed form and find that it is always hidden inside the event horizon in four dimensions, whereas for $D\geq5$ it emerges from the horizon for near-extremal charges and becomes visible to a distant observer. The null and weak energy conditions are satisfied everywhere, the strong energy condition is violated only inside the horizon, and the dominant energy condition is violated at large distances only for $\gamma>D-3$. Finally, since the theory is fixed, the first law of black hole thermodynamics holds in its standard form; we derive the horizon potential in closed form, the extended first law with the couplings as thermodynamic variables, and the corresponding Smarr formula.

Pulsar timing array (PTA) collaborations have reported evidence for a stochastic gravitational wave (GW) background in the nHz band. Should scalar-induced GWs, sourced at second order by enhanced primordial curvature perturbations, contribute to this signal, a coincidence of scales makes them directly sensitive to the softening of the equation of state around the QCD crossover. We solve the tensor and scalar equations of motion across the Standard Model (SM) thermal history and tabulate the transfer functions for direct use in present and future PTA analyses. We show that this SM effect modifies the height of the induced spectrum by up to $\approx55\%$ across the PTA band relative to the radiation-domination expectation, with either sign depending on whether the source modes cross the horizon before or after the crossover. Fitting the NANOGrav 15-year data with a broken-power-law curvature power spectrum, we find that including the crossover shifts the inferred peak amplitude and scale by an amount that could already be relevant for the comparison with primordial black hole overproduction bounds. The importance of this SM effect will grow as the statistical uncertainty on the amplitude and scale shrinks with future, more sensitive PTA datasets, at which point neglecting it could significantly bias the inference.

We study the quasinormal mode spectrum and wave-scattering properties of the four-dimensional rational regular black hole recently obtained in non-polynomial (quasi-topological) gravity. Using third-order WKB together with two independent cross-checks a Frobenius–Riccati shooting method and a time-domain evolution we compute the fundamental scalar, electromagnetic, and axial gravitational-type quasinormal frequencies, validating our approach against known Schwarzschild results. We find that as the geometry approaches extremality, its dimensionless ringing frequency is systematically suppressed relative to the Schwarzschild value, a direct and quantifiable imprint of the non-polynomial regularization that we trace, via the photon-sphere correspondence, to the response of the effective potential near the horizon. A Fisher-matrix estimate indicates that this suppression could in principle be resolved by a space-based detector such as LISA for a sufficiently massive and nearby source. We further compute, for the first time, the axial gravitational-type ringdown spectrum of this geometry using the Regge–Wheeler master equation, while noting that a full stability analysis of the underlying gravity theory remains an open problem requiring the linearized field equations of the modified action. Finally, we examine the greybody transmission factors across spins $s=0,1,2$ and show that they follow the same systematic ordering and near-extremal enhancement as the ringing-frequency suppression, pointing to a common physical origin. Together, these results give one of the first quantitative characterizations of the observational signatures and the remaining open theoretical questions of a regular black hole in modified gravity.

Several hundreds of gravitational wave events have been detected. They are good tools to investigate various fundamental problems, including tests of general relativity (GR) theory. Among the GR test schemes based on gravitational wave data, the parameterized scheme has been extensively studied by many groups including the LIGO-VIRGO-KAGRA collaboration (LVK). Existing results indicate that the deviation parameters from GR are small. In the current paper, we would like to estimate whether these deviation parameters are small enough to confirm GR. We transform the deviation parameters into mass–mass relations and, by comparing the resulting mass–mass relations from different parameters, we can estimate the probabilities that the data favor the GR theory. Applying our null test scheme to five O1/O2 binary black hole events, we find some GR deviation signatures but they are not conclusive yet. We also find no monotonic correlation between the network SNR and the strength of GR deviation, suggesting that the observed tensions are not simply driven by signal strength. Our null test scheme can be easily generalized and applied to all gravitational wave observation data.

We investigate linear matter-density perturbations in full quadratic gravity. Working in the longitudinal gauge and adopting the sub-horizon and quasi-static approximations, we obtain an algebraic expression for the effective gravitational coupling in the theory and the equation for evolution of matter density perturbations. We identify the positive-real poles of this coupling and map their dependence on the quadratic couplings. In the formal infrared continuation, the reduced coupling recovers its general-relativistic value, whereas at large physical wavenumber it is suppressed when both quadratic couplings are nonzero; the single-coupling limits reproduce distinct behaviors. Numerical integrations of the growing mode are performed up to the first pole and show that pole-free regions of the reduced system can support regular matter-growth solutions, while poles obstruct a continuous late-time evolution. Our results show that quadratic gravity cannot generally reproduce the standard cosmological evolution of perturbations and in the regions where it can, it closely matches the behavior of $f(R)$ theories.

Based on a two-parameter perturbative framework, we derive perturbative formulae for the radii of massive particle spheres and massive shadow radii, where an extremal black hole is employed as the perturbative background. Adopting extremal black holes as the perturbative background enables energy-dependent scans to probe stronger gravitational regimes. Using these formulae, we perform perturbative analyses of the massive shadows for several non-standard black holes, and obtain the expansion expressions near the photon sphere of the extremal black hole. It is found that, although the dependence on the energy parameter $\epsilon$ is the same across models, the coefficients associated with the deviation parameter $\delta$ differ significantly, which can serve to distinguish among different black hole models. Furthermore, we construct a two-point Padé approximant in the form of a continued fraction which achieves a globally accurate approximation for the massive shadow radius of the black hole over the entire parameter range. Numerical tests show that the relative errors based on this approximant are small enough to provide a reliable foundation for model-independent reconstruction of metric parameters.

Detecting quadratic quasi-normal modes in black hole ringdowns would provide evidence for nonlinear gravitational dynamics, while measuring their properties would enable tests of the corresponding predictions of general relativity. Specifically, second-order black hole perturbation theory predicts that their amplitudes scale with the product of the amplitudes of their parent linear modes, with a coupling coefficient that depends on the spin of the remnant black hole. This mode-specific coupling coefficient has not yet been directly measured from gravitational wave data. Here, we use Bayesian inference on the GW250114 ringdown, modeled with the $220$, $221$, and $220\times220$ modes, to infer the amplitude-coupling coefficient of the $220\times220$ mode. The inferred coefficient is consistent with predictions from numerical relativity fits and second-order perturbation theory; no comparison shows a deviation exceeding $1.3 \sigma$. Although the current data cannot distinguish among these spin-dependent predictions, this measurement establishes a basis for more stringent observational tests of nonlinear gravitational dynamics in black-hole ringdowns.

We study the quasinormal modes (QNMs) of Type II perturbations for dilaton-Euler-Heisenberg (dEH) black holes. These perturbations consist of coupled even-parity gravitational and dilaton perturbations together with odd-parity electromagnetic perturbations. The background is described by mass $M$, magnetic charge $Q_m$, and dilaton coupling difference $\zeta=\alpha-\beta$ to the Euler-Heisenberg term. To find the QNM frequencies, we need to find the parameter space of $(\zeta, Q_m/M)$ that is free from vector ghosts. For the $\ell=1$ mode, the radiative block couples the dilaton and axial electromagnetic amplitudes, whereas the $\ell=2$ mode also contains metric amplitudes, so the frequencies are obtained from matrix-valued boundary-value problems. We calculate QNM frequencies for the fundamental ($n=0$) branch with direct integration and the Chebyshev pseudospectral method, and compare the two computations wherever their charge intervals overlap. We find that the $\ell=1,2$ modes in the $n=0$ branch remain damped over the ghost-free parameter domain, supporting the stability of dEH black holes against Type II perturbations. The Type I sector will be presented elsewhere; no claim is made here about that sector, higher multipoles, or overtones.

We propose the turning-point count-discrepancy indicator (TPCD) for diagnosing orbital chaos from a single trajectory in relativistic Hamiltonian systems with two oscillatory degrees of freedom. TPCD measures the largest cumulative departure of one turning-event count from its mean rate per reference cycle, requiring neither a neighboring orbit nor a phase-space partition and applying to both massive particles and photons. We establish its long-time behavior under explicit event–phase assumptions. Rigid phases with an exact event–phase correspondence obey a strict discrepancy bound of unity, and linearizable regular tori with bounded degree-one phase deformations obey a finite, orbit-dependent bound; both imply that the normalized indicator decays to zero as the record grows. A diffusive fluctuation mechanism instead yields a Brownian-bridge scaling and a finite statistical scale. Integrable Kerr motion validates the construction, recovering prescribed frequency ratios from event counts to within $2.6\times10^{-5}$ for six targets, including an irrational ratio. In charged-particle scans around a Kerr black hole in an external test magnetic field, TPCD and the fast Lyapunov indicator agree for all 80 sampled trajectories. In the Schwarzschild–Melvin photon model, a trajectory with elevated finite-time TPCD but low fast-Lyapunov values is identified as regular once its indicator trends downward over an extended integration, showing that finite-time values must be read together with their long-time trend.

Accretion disk surrounding around compact astrophysical objects serve as a powerful laboratories for testing gravitational theories in the strong-field regimes. In this work, we systematically investigated the structural, dynamical, and thermodynamic properties of the steady-state thin accretion disk within the framework of $f(R,T)$ gravity by adopting the linear functional form $f(R,T) = R + 2\chi T$ coupled with the pressure-free dust fluid matter Lagrangian ($\mathcal{L}_m = -\rho$). We demonstrate that the modified field equations seamlessly reduce to standard General Relativity in the pure vacuum limit ($T_{\mu\nu} = 0$). Utilizing a perturbative approach for a small coupling parameter ($\vert{}\chi\vert{} \ll 1$), we derive the first-order corrections to the background Schwarzschild metric potentials, which subsequently govern the modified particle kinematics, the specific energy $E$, the specific angular momentum $L$, and the Keplerian angular velocity $\Omega$. Our analysis reveals that the geometry-matter coupling parameter $\chi$ significantly shifted towards the location of the Innermost Stable Circular Orbit ($r_{isco}$) and alters the radial energy dissipation rate $F(r)$ and the effective surface temperature profile $T(r)$ across the Shakura-Sunyaev thin disk. Furthermore, the resulting modifications in the Spectral Energy Distribution (SED) and overall radiative efficiency $\eta$ provide robust theoretical signature.

Gravitational waves (GWs) can induce effective electromagnetic currents when interacting with external electric and magnetic fields, as described by linearized gravity modifications to Maxwell's equations. This coupling enables a novel detection approach for high-frequency gravitational waves (HFGWs) using axion haloscope experiments. Here we present the MAGnetic Gravitational wave Induction Experiment (MAGGIE), the first lumped-element HFGW detector proposed in Europe, designed to probe HFGWs in the kHz-MHz regime by leveraging a 14 T solenoidal magnet at the University of Hamburg. The GW-induced magnetic flux is captured by a custom-designed pickup loop optimized for the expected symmetry of the effective current. A figure-8-shaped geometry, oriented to break the azimuthal symmetry, is implemented, together with a blind-loop configuration, for real-time noise rejection and calibration. The readout scheme is tailored for continuous signals and time-domain transient searches, using waveform templates for primordial black hole (PBH) mergers. The expected experimental reach in terms of strain spectral noise density at the 40 MHz high-frequency end is projected to reach $\sim 4 \times 10^{-16}/\sqrt{\mathrm{Hz}}$ for transient searches, and in terms of strain, projected to reach $\sim 10^{-19}$ for 1 year of continuous search. This allows MAGGIE to constrain currently unexplored regions of the HFGW parameter space.

Parity violation in the early Universe may lead to a chiral gravitational wave background. We show that, for finite-time parity-violating sources, the low-frequency tail of the generated chiral gravitational wave background is universally unpolarized, and that the chirality fraction decreases linearly with frequency. Our result generally applies to any parity-violating gravitational-wave source that appears quadratically in the wave equation. We consider both Gaussian sources and explicit trispectrum templates. We attribute the lack of chirality of the low-frequency tail to the fact that, on large scales, gravitational wave generation is almost equally inefficient for both helicities.

Particle accelerators have carried fundamental physics to remarkable precision, but the Planck scale remains fifteen orders of magnitude beyond any foreseeable collider. We argue that this impasse is not a temporary shortfall but an insurmountable one. The way out runs through condensed matter physics: laboratory tests of generalized uncertainty relations, analog systems, and — most provocatively — frameworks in which spacetime itself is a condensate. We review the string-loop picture, in which quantization turns the world sheet into a three-dimensional fractal, the corpuscular description of black holes as self-sustained graviton condensates, and the phase-transition structure connecting the Riemannian phase to a pre-geometric one. The conclusion is deliberately provocative: the question of quantum gravity is no longer a question of energy, but of crossing the bridge.

The high-precision observations from the $Planck$ and BICEP/$Keck$ have imposed stringent bounds on the tensor-to-scalar ratio, ruling out many standard inflationary models. The recent bound from Atacama and SPT along with DESI provides a different viable landscape, making it difficult for the inflationary models to sustain. Amid these increasingly stringent and conflicting observational constraints, modified gravity theories offer a compelling avenue to restore the viability of inflationary models. With this motivation, we study mutated hilltop inflation within $f(\phi, T)$ gravity, where a scalar field is non-minimally coupled to the trace of the energy-momentum tensor. The scalar spectral index $n_s$, the tensor spectral index $n_t$, and the tensor-to-scalar ratio $r$ are calculated, and their trajectories are presented in the $n_s-r$ plane. The results are then confronted with the latest combined CMB bounds from $Planck$, BICEP/$Keck$, ACT DR6, SPT, and DESI DR2. We report that MHI can predict $r \sim 10^{-3}$ and is compatible with the latest CMB & DESI bound SPA+BK+DESI at $ 1\sigma $ CL.

We construct a holographic open quantum system by coupling two CFTs through a double trace deformation and tracing out the bath degrees of freedom within the Schwinger-Keldysh formalism. This procedure yields an effective description of the nonunitary reduced dynamics of the remaining sector, whose linear response is encoded in its retarded Green's function. Working perturbatively in the double trace coupling, we derive closed form expressions for the quasinormal modes of several coupled geometries involving pure AdS and rotating or nonrotating BTZ black holes, considering both possible system/bath assignments. For the nonrotating configurations previously investigated in arXiv:2509.14312, our analytic expressions agree with the numerical results, while our treatment extends the analysis to configurations involving rotating BTZ black holes. Finally, moving beyond perturbation theory, we derive a relation between dissipation and transmission in the general AdS-BTZ setup.

We study the conservative and dissipative forces on a Newtonian binary interacting with a nonrelativistic ultralight scalar medium. We derive integral expressions for the instantaneous force and the orbit-averaged energy flux generated by wake perturbations of a general background field. For a homogeneous coherent background, the conservative force suffers from a well-known infrared divergence. We trace this divergence to the failure of perturbation theory about a constant scalar state and identify the gravitational Bohr scale at which the homogeneous approximation breaks down. In the long-wavelength regime, the leading dissipation into the medium is quadrupolar, and we obtain an explicit expression for the leading energy flux from an eccentric binary. In the short-wavelength regime, we identify a local ”hard” region in momentum space that produces a universal Coulomb logarithm, while the nonlogarithmic contribution remains orbit dependent. We then promote the background to a stochastic ensemble with a general velocity distribution. Three independent length scales, together with a derived geometric-mean scale, produce eight distinct scale hierarchies, for which we characterize the ensemble-mean response. Finally, we derive the two-point correlation function of the orbit-averaged energy flux and evaluate it in ”coherent-response” regimes. Wave interference produces order-unity density fluctuations, so the flux in an individual realization can differ substantially from its ensemble mean.