Binary systems of compact objects in close orbit around a supermassive black hole (SMBH) may form in galactic nuclei, providing a unique environment to probe strong-gravity tidal effects on the binary’s dynamics. In this work, we investigate precession resonances arising between the periastron precession frequency of a binary system and its orbital frequencies around the SMBH. By modeling the SMBH as a Schwarzschild black hole, we find that relativistic effects in the tidal field give rise to a significantly richer resonance spectrum compared to the Newtonian case. This result is supported by both perturbative and numerical analyses of the quadrupolar tidal interaction in the strong-gravity regime. Our results reveal new signatures for strong-gravity effects in such triple systems, with potential implications for gravitational-wave astronomy.

Recently, we introduced the Lorentzian-Euclidean black hole, a static and spherically symmetric solution of vacuum Einstein equations that exhibits a change in metric signature across the event horizon. In this framework, the analysis of radial trajectories of freely falling bodies proves that the central singularity can be avoided via a mechanism we interpret as atemporality, which is responsible for the shift of the time variable from real to imaginary values. In this paper, we further explore this model by first examining the behavior of null geodesics. Our investigation requires a set of signature-adaptive coordinate changes that generalize the local Lorentz transformations underlying general relativity. We find that photon orbits, like their massive counterparts, cannot traverse the event horizon, thereby strengthening the previous result on the impossibility to reach the $r=0$ singularity. Additionally, we discuss the causal structure of the spacetime, provide the corresponding Penrose diagram, and analyze the process of matter accretion in the outer region of the black hole.

We compute the scattering angle for a scalar neutral probe undergoing unbound motion around a topological star, including self-force effects. Moreover, we identify the “electromagnetic” source of the background as a Papapetrou field compatible with the isometries and characterize topological stars by studying their sectional curvature, geometric transport along special curves, and gravitational energy content in terms of the superenergy tensors.

Nowadays, one of the well-known serious challenges in cosmology is the Hubble tension, namely the discrepancy between the Hubble constants from the local observation of Type Ia supernova (SNIa) and the high-$z$ observation of cosmic microwave background (CMB). Here, we are interested in alleviating the Hubble tension with a local void. The key idea is assuming that we live in a locally underdense void, where one will feel a faster expansion rate compared to the cosmic average. In the literature, it was found that a local void cannot satisfyingly alleviate the Hubble tension, since it is not preferred over the $\mathrm{\Lambda }\mathrm{CDM}$ model by the observations such as the Pantheon SNIa sample, especially in terms of the information criteria AIC and BIC. In the present work, we try to alleviate the Hubble tension with a local void and transitions of the absolute magnitude $M$, by using the $\mathrm{Pantheon}+$ SNIa sample alone or jointly with the CMB data of Planck 2018. We find that the Hubble tension can be satisfyingly alleviated, while the $\mathrm{\Lambda }\mathrm{LTB}$ void models are strongly preferred by the observations.

The optical depth to reionization, a key parameter in the standard $\mathrm{\Lambda }\mathrm{CDM}$ cosmological model—which describes the Universe using six parameters and includes a cosmological constant ($\mathrm{\Lambda }$) and cold dark matter (CDM)—can be computed within astrophysical frameworks for star formation by modeling the evolution of the intergalactic medium. Accurate evaluation of this parameter is thus crucial for joint statistical analyses of cosmic microwave background (CMB) data and late-time probes such as the 21 cm power spectrum, requiring consistent integration into cosmological solvers. However, modeling the optical depth with sufficient precision in a computationally feasible manner for Markov chain Monte Carlo (MCMC) analyses is challenging due to the complexities of the nonlinear astrophysics. We introduce Neural Network Emulator for Reionization and Optical depth (nnero), a framework that leverages neural networks to emulate the evolution of the free-electron fraction during cosmic dawn and reionization. We demonstrate its effectiveness by simultaneously constraining cosmological and astrophysical parameters in both standard cold dark matter and noncold dark matter scenarios, including models with massive neutrinos and warm dark matter, showcasing its potential for efficient and accurate parameter inference.

The Indian Dark matter search Experiment (InDEx) has been initiated at Jaduguda Underground Science Laboratory to explore the low mass region of dark matter. The detectors used by InDEx are superheated droplet detectors with active liquid ${\mathrm{C}}_2{\mathrm{H}}_2{\mathrm{F}}_4$. The run1 of InDEx was with $2.47\mathrm{kg}-\mathrm{days}$ of exposure at a threshold of 5.87 keV. In the present work, the run2 of InDEx, the detectors were set at 1.95 keV thresholds with an active liquid mass of 70.4 g. For a runtime of 102.48 days, the experimental results set constraint on spin-independent cross section at $(1.55+(+0.62-0.32{)}_{\mathrm{stat}}+(+0.03-0.02{)}_{\mathrm{sys}})\times {10}^{-40}{\mathrm{cm}}^2$ (90% C.L.) and spin-dependent limit on F at $(7.97+(+3.44-1.78{)}_{\mathrm{stat}}+(+0.15-0.18{)}_{\mathrm{sys}})\times {10}^{-38}{\mathrm{cm}}^2$ (90% C.L.) for $20.4\mathrm{GeV}/{\mathrm{c}}^2$ and $21.0\mathrm{GeV}/{\mathrm{c}}^2$ weakly interacting massive particle (WIMP) mass, respectively. There is a shift of the most sensitive WIMP mass towards the lower region and an improvement of the sensitivity limit over the InDEx run1.

Using relative stellar astrometry for the detection of coherent gravitational wave sources is a promising method for the microhertz range, where no dedicated detectors currently exist. Compared to other gravitational wave detection techniques, astrometry operates in an extreme high-baseline-number and low-signal-to-noise ratio per baseline limit, which leads to computational difficulties when using conventional Bayesian search techniques. We extend a technique for efficiently searching pulsar timing array datasets through the precomputation of inner products in the Bayesian likelihood, showing that it is applicable to astrometric datasets. Using this technique, we are able to reduce the total dataset size by up to a factor of $\mathcal{O}(100)$, while remaining accurate to within 1% over 2 orders of magnitude in gravitational wave frequency. Applying this technique to simulated astrometric datasets for the Kepler Space Telescope and Nancy Grace Roman Space Telescope missions, we obtain forecasts for the sensitivity of these missions to coherent gravitational waves. Due to the low angular sky coverage of astrometric baselines, we find that coherent gravitational wave sources are poorly localized on the sky. Despite this, from ${10}^{-8}\mathrm{Hz}$ to ${10}^{-6}\mathrm{Hz}$, we find that Roman is sensitive to coherent gravitational waves with an instantaneous strain above $h_0\simeq {10}^{-11.4}$, and Kepler is sensitive to strains above $h_0\simeq {10}^{-12.4}$. At this strain, we can detect a source with a frequency of ${10}^{-7}\mathrm{Hz}$ and a chirp mass of ${10}^9{M}_{\odot }$ at a luminosity distance of 3.6 Mpc for Kepler and 0.3 Mpc for Roman. We finally discuss possible strategies for improving on these strain thresholds.

The arrival time of electromagnetic signals traveling in chiral cosmic media is investigated in the context of Maxwell-Carroll-Field-Jackiw electrodynamics. Considering the interstellar medium (ISM) as a cold, ionized, chiral plasma, we derive the time delay between two traveling signals, expressed in terms of a modified dispersion measure (DM) which receives additional contribution from the chiral parameters. Faraday rotation angle is also addressed in this chiral plasma scenario, yielding modified rotation measures (RM). Using DMs data from five pulsars, we establish constraints on the chiral parameter magnitude at the order of ${10}^{-23}–{10}^{-22}\mathrm{GeV}$. On the other hand, the Faraday rotation retrieved from RM measurements implied upper constraints as tight as ${10}^{-36}\mathrm{GeV}$.

We investigate TeV scale seesaw scenarios at ${\mu }^{+}{e}^{-}$ and ${\mu }^{+}{\mu }^{+}$ colliders in the $\mu \mathrm{TRISTAN}$ (Transposable Ring Intersecting STorage Accelerator in Nippon) experiment. In a minimal type I seesaw scenario, we consider two generations of Standard Model (SM) singlet heavy Majorana–type right-handed neutrinos, which couple with SM-gauge bosons through light-heavy neutrino mixing. We discuss the prospects of probing heavy neutrinos via the processes such as ${\mu }^{+}{e}^{-}\to \nu N_i\to e^{+}jj\nu$ or ${\mu }^{-}jj\nu$ for $\sqrt{s}=346\mathrm{GeV}$ and $1{\mathrm{ab}}^{-1}$ luminosity. Studying these process, we estimate limits on the light-heavy neutrino mixing angles as a function of heavy neutrino mass, which could be 2 orders of magnitude stronger than electroweak precision data. Further, we study the effect of a doubly charged scalar boson $(H^{++})$ from the type II seesaw scenario in ${\mu }^{+}{\mu }^{+}$ collision at $\sqrt{s}=2\mathrm{TeV}$. In this case, we consider ${\mu }^{+}{\mu }^{+}\to {\ell }_i^{+}{\ell }_j^{+}$ and ${\mu }^{+}{\mu }^{+}\to H^{++}Z/\gamma$ processes followed by the same-sign dilepton decay of $H^{++}$. We find that events involving $e^{+}{e}^{+}$ among these final states can probe the neutrino mass ordering in the $\mu \mathrm{TRISTAN}$ experiment at $5\sigma$ significance. In addition, we study the production of positively charged triplet fermions in $\mu \mathrm{TRISTAN}$ following the ${\mu }^{+}{\mu }^{+}\to {\mu }^{+}{\mathrm{\Sigma }}^{+}$ process, where ${\mathrm{\Sigma }}^{+}$ decays into ${\mu }^{+}jj$ mode through $Z$ boson exchange. Considering a triplet at 1 TeV and studying SM backgrounds, we estimate the discovery potential of the ${\mu }^{+}{\mu }^{+}jj$ signal at $\mu \mathrm{TRISTAN}$ with respect to projected luminosity.

We report the first measurement of neutron capture multiplicity in neutrino-oxygen neutral-current quasielasticlike interactions at the gadolinium-loaded Super-Kamiokande detector using the T2K neutrino beam, which has a peak energy of about 0.6 GeV. A total of 30 neutral-current quasielasticlike event candidates were selected from T2K data corresponding to an exposure of $1.76\times {10}^{20}$ protons on target. The $\gamma$ ray signals resulting from neutron captures were identified using a neural network. The flux-averaged mean neutron capture multiplicity was measured to be ${1.37\pm 0.33(\mathrm{stat}.)}_{-0.27}^{+0.17}(\mathrm{syst}.)$, which is compatible within 2.3 sigma than predictions obtained using our nominal simulation. We discuss potential sources of systematic uncertainty in the prediction and demonstrate that a significant portion of this discrepancy arises from the modeling of hadron-nucleus interactions in the detector medium.

We study defect conformal field theory (CFT) within the framework of holographic duality, emphasizing the impact of corner contributions. We model distinct conformal defects using interface branes that differ in tensions and are connected by a corner. Employing the relationship between CFT scaling dimensions and Euclidean gravity actions, we outline a general procedure for calculating the anomalous dimensions of defect changing operators at nontrivial cusps. Several analytical results are obtained, including the cusp anomalous dimensions at big and small angles. While $1/\varphi$ universal divergence appears for small cusp angles due to the fusion of two defects, more interestingly, we uncover a bubble phase rendered by a near zero angle cusp, in which the divergence is absent.

In this work, we study the spectrum of the Carrollian superstring in the flipped vacuum (the highest-weight representation) in detail. The target spacetime of the Carrollian string has a generalized Carrollian symmetry, and it is composed of both Poincaré directions and Carrollian directions. We explicitly show that two homogeneous Carrollian superstring theories, one with compactified Poincaré direction and the other with compactified Carrollian direction, share the same homogenous super-${\mathfrak{bms}}_3$ symmetry, and their zero modes can be identified under the usual T-duality transformation. Moreover, we investigate the spectrum of a general Carrollian superstring in the flipped vacuum. As the string can still have nonvanishing winding along the spatial direction in the infinite radius limit, the spectrum of the Carrollian strings in the flat background is no longer truncated. We furthermore construct the vertex operators of gravitons and discuss their scattering amplitudes. We find that the form of 3-point amplitudes differs from that of the usual tensile superstrings only by a simple function, which reflects the ultralocal nature of Carrollian physics.

One of the most significant debates of our time is whether our macroscopic world (i) naturally emerges from quantum mechanics or (ii) requires new physics. We argue for the latter and propose a simple gravitational self-decoherence mechanism. For this purpose, we postulate the existence of a Heisenberg cut such that particles with masses $m$ much smaller and larger than a critical mass $M_{\mathrm{C}}$ (of the order of the Planck mass $M_{\mathrm{P}}$) would be necessarily treated according to quantum and classical rules, respectively. Our effective model is designed to capture the new physics that free quantum particles would experience as their masses approach $M_{\mathrm{C}}$. The purity loss for free quantum particles is evaluated and shown to be highly inefficient for quantum particles with $m\ll M_{\mathrm{C}}$ but very effective for those with $m\sim M_{\mathrm{C}}$. The physical picture behind it is that coherence would (easily) leak from heavy enough particles to (nonobservable) spacetime quantum degrees of freedom. Finally, we contextualize our proposal with state-of-the-art experiments and show how it can be tested in a future Stern-Gerlach-like experiment.

In this paper, we discuss the concept of bulk reconstruction, which involves mapping bulk operators into CFT operators to understand the emergence of spacetime and gravity. We argue that the $N=\infty$ approximation fails to capture crucial aspects of gravity, as it does not respect gauge invariance and lacks direct connections between energy and boundary metrics. Key concepts such as entanglement wedge reconstruction and holographic error correction codes, which are based on the $N=\infty$ theory, may be incorrect or require significant revision when finite $N$ effects are considered. We present explicit examples demonstrating discrepancies in bulk reconstructions and suggest that a gauge-invariant approach is necessary for an accurate understanding.

The graviton $\mathcal{S}$ matrix has a famous soft pole. We show that the $\mathcal{S}$ matrix for cylindrical gravitational waves has a soft zero. The soft pole for ordinary gravitons comes from a Ward identity for supertranslation symmetry at asymptotic infinity. We show that the soft zero for cylindrical gravitational waves comes from a Ward identity for Geroch symmetry at asymptotic infinity. Because it is a zero rather than a pole, there is no memory effect. Overall, this soft zero is a manifestation of Geroch symmetry and of the extraordinary simplicity of cylindrical gravitational waves.

In field theory, domain walls are constructed by embedding localized field configurations varying in one space dimension, such as the ${\varphi }^4$ kink, in two or three space dimensions. At the classical level, the kink mass straightforwardly turns into the energy per unit length or area, known as the domain wall tension. The quantum contribution to the tension is more difficult to compute, because the quantum fluctuations about the domain wall in the additional coordinates must be included. We show that spectral methods, making use of scattering data for the interaction of quantum fluctuations with the domain wall background, are an efficient way to compute the leading quantum correction to the domain wall tension. In particular we demonstrate that within this approach it is straightforward to pass from one renormalization scheme to another.

We investigate on the diffeomorphism invariance of the effective gravitational action, focusing in particular on the path integral measure. In the literature, two different measures are mainly considered, the Fradkin-Vilkovisky and the Fujikawa one. With the help of detailed calculations, we show that, despite claims to the contrary, the Fradkin-Vilkovisky measure is diffeomorphism invariant, while the Fujikawa measure is not. In particular, we see that, contrary to naïve expectations, the presence of $g^{00}$ factors in the Fradkin-Vilkovisky measure is necessary to ensure the invariance of the effective gravitational action. We also comment on results recently appeared in the literature, and show that formal calculations can easily miss delicate points.

This paper investigates gravitational lensing in the strong deflection limit, focusing particularly on higher-order images produced near compact objects such as black holes and their observable impact through the visibility function. Employing a robust parametrization framework proposed by Rezzolla and Zhidenko, the study systematically explores deviations from the Schwarzschild metric. A detailed theoretical analysis of interferometric observables is provided, highlighting how higher-order images imprint distinctive, measurable patterns in the visibility function, notably characterized by a staircaselike structure. By parametrically varying metric coefficients, the analysis reveals clear dependencies between spacetime deviations and key observational signatures, specifically the step heights and periodicities in the interferometric visibility. The results enhance the theoretical groundwork for interpreting data from advanced interferometric observations, potentially enabling precise tests of general relativity and the discrimination among alternative gravitational theories.

We combine numerical analysis with the inverse scattering method to study the periodic analog of Kerr solution. The periodic analog of the Schwarzschild solution is known to be regular and exhibit Kasner asymptotic behavior for an arbitrary size of event horizon not exceeding the period. The previous numerical analysis of the rotating version of the periodic Schwarzschild black hole in Peraza et al. [Classical Quantum Gravity 40, 175010 (2023).] based on the heat flow, together with analytic results by Peraza and Reiris [Classical Quantum Gravity 42, 085008 (2025).] shows that there exist obstructions to putting the periodic Schwarzschild solution in rotation in a certain parameter range. In this paper, we apply an efficient numerical approach based on the inverse scattering method, interpreting the periodic Kerr solution as an infinite chain of solitons. This allows one to completely describe the existence domain in the space of physical parameters (the period, mass, and angular momentum); we study the dependence of Kasner exponent and the shape of the ergosphere on parameters of the problem.

Gravitational waves offer new ways to test general relativity (GR) in the strong-field regime, including tests involving the ringdown phase of binary black hole mergers, characterized by oscillating and quickly decaying quasinormal modes (QNMs). Recent advances have extended QNM calculations to several theories beyond GR through the development of the modified Teukolsky formalism, including higher derivative gravity and dynamical Chern-Simons (dCS) gravity. Using the modified Teukolsky formalism, we previously derived radial second-order differential equations governing curvature and scalar field perturbations in dCS gravity at leading order in spin. In this work, we compute the QNM frequency shifts for slowly rotating black holes in dCS gravity from these modified Teukolsky equations, and (1) show that the radial equations for Weyl scalars ${\mathrm{\Psi }}_{0,4}$ can be separated into even- and odd-parity parts, confirming that the scalar field couples only to the odd-parity sector; (2) extend the eigenvalue perturbation method to coupled fields; (3) compute the QNM spectrum, obtaining consistent results across independent calculations using different radiation gauges; (4) calculate the overtones in the QNM spectra for the first time in dCS gravity; (5) show that our findings align with previous metric perturbation studies and mark the first QNM spectrum calculation in a nonminimally coupled scalar-tensor theory via the modified Teukolsky formalism. This work lays the foundation for studying fast-rotating black holes in dCS gravity, advancing black hole spectroscopy in beyond-GR contexts.