Bounce Point Research Articles

Theoretical insights and implications of bouncing cosmology in f(R,T2)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f(\\mathcal {R},\ extrm{T}^2)$$\\end{document} theory

In this paper, we explore cosmological bouncing solutions to investigate the cosmic evolution in the framework of energy-momentum squared gravity. We consider flat Friedmann–Robertson–Walker spacetime with a perfect matter distribution. We assume two different functional forms of f(R,T2)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f(\\mathcal {R},T^2)$$\\end{document} gravity model to examine the impact of this modified framework in the evolution of the universe. Furthermore, we consider a specific scale factor to investigate different cosmological parameters, analyzing the evolutionary behavior of the universe in this gravity. We also perform stability analysis using the perturbation technique. Our findings indicate that the null energy condition violates at the bounce point and equation of state parameter exhibits characteristics of a quintessence era or phantom regimes. These aspects highlight the complex interplay between energy conditions and stability in bouncing cosmological model. We conclude that the f(R,T2)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f(\\mathcal {R},T^2)$$\\end{document} theory successfully provides viable alternatives to the standard cosmological scenarios, offering insights into the early universe and the nature of gravity.

A comprehensive analysis of cosmic evolution in f(Q,T) theory

This paper examines the bouncing cosmology in f(Q,T) theory, where Q represents non-metricity and T denotes the trace of energy-momentum tensor. The aim of this research is to study the phenomenon of a cosmic bounce, where contraction, bounce point and expansion occur at t < 0, t = 0 and t > 0, respectively. In this perspective, we consider Bianchi Type-I spacetime with perfect matter configuration to study the mysterious universe. Further, we analyze the behavior of cosmological parameters such as scale factor, Hubble parameter, deceleration parameter and equation of state parameter using three distinct functional forms of f(Q,T) theory. A comprehensive examination of energy conditions is analyzed to study the matter bounce in this modified framework. Our findings indicate that the acceleration takes place in the vicinity of the bouncing point and ensures the presence of a nonsingular bounce in this theory.

Effects of 2.5-D ultra-low and ultra-high velocity zones on flip-reverse-stacking (FRS) of the ScS wavefield

SUMMARY Within the last decade, thin ultra-low velocity zone (ULVZ) layering, sitting directly on top of the core–mantle boundary (CMB), has begun to be investigated using the flip-reverse-stack (FRS) method. In this method, pre- and post-cursor arrivals that are symmetrical in time about the ScS arrival, but with opposite polarities, are stacked. This same methodology has also been applied to high velocity layering, with indications that ultra-high velocity zones (UHVZs) may also exist. Thus far, studies using the FRS technique have relied on 1-D synthetic predictions to infer material properties of ULVZs. 1-D ULVZ models predominantly show a SdS precursor that reflects off the top of the ULVZ and an ScscS reverberation within the ULVZ that arrives as a post-cursor. 1-D UHVZ models are more complex and have a different number of arrivals depending on a variety of factors including UHVZ thickness, velocity contrast, and lateral extent. 1-D modelling approaches assume that lower mantle heterogeneity is constant and continuous everywhere across the lower mantle. However, lower mantle features display lateral heterogeneity and are either finite in extent or display local thickness variations. We examine the interaction of the ScS wavefield with ULVZs and UHVZs in 2.5-D geometries of finite extent. We show that multiple additional arrivals exist that are not present in 1-D predictions. In particular, multipath ScS arrivals as well as additional post-cursor arrivals are generated. Subsequent processing by the FRS method generates complicated FRS traces with multiple peaks. Furthermore, post-cursor arrivals can be generated even when the ScS ray path does not directly strike the heterogeneity from above. Analysing these predictions for 2.5-D models using 1-D modelling techniques demonstrates that a cautious approach must be adopted in utilization and interpretation of FRS traces to determine if the ScS wavefield is interacting with a ULVZ or UHVZ through a direct strike on the top of the feature. In particular, traveltime delays or advances of the ScS arrival should be documented and symmetrical opposite polarity arrivals should be demonstrated to exist around ScS. The latter can be quantified by calculation of a time domain multiplication trace. Because multiple post-cursor arrivals are generated by finite length heterogeneities, interpretation should be confined to single layer models rather than to interpret the additional peaks as internal layering. Furthermore, strong trade-offs exist between S-wave velocity perturbation and thickness making estimations of ULVZ or UHVZ elastic parameters highly uncertain. We test our analysis methods using data from an event occurring in the Fiji-Tonga region recorded in North America. The ScS bounce points for this event sample the CMB region to the southeast of Hawaii, in a region where ULVZs have been identified in several recent studies. We see additional evidence for a ULVZ in this region centred at 14°N and 153°W with a lateral scale of at least 250 km × 360 km. Assuming a constant S-wave velocity decrease of −10 or −20 per cent with respect to the PREM model implies a ULVZ thickness of up to 16 or 9 km, respectively.

Cosmographic and matter bounce scenario in modified torsion gravity

In this manuscript, we investigate the matter bounce cosmological implications in the context of f(T,τ) gravity (T represents the torsion scalar whereas τ is the trace of the energy–momentum tensor). We construct the field equations for spatially flat homogeneous and isotropic space–time with perfect fluid configuration. We consider the two particular forms of scale factor inspired by loop-quantum cosmology to investigate the matter bounce mechanism. In this regard, we examine the graphical behavior of cosmological as well as dynamical parameters at and in the neighborhood of bounce point. Furthermore, the stability via perturbation technique and energy conditions are also discussed. It is concluded that all the cosmographic tests favor the bouncing cosmology for an appropriate choice of model parameters. It is worth mentioning that considered models are stable and energy conditions are violated at the bouncing point as well as in its vicinity leading to a successful bouncing scenario.

Analyzing quantum gravity spillover in the semiclassical regime

One of the standard approaches of incorporating the quantum gravity (QG) effects into the semiclassical analysis is to adopt the notion of a quantum-corrected spacetime arising from the QG model. This procedure assumes that the expectation value of the metric variable effectively captures the relevant QG subtleties in the semiclassical regime. We investigate the viability of this effective geometry approach for the case of dust dominated and a dark energy dominated universe. We write the phase space expressions for the geometric observables and construct corresponding Hermitian operators. A general class of operator ordering of these observables is considered, and their expectation values are calculated for a unitarily evolving wave packet. In the case of dust dominated universe, the expectation value of the Hubble parameter matches the “semiclassical” expression, the expression computed from the scale factor expectation value. In the case of the Ricci scalar, the relative difference between the semiclassical expression and quantum expectation is maximum at singularity and decays for late time. For a cosmological constant driven universe, the difference between the semiclassical expressions and the expectation value is most pronounced far away from the bounce point, hinting at the persistent quantum effect at the late time. The parameter related to the shape of the distribution appears as a control parameter in these models. In the limit of a sharply peaked distribution, the expectation value of the observables matches with their semiclassical counterpart, and the usage of effective geometry approach is justified.

Field experiments: How well can seismic monitoring assess rock mass falling?

Rockfall tracking and mass estimation are critical for effective hazard response. While seismic techniques have been applied to the field, systematic analysis remains lacking. In this study, we conducted rockfall experiments using seismic networks to investigate tracking and mass estimation issues. Two methods, time-based and amplitude-based source location methods, were used to identify initial rock-ground impact points. The high-fitness areas (>0.95) overlapping between the two methods showed a location error of <8.7 m for initial rock-ground impact points. However, the combined methods could not track bounce points on the slope. To address the limitation, we further defined a seismic energy ratio, a ratio between target and reference stations. When a reference station was far away from rockfall, the rising or decreasing of high-frequency (70–99 Hz) seismic energy ratios reflected the rock approaching or departing from the target station, respectively. Additionally, the peak low-frequency (1–40 Hz) seismic energy ratio of ground stations indicated the timing of the initial rock-ground impact. Successful implementation of these methods depends on optimal station distribution. The study recommends that different source-to-station distances with triangular station geometry and stations at different elevations provide a reliable location determination and straightforward rockfall tracking. Furthermore, the lowest frequency(fL) satisfied the power spectral threshold of −100 dB during the rockfall moving process correlated with the rock mass(M), M = 38,989FL-2.03 with R2 of 0.62. The relation was practical for estimating rockfalls' mass for future monitoring in the same slope. Our experiments demonstrate that this seismic technique can effectively assist in rockfall assessment.

Higher order theory of quasi-isodynamicity near the magnetic axis of stellarators

The condition of quasi-isodynamicity is derived to second order in the distance from the magnetic axis. We do so using a formulation of omnigenity that explicitly requires the balance between radial particle drifts at opposite bounce points of a magnetic well. This is a physically intuitive alternative to the integrated condition involving distances between bounce points, used in previous works. We investigate the appearance of topological defects in the magnetic field strength (puddles). A hallmark of quasi-isodynamic fields, the curved contour of minimum field strength, is found to be inextricably linked to these defects. Our results pave the way to construct solutions that satisfy omnigenity to a higher degree of precision and also to simultaneously consider other physical properties, like shaping and stability.

Enhance primordial black hole abundance through the non-linear processes around bounce point

The non-singular bouncing cosmology is an alternative paradigm to inflation, wherein the background energy density vanishes at the bounce point, in the context of Einstein gravity. Therefore, the non-linear effects in the evolution of density fluctuations (δρ) may be strong in the bounce phase, which potentially provides a mechanism to enhance the abundance of primordial black holes (PBHs). This article presents a comprehensive illustration for PBH enhancement due to the bounce phase. To calculate the non-linear evolution of δρ, the Raychaudhuri equation is numerically solved here. Since the non-linear processes may lead to a non-Gaussian probability distribution function for δρ after the bounce point, the PBH abundance is calculated in a modified Press-Schechter formalism. In this case, the criterion of PBH formation is complicated, due to complicated non-linear evolutionary behavior of δρ during the bounce phase. Our results indicate that the bounce phase indeed has potential to enhance the PBH abundance sufficiently. Furthermore, the PBH abundance is applied to constrain the parameters of bounce phase, providing a complementary to the surveys of cosmic microwave background and large scale structure.

Torsional birefringence in metric-affine Chern-Simons gravity: gravitational waves in late-time cosmology

In the context of the metric-affine Chern-Simons gravity endowed with projective invariance, we derive analytical solutions for torsion and nonmetricity in the homogeneous and isotropic cosmological case, described by a flat Friedmann-Robertson-Walker metric. We discuss in some details the general properties of the cosmological solutions in the presence of a perfect fluid, such as the dynamical stability and the emergence of big bounce points, and we examine the structure of some specific solutions reproducing de Sitter and power law behaviours for the scale factor. Then, we focus on first-order perturbations in the de Sitter scenario, and we study the propagation of gravitational waves in the adiabatic limit, looking at tensor and scalar polarizations. In particular, we find that metric tensor modes couple to torsion tensor components, leading to the appearance, as in the metric version of Chern-Simons gravity, of birefringence, characterized by different dispersion relations for the left and right circularized polarization states. As a result, the purely tensor part of torsion propagates like a wave, while nonmetricity decouples and behaves like a harmonic oscillator. Finally, we discuss scalar modes, outlining as they decay exponentially in time and do not propagate.

Constraints on the martian crust away from the InSight landing site

The most distant marsquake recorded so far by the InSight seismometer occurred at an epicentral distance of 146.3 ± 6.9o, close to the western end of Valles Marineris. On the seismogram of this event, we have identified seismic wave precursors, i.e., underside reflections off a subsurface discontinuity halfway between the marsquake and the instrument, which directly constrain the crustal structure away (about 4100−4500 km) from the InSight landing site. Here we show that the Martian crust at the bounce point between the lander and the marsquake is characterized by a discontinuity at about 20 km depth, similar to the second (deeper) intra-crustal interface seen beneath the InSight landing site. We propose that this 20-km interface, first discovered beneath the lander, is not a local geological structure but likely a regional or global feature, and is consistent with a transition from porous to non-porous Martian crustal materials.

High-resolution mid-mantle imaging with multiple-taper SS-precursor estimates

SUMMARY SS-precursor imaging is used to image sharp interfaces within Earth’s mantle. Current SS-precursor techniques require tightly bandpassed signals (e.g. 0.02–0.05 Hz), limiting both vertical and horizontal resolutions. Higher frequency content would allow for the detection of finer structure in and around the mantle transition zone (MTZ). Here, we present a new SS-precursor deconvolution technique based on multiple-taper correlation (MTC). We show that applying MTC to SS-precursor deconvolution can increase the frequency cut-off up to 0.5 Hz, which potentially sharpens vertical resolution to ∼10 km. Furthermore, the high-pass frequency can be lowered (≪ 0.01 Hz), allowing more long-period energy to be included in the calculation, to better constrain the signal and reduce side lobes. Our method is benchmarked on full-waveform synthetic seismograms computed via AxiSEM3D for the PREM 1-D Earth model. We apply our novel MTC-SS-precursor deconvolution to ∼7000 seismograms recorded at broad-band borehole sensors of the Global Seismographic Network with source–receiver bounce points in the North-Central Pacific Ocean. The MTZ in this region appears to be thin, which agrees with previous results. We do not observe the 520-km discontinuity in our SS-precursor estimates. Additionally, we detect a low-velocity zone above the MTZ to the north of the Hawaiian Islands that has previously been inferred from asymmetry in side lobe amplitudes. Our high-frequency analysis demonstrates this feature to be a sharp interface (≤ 10-km thickness), rather than a thick wave speed gradient.

Investigation of Effects of Near‐Surface Complexities on Measurement of Mantle Discontinuity Using SS and Its Precursors

AbstractSeismic observations of the mantle discontinuities at depths of 410 and 660 km provide crucial information to constrain the thermal and compositional states and further shape our understanding of mantle dynamics. The topography and impedance contrast of the mantle discontinuities, at both global and regional scales, have been imaged through differential travel times and amplitude ratios between SS and its precursors. However, a relatively simple reference phase (SS) is often assumed, which may overlook potential bias caused by near‐surface complexities. To assess these effects, we developed an algorithm to simulate the waveforms of SS and its precursors efficiently based on the propagation matrix method. We found that the crustal thickening increases the differential travel times but does not change the amplitude ratios much, while the sedimentary layer significantly affects both the differential travel times and amplitude ratios. The global anomalies of differential travel times and amplitude ratios are mapped incorporating earth models from CRUST1.0 and PREM. The time shift can reach ∼9 s, which will bring a depth bias of mantle discontinuity of more than 20 km, while the amplitude ratio anomaly is up to ∼40%, which will make the impedance contrast of the mantle discontinuity overestimated. The near‐surface complexities cannot be ignored in the investigation of mantle discontinuities with SS precursors, especially when the bounce points located in the regions with sediment.

Elliptically polarized sum-frequency generation by total-internal-reflection-based optical rotation quasi-phase-matching technique.

This work demonstrates the elliptically polarized radiation of sum-frequency generation by the total-internal-reflection-based optical rotation quasi-phase-matching technique that allows polarization rotation in the presence of an external electric field across a rectangular-shaped, Y2O3-coated, MgO-doped LiNbO3 crystal. This thin film is employed to control the phase shifts generated by p- and s-polarized light propagation at each bounce point of the slab-film interface. The computer-aided simulation yielded a high conversion efficiency of 7.5%/W/cm, reflecting the combined field impact of the optical rotation quasi-phase-matching and fractional quasi-phase-matching approaches. Moreover, the conversion yielding limiting factors such as absorption, surface roughness, and the interference effect of the nonlinear law of reflection have also been included to simulate the generation of a 770 nm wavelength. A comparative assessment of the generated linearly and elliptically polarized radiations has also been addressed.

Total-internal-reflection-based optical rotation quasi-phase-matching technique realized in a biaxial electro-optic chiral material to generate highly efficient second harmonic

This work demonstrates a total-internal-reflection-based optical rotation quasi-phase-matching technique availing polarization rotation under the influence of an external electric field in a rectangular-shaped potassium niobate crystal coated with a thin layer of gallium phosphide. This thin film is utilized to regulate the phase shifts caused by the propagation of p - and s -polarized light at the slab-film interface at each total internal reflection bounce point. The peak conversion efficiency of 3.3%/W/cm, obtained from the computer-aided simulation, reflects the combined field effect of optical rotation quasi-phase-matching and fractional quasi-phase-matching techniques. The effect of optical losses and the nonlinear law of reflection have also been included to simulate the generation of a second-harmonic wavelength of 2150 nm.

Loop Quantum Cosmological Model From ADM Hamiltonian

The loop quantum cosmological model from ADM Hamiltonian is studied in this article. We consider the spatially flat homogeneous FRW model. It turns out that the modified Friedmann equation keeps the same form as the APS LQC model. However, the critical matter density for the bounce point is only a quarter of the previous APS model, that is, ρcL=ρc4. This is interesting because the lower critical bounce density means the quantum gravity effects will get involved earlier than the previous LQC model. Besides, the lower critical density also means the detection of quantum gravity effects easier than the previous model.

Spectral Element Modeling of Acoustic to Seismic Coupling Over Topography

AbstractAcoustic waves in the atmosphere are commonly recorded on seismometers as they couple into the ground. These signals, here called ground coupled airwaves, are not commonly considered in numerical modeling of infrasound propagation, which often assumes a rigid unmeshed boundary. Starting from an analytically‐tractable spherical wave model, we analyze how the coupling of an acoustic wave into a planar elastic halfspace changes over a wide range of scenarios. We use energy admittance to quantify acoustic to seismic coupling over both a planar elastic halfspace and meshed topography. Our spectral element and analytic calculations have different maxima as a function of incidence angle, with very high admittance values for near‐vertical incidence (maximum ≈78%). Energy admittance calculations at shallow incidence angles are much smaller (less than 1%). In simulations over the complex topography of Sakurajima Volcano, we attribute the variable spatial pattern of energy admittance to changes in earth parameters between each model. The observed pressure difference over the simulated 15 km region appears to be . While this value is relatively small, the cumulative addition over 100s of km and multiple acoustic bounce points may be significant. Acoustic to seismic coupling along the propagation path may bias long distance yield estimates, particularly when infrasound propagates over regions with steep topography or particularly slow seismic velocities, such as alluvial planes.

Energy conditions in the new model of loop quantum cosmology * *Supported by the National Natural Science Foundation of China (NSFC) (11775082, 12047519, 11875006, 11961131013), China Postdoctoral Science Foundation (2021M691072)

Recently, a de-Sitter epoch has been found in the new model of loop quantum cosmology, which is governed by the scalar constraint with both Euclidean and Lorentz terms. The singularity free bounce in the new LQC model and the emergent cosmology constant strongly suggest that the effective stress-energy tensor induced by quantum corrections must violate the standard energy conditions. In this study, we perform an explicit calculation to analyze the behaviors of specific representative energy conditions, i.e., average null, strong, and dominant energy conditions. We reveal that the average null energy condition is violated at all times, while the dominant energy condition is violated only at a period around the bounce point. The strong energy condition is violated not only at a period around the bounce point but also in the whole period from the bounce point to the classical phase corresponding to the de Sitter period. Our results will shed some light on the construction of a wormhole and time machine, which usually require exotic matter to violate energy conditions.

Evaluation of a Statistical Approach for Extracting Shallow Water Bathymetry Signals from ICESat-2 ATL03 Photon Data

In this study we present and validate a simple empirical method to obtain bathymetry profiles using the geolocated photon data from the Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) mission, which was launched by NASA in September 2018. The satellite carries the Advanced Topographic Laser Altimeter System (ATLAS), which is a lidar that can detect single photons and calculate their bounce point positions. ATLAS uses a green laser, causing some of the photons to penetrate the air–water interface. Under the right conditions and in shallow waters (&lt;40 m), these photons are reflected back to ATLAS after interaction with the ocean bottom. Using ICESat-2 data from four different overflights above the Heron Reef, Australia, a comparison with SDB data showed a median absolute deviation of approximately 18 cm and Root Mean Square Errors (RMSEs) down to 28 cm. Crossovers between two different overflights above the Heron Reef showed a median absolute difference of 13 cm. For an area north-west of Sisimiut, Greenland, the comparison was done with multibeam echo sounding data, with RMSEs down to between 35 cm, and correspondingly showed median absolute deviations between 33 and 49 cm. The proposed method works well under good conditions with clear waters such as in the Great Barrier Reef; however, for more difficult areas a more advanced machine learning technique should be investigated in order to find an automated method that can distinguish between bathymetry and other signals and noise.

An anisotropic bouncing universe in non-local gravity

We show that it is possible to realize a cosmological bouncing solution in an anisotropic but homogeneous Bianchi-I background in a class of non-local, infinite derivative theories of gravity. We show that the anisotropic shear grows slower than in general relativity during the contraction phase, peaks to a finite value at the bounce point, and then decreases as the universe asymptotes towards isotropy and homogeneity, and ultimately to de Sitter. Along with a cosmological constant, the matter sector required to drive such a bounce is found to consist of three components — radiation, stiff matter and k-matter (whose energy density decays like the inverse square of the average scale factor). Generically, k-matter exerts anisotropic pressures. We will test the bouncing solution in local and non-local gravity and show that in the latter case it is possible to simultaneously satisfy positivity of energy density and, at least in the late time de Sitter phase, avoid the introduction of propagating ghost/tachyonic modes.

Highly efficient second-harmonic generation in an electro-optic chiral material using total-internal-reflection-based optical rotation quasi-phase-matching technique

The present work introduces the concept of a total-internal-reflection (TIR)-based optical rotation, quasi-phase-matching (QPM) technique for generating highly efficient second-harmonic optical output in a rectangular slab of magnesium oxide-doped lithium niobate crystal coated with a thin layer of yttrium oxide. Combinational effects of TIR optical rotation QPM and fractional QPM techniques are experienced at certain bounce points inside the slab, thereby enhancing the conversion efficiency. In this analysis, the thin-film is used for controlling the phase-shifts accompanying the propagation of p- and s-polarized light at the slab-film interface during TIR. A conversion efficiency of 32%, corresponding to a second-harmonic wavelength of 532 nm has been observed using computer-aided simulation. Optical losses such as surface roughness, absorption, and interference effect due to the nonlinear law of reflection have also been incorporated.