In the quest to unravel the dark sector, feebly interacting freeze-in dark matter presents an intriguing possibility, plausibly explaining the consistent null results from various dark matter experiments. We propose a unique imprint in the form of gravitational waves generated during the freeze-in production of dark matter from heavy particle decay in the early Universe. This characteristic gravitational wave signature can serve as a powerful probe for freeze-in dark matter. Our study indicates that future high-frequency gravitational wave experiments can detect these waves, offering a novel avenue to critically test the underlying conditions and requirements of this dark matter paradigm, which typically lie beyond the reach of current and planned dark matter detection experiments.

We study quasinormal modes and echoes of symmetric and asymmetric black bounce solutions generated by anisotropic fluids within the framework of general relativity. We derive the effective potential governing massless scalar fields and compute the corresponding quasinormal mode spectra using three independent methods: sixth-order WKB, Pöschl-Teller and time-domain evolution. Our results show that symmetric black bounce configurations with horizons yield a standard single-barrier potential, while horizonless solutions may exhibit multiple potential barriers that generate gravitational wave echoes. These echoes are sensitive to model parameters such as the fluid energy density and the regularizing parameter a that defines the minimal 2-sphere. The asymmetric models considered recover the Reissner-Nordström solution in their external region but can be bounded or unbounded in the inside, depending on the sign of a parameter. Both cases have similar qualitative properties as far as wave emission is concerned but show no echoes. This makes it very difficult to distinguish them from standard Reissner-Nordström configurations.

A particularly compelling aspect of the GW190521 event detected by the LIGO-Virgo-KAGRA (LVK) collaboration is that it has an extremely short duration, and lacks a clearly identifiable inspiral phase usually observed in the binary black holes (BBHs) coalescence. In this work, we hypothesize that GW190521 might represent a single, isolated gravitational wave (GW) echo pulse from the wormhole, which is the postmerger remnant of BBHs in another universe and connected to our universe through a throat. The ringdown signal after BBHs merged in another universe can pass through the throat of wormhole and be detected in our universe as a short-duration echo pulse. Our analysis results indicate that our model yields a network signal-to-noise ratio comparable to that of the standard BBHs merger model reported by the LVK collaboration. For GW190521, Bayesian model selection yields ln ℬEcho BBH ≃ -2.9, indicating that the data favor the BBH hypothesis over our echo-for-wormhole model.

Marine oil film identification based on GLOH, K-Means and adaptive threshold.

Structural health monitoring has always relied on nondestructive testing for detecting deterioration in structures. Individually, various sensors like Electrical Resistivity, Ground Penetrating Radar, Half-Cell Potential, Impact Echo, and Ultrasonic Surface Waves give valuable yet sometimes incomplete perspectives which show deterioration according to one sensor reading; leaving out deterioration according to other sensor perspectives. This study aims to develop a data fusion framework where information from these diverse sensor readings is integrated to improve deterioration assessment. By aligning the sensor readings, quantifying inter-sensor dependencies, and applying graph-based learning methods, the study extracts deterioration patterns that emerge only when sensors are analyzed jointly. Field data was collected at the Bridge Evaluation and Advanced Structural Testing (BEAST) lab at Rutgers University, and the data was aligned. Using direct spatial interpolation of the raw sensor readings, thereby avoiding signal transformation, sensor readings were transformed into vectors. Using Principal Component analysis, we linearly extracted shared deterioration patterns without assumptions/ labels. Fused deterioration heat maps and quantitative deterioration index trends linking deterioration growth to calendar time interpolation. The resulting fused deterioration heat maps and deterioration index trends provided a clear visualization of deterioration progression from different sensor perspectives over time and space.

The ultrasonic array is widely used for non-destructive evaluation as it has a large inspection coverage and is sensitive to small defects. The Time-Reversal Multi-Signal Classification method can achieve super-resolution imaging for defects whose size is below the Rayleigh diffraction limit with full-matrix capture (FMC) data. However, the FMC only uses a single element in the transmit stage and hence degrades the signal-to-noise ratio of echo waves in multilayer structures, which leads to poor resolution ability of practical imaging. Additionally, the FMC consumes a long acquisition time and produces a large data volume, which decreases the scanning speed of automatic detection equipment. Focusing on these problems, a novel imaging method called virtual-source excitation super-resolution imaging (VSE-SRI) is proposed. The VSE-SRI applies a group of elements to transmit energy, by adjusting the delay time of each element, the energy can be concentrated in the layer of detected material, which contains defects, and the energy attenuation in the transmit stage is greatly decreased, which improves the SNR of signals and the resolution ability of images. More importantly, with a higher SNR, equal resolution performance can be realized with fewer transmit times. Experiments demonstrate that the VSE-SRI can resolve the two 1 mm holes with a distance of 0.60 Rayleigh limit in a three-layer composite structure. Compared with traditional FMC-SRI, the peak-to-center intensity difference increases from 1.02 dB to 8.38 dB, and the data volume decreases from 64×64 A-scan signals to 64×3 A-scan signals. It is promised that the proposed VSE-SRI method can achieve faster and more robust super-resolution imaging for multilayer composite structures in high-speed automatic detection situations.

The article presents the radar reflectivity of complex objects (navigation objects in atmospheric formations) during their observation by a shipborne radar polarisation complex (SRPC) in the form of an equation relating three matrices. Two matrices determine the energy and parametric characteristics of fully polarised waves that irradiate a complex radar observation object, the elements of which are real Stokes energy parameters, while the third matrix of Müller scattering of echo signals of partially polarised waves of a complex object determines its scattering properties, the elements of which are the effective scattering areas of the navigation object and atmospheric formation. The reflectivity of the navigation object is represented by four linear equations and allows determining the effective reflective properties of its surface by measuring the SRPC of four Stokes parameters of the reflected wave echo signals. Taking into account the scattering properties of atmospheric formations, their reflective properties are substantiated and presented in the form of a matrix consisting of 16 elements, which are the average effective scattering surfaces of atmospheric formation particles for the radar volume, and its polarisation properties are determined by four consecutive irradiations with fully polarised waves of specific polarisation and measurement of the Stokes parameters for each polarisation of the irradiating wave. The reflective properties of a complex object are considered from the point of view of distinguishing the polarisation structure of its echo signals, the individual characteristics of the observed SRPC navigation object against the background of the echo signal of atmospheric formation, and are presented in the form of a Mueller matrix.

Abstract Recent discovery of a compact binary coalescence through GW 230529 by LIGO has indicated the merger event of a compact object of mass between $$2.5{-}4.5~M_{\odot }$$ 2.5 - 4.5 M ⊙ with a neutron star of mass between $$1.2{-}2.0~M_{\odot }$$ 1.2 - 2.0 M ⊙ . The mass of the unknown compact object makes it within the heaviest neutron star never tracked out or the lightest black hole ever detected. Here, we have shown that such a mass gap neutron star with this observed mass ( $$2.5{-}4.5~M_{\odot }$$ 2.5 - 4.5 M ⊙ ) can be explained consistently by f ( R ) gravity. We have also adopted the presence of pressure anisotropy inside the neutron star which supports a massive neutron star with mass more than $$2.6~M_{\odot }$$ 2.6 M ⊙ , compatible with LIGO data ( GW 190814). Further, the modified Tollman–Oppenheimer–Volkoff equations have acknowledged such a compact stellar structure that can produce gravitational wave echoes (frequencies remain in the range of $$3{-}6$$ 3 - 6 kHz).

To accurately simulate the progression of pipeline corrosion, this paper proposes a three-dimensional corrosion modeling method for curved random surfaces based on spatial frequency composition. It applies this method to the inner surface of layered pipelines to emulate both the morphological characteristics and the evolution of internal corrosion. Combined with ultrasonic guided wave technology, the approach enables quantitative assessment of internal corrosion in layered pipelines. First, trigonometric series expansion and nonlinear polynomial superposition are used to characterize the roughness and curvature of the corroded surface, respectively, establishing a mathematical model capable of accurately representing complex corrosion morphologies. Next, a COMSOL–ABAQUS co-modeling approach is employed to build a finite element model of a three-layer composite pipeline consisting of a steel pipe, an insulating layer, and an anti-corrosion layer, with curved random-surface corrosion on the inner surface of the steel pipe. Finally, a wavelet packet decomposition algorithm is applied to extract features from the guided wave echo signals, creating a damage index matrix to correlate the corrosion area with the damage index quantitatively. The results show that the damage index increases steadily with the corrosion area, confirming the effectiveness of the proposed method. This study provides an alternative technical approach for high-fidelity modeling and precise assessment of pipeline corrosion detection.

Carbon fiber reinforced plastic (CFRP) is widely used in fields such as aircraft and construction due to its advantages of light weight, high hardness, wear and corrosion resistance. To quantitatively detect internal defects in CFRP panels, a THz reflective defect detection system was built. A defect analysis algorithm based on multi-layer gradient threshold is proposed. Based on the manufacturing process of CFRP, a THz wave echo function model for multi-layer CFRP structures was derived. A CFRP plate containing debonding defects and crack defects was trial produced, to obtain its THz time-domain spectrum. The experiment completed the reconstruction of THz two-dimensional images of CFRP plates. The test results show that three different depths of test data can be detected, and the calculated diameters of the debonding defect areas are 9.12, 9.86, and 9.93 mm, respectively, with a relative error mean of 3.63%. Crack defects can also be effectively identified, but the quality of their test images will decrease with increasing pre-embedded depth. The linearity between defect depth and testing frequency is 0.98, which verifies the possibility of quantitative calculation. The test results of power spectral density and defect depth show that it has good uniformity.

Temporal modulation of material parameters offers unprecedented control over wave dynamics, enabling phenomena beyond the capabilities of static systems. Here we introduce and analyze a robust mechanism for time rewinding, whereby a temporally evolved wave is fully restored to its original state through a carefully engineered sequence of temporal modulations. In electromagnetic systems, time rewinding emerges from impedance-matched or anti-matched hierarchical bilayer structures with matched modulation durations, exploiting total transmission or reflection and reversed phase accumulation. In Dirac systems, it arises via complete interband transition driven by time-dependent vector potentials. Unlike time-reversal holography or quantum time mirrors, which produce wave echoes but only partial waveform recovery, our approach achieves deterministic and complete reconstruction of the entire wave state, including both amplitude and phase. Analytical conditions for robust amplitude and phase restoration are derived and validated through simulations of discrete and continuous modulations, demonstrating resilience to modulation complexity and temporal asymmetry. These findings establish a versatile platform for secure information retrieval, temporal cloaking, programmable metamaterials, and wave-based logic devices.

Testing loop quantum gravity with Gaia BH1: predicted orbital anomaly, gravitational wave echoes, and thermodynamic shifts

This work presents a maximized formulation of Pole Theory, a discrete scalar field framework unifying quantum mechanics, general relativity, and cosmology. The universe is modeled as emerging from a state of absolute zero—defined not as physical emptiness but as a pre-geometric null state in which pole–antipole pairs exist in Grassmann-type superposition, satisfying antisymmetric relations such as θ₍ᵢⱼ₎ = −θ₍ⱼᵢ₎ and θ² = 0. At a critical pole density, internal symmetry collapses, initiating the Big Bang as a curvature-pressure singularity. Time and space emerge through asymmetric field evolution governed by a scalar polar field φ(x, t), defined as the product of two components: T(x, t) (polar tension) and Kₜₕₑₜₐ(x, t) (curvature phase). Thus, φ(x, t) = T(x, t) × Kₜₕₑₜₐ(x, t). From this, a unified scalar field equation is derived of the form: □φ + m²φ + λφ⁴ = (8πG / c⁴) × (T^φ / φ) × R Where □ is the d’Alembert operator, m is the effective mass term arising from field locking, λ is the self-interaction coefficient of the polar field, and R is the Ricci curvature scalar. This equation embeds the structure of quantum fields, gravitational dynamics, and cosmological inflation within a discrete geometric context. Energy, mass, charge, and spin emerge as collective properties of pole density, symmetry, and locking configuration. Reduction of the polar field under symmetry constraints leads naturally to the Dirac equation, the Fermi Lagrangian, and SU(1) × SU(2) × SU(3) gauge representations. Gravitational phenomena—including black hole curvature, evaporation, and wave propagation—are modeled as macroscopic oscillations and collapses of the polar field. The cosmological constant Λ arises dynamically as the initial imbalance of superposed pole symmetry, expressible as Λ ≈ ∂²φ / ∂t² ÷ φ evaluated near the Big Bang. The theory predicts observable gravitational wave echoes, pole-field-dependent deviations in collider experiments, and anisotropic field memory in the cosmic microwave background. Falsifiability is achievable at both Planck-scale energy thresholds and astrophysical scales through structured phase interference. This paper refines and expands upon the original pole theory framework, incorporating pre-Big Bang geometry, quantum emergence, Hamiltonian formulation, decoherence, and the eventual return to zero via symmetry relaxation. It concludes with philosophical reflections on duality, causality, and the encoding of universal memory within the polar curvature field.

Optical frequency domain reflectometry (OFDR) has been widely used in many fields. Nevertheless, the echo signal in OFDR measurements is usually weak. To enhance echo wave for high signal-to-noise ratio (SNR), the emission power of the laser is typically high. To improve the sensitivity to the echo wave and spatial resolution of OFDR, a laser feedback OFDR(LF-OFDR) system is proposed in this paper. Employing the spontaneous enhancement in laser cavity, LF-OFDR provides an improved SNR of about 27 dB without extra amplifiers compared with the conventional OFDR. Consequently, targets at 1.5 km are detected utilizing only a 0.7 nW probe beam. Employing an auxiliary interferometer to suppress the spectral broadening caused by the nonlinearity of frequency modulation, the signal peaks can be effectively calculated at 1.5 km. The spatial resolution in the range of 1.5 km is 1.53 mm. Advanced in photon consumption and resolution, the proposed method promises more economical and precise development of OFDR.

Surface ships are mostly elastic structures with curved surfaces and certain openings. The incidence of plane waves on curved surfaces excites various surface elastic waves on the shell. The presence of the air–water interface and shell openings alters the types of surface waves on the shell and their propagation paths, thereby influencing the backscattering acoustic field of partially submerged targets at a flat interface. This paper takes a partially submerged open cylindrical shell at the air–water interface as the research model, revealing the excitation and reradiation mechanisms of the flexural waves a0−, a0+, and a0 by a horizontally incident plane wave on a target at a flat interface. The influence of the immersion depth of the cylindrical shell and the size of opening on its acoustic scattering characteristics is investigated, and theoretical prediction formulas are provided, which are validated through experiments. The results indicate that the flexural waves can cross the air–water interface, be reflected at the open end of the cylindrical shell, and propagate circumferentially along the shell, forming periodic pulse echoes. By considering the multipath of flexural wave echoes, an estimation formula for the arrival times of each echo is derived. Utilizing the phase matching theory, expressions for the resonant frequencies of flexural waves and the phase conditions for mutual interference of paths are obtained. The accuracy of the prediction formulas is verified through simulations and experiments.

This research paper introduces a novel framework modelling space-time as a compressible fluid, unifying general relativity, quantum mechanics, and cosmology. Gravity emerges from pressure gradients as mass creates low-pressure voids in the fluid. Time is entropy flow, with dilation in suppressed entropy regions. Black holes are cavitation zones with finite-density cores, resolving singularities, while wormholes form stable pressure tunnels without exotic matter. Quantum phenomena, like entanglement and tunnelling, arise as fluid oscillations and pressure collapses. The model derives Einstein’s field equations as a fluid state law and accurately predicts planetary orbits, such as Mercury, Mars, Venus, and Earth (e.g., Earth’s orbit within 0.011% error) (Appendix B), aligning with observations like lensing and redshift. Novel predictions include chromatic lensing, gravitational wave echoes, and CMB anisotropies. This intuitive, observationally robust theory offers a cohesive framework for understanding the universe’s fundamental dynamics across scales.

Abstract non-reflective boundary condition is required by aeroacoustic direct numerical simulation. The commonly used methods are characteristic-based and far-field-based non-reflecting boundaries, whose performance will be attenuated by nonlinear flows at the outlet. Large Eddy Simulation further directly computes the compressible flow noise of a cylinder at Mach number 0.4. The limitations of characteristic-based and far-field-based non-reflecting methods are discussed. The impact of numerical dissipation and viscosity diffusion of the damping layer on the radiated sound field and the near Stream field are investigated. The results indicated that the hybrid non-reflective boundary effectively absorbs the reflected sound waves, reduces the interference noise between the radiated waves and the echo waves in the watershed, and obtains a more reasonable dipole acoustic radiation feature. The echo waves from the outlet are reduced, and a more reliable dipole radiation pattern is obtained. More echo waves are absorbed by the multi-dimensional stretched grids in the damping layer, as acoustic waves propagate in multi-dimensions are better matched. However, the impacts of the hybrid non-reflective boundary on the cylinder’s near-field flows are negligible.

The Hydroxyl-Terminated Polybutadiene (HTPB) liner in solid rocket motors is a viscoelastic material formed through the polymerization and gelation of HTPB and toluene diisocyanate. The semi-cured state of the HTPB liner plays a critical role in determining the optimal timing for propellant loading. However, due to the thinness of the HTPB layer, traditional nonlinear ultrasonic testing methods suffer from insufficient resolution and time-domain aliasing of detection echoes, limiting their effectiveness in real-world applications. To address these challenges, this study proposes a novel Aluminum Plate - HTPB liner - Aluminum Plate detection structure for real-time monitoring of the HTPB curing process. First, the theoretical framework of nonlinear ultrasonic beam mixing is analyzed. Then, through both simulations and experimental studies on LY12 aluminum alloy, the most suitable mixing modes for beam mixing monitoring are determined. Based on these findings, a non-collinear mixing monitoring system incorporating the proposed detection structure is designed and implemented to track the curing progression of the HTPB liner in real time. Experimental results demonstrate that the mixed-frequency signals generated within the designed structure exhibit high sensitivity to changes in the curing state of the HTPB layer. In addition, this approach effectively prevents time-domain aliasing between the mixed initial wave and primary echo. Furthermore, the detected signal parameters align well with theoretical predictions, confirming the feasibility of this method for accurate monitoring of the HTPB curing process.

Non-invasive imaging modalities that identify rupture-prone atherosclerotic plaques hold promise to improve patient risk stratification and advance early intervention strategies. Here, phase-changing peptide nanoemulsions are developed as theranostic contrast agents for synchronous ultrasound detection and therapy of at-risk atherosclerotic lesions. By targeting lipids within atherogenic foam cells, and exploiting characteristic features of vulnerable plaques, these nanoemulsions preferentially accumulate within lesions and are retained by intraplaque macrophages. It is demonstrated that acoustic vaporization of intracellular nanoemulsions promotes lipid efflux from foam cells and generates echogenic microbubbles that provide contrast-enhanced ultrasound identification of lipid-rich anatomical sites. In Doppler mode, stably oscillating peptide nanoemulsions induce random amplitude and phase changes of the echo wave to generate transient color imaging features, referred to as ‘twinkling’. Importantly, acoustic twinkling is unique to these peptide emulsions, and not observed from endogenous tissue bubble nuclei, generating diagnostic features that offer unprecedented spatial precision of lesion identification in 3D.

Abstract Aging degradation is the main form of failure of rubber in service, leading to a decline in its physical and mechanical properties. This paper presents an efficient method for assessing the aging degradation of rubber using the quasi-static component (QSC) of ultrasonic longitudinal waves induced by acoustic radiation. The experiments quantitatively observe the response of the QSC pulse to different levels of aging degradation. A pulse-echo ultrasonic transducer is employed to simultaneously capture the primary longitudinal wave (PLW) and QSC echoes, enabling the determination of the acoustic nonlinearity parameter of QSC with a single transducer excitation. The results suggest that, in comparison to traditional linear ultrasonic techniques based on attenuation coefficient and wave velocity measurements, the relative acoustic nonlinear parameter of QSC proves to be more sensitive to aging degradation in rubber. Particularly, the amplitude of the QSC pulse undergoes a significant change with increasing aging degradation, even when the PLW tone burst is completely attenuated. These findings confirm the effectiveness of QSC as a method for evaluating aging degradation in highly attenuative materials.