924 reads in the past 30 days
Life on a closed timelike curveDecember 2024
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959 Reads
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1 Citation
Published by IOP Publishing
Online ISSN: 1361-6382
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Print ISSN: 0264-9381
Disciplines: Astronomy & Astrophysics, Physics, Multidisciplinary, Physics, Particles & Fields, Quantum Science & Technology
924 reads in the past 30 days
Life on a closed timelike curveDecember 2024
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959 Reads
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1 Citation
67 reads in the past 30 days
White paper and roadmap for quantum gravity phenomenology in the multi-messenger eraJanuary 2025
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461 Reads
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29 Citations
The unification of quantum mechanics and general relativity has long been elusive. Only recently have empirical predictions of various possible theories of quantum gravity been put to test, where a clear signal of quantum properties of gravity is still missing. The dawn of multi-messenger high-energy astrophysics has been tremendously beneficial, as it allows us to study particles with much higher energies and travelling much longer distances than possible in terrestrial experiments, but more progress is needed on several fronts. A thorough appraisal of current strategies and experimental frameworks, regarding quantum gravity phenomenology, is provided here. Our aim is twofold: a description of tentative multimessenger explorations, plus a focus on future detection experiments. As the outlook of the network of researchers that formed through the COST Action CA18108 ‘Quantum gravity phenomenology in the multi-messenger approach (QG-MM)’, in this work we give an overview of the desiderata that future theoretical frameworks, observational facilities, and data-sharing policies should satisfy in order to advance the cause of quantum gravity phenomenology.
25 reads in the past 30 days
Pupil aberrations correction of the afocal telescope for the TianQin projectSeptember 2023
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453 Reads
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4 Citations
24 reads in the past 30 days
Observational constrained F(R, \mathcal{G}) gravity cosmological model and the dynamical system analysisOctober 2023
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172 Reads
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18 Citations
24 reads in the past 30 days
Free-falling motion of an elastic rigid rod towards a Schwarzschild black holeSeptember 2024
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43 Reads
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2 Citations
Classical and Quantum Gravity is an established journal for physicists, mathematicians and cosmologists in the fields of gravitation and the theory of spacetime. The journal is now the acknowledged world leader in classical relativity and all areas of quantum gravity.
February 2025
In this article we provide a more detailed account of the geometry and topology of the composite bundle formalism introduced by Tresguerres (2002 Phys. Rev. D 66 064025) to accommodate gravitation as a gauge theory. In the first half of the article we identify a global structure required by the composite construction which not only exposes how the ordinary frame and tangent bundle expected in general relativity arise but provides the link or connection between these ordinary bundles and the gauge bundles of the composite bundle construction. In the second half of the article we discuss implications of this method of constructing gravity as a fiber bundle for the global structure of spacetime. We find that the underlying manifold of the composite bundle construction is expected to admit, not only a spin structure but also a string structure. As a consequence of our work we are able to extend past work on global structures of physically reasonable spacetime manifolds. It has been shown that in four spacetime dimensions, that if an oriented, Lorentzian, four dimensional manifold is stably casual that it is parallizable, and hence admits a spin structure which allows for chiral spinors. We may now add to this that such a manifold also admits a string structure.
February 2025
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2 Reads
The Eisenhart lift of Riemannian type describes the motion of a particle as a geodesic in a higher-dimensional Riemannian manifold with one additional coordinate. It has recently been generalized to a scalar field system by introducing one additional vector field. We apply this approach to a scalar field system in the Friedmann–Lemaitre–Robertson–Walker universe and classify the symmetries of the system. In particular, for a scalar field potential consisting of the square of a combination of exponential functions with specific index e±64ϕ, we find nontrivial (conformal) Killing vector fields and Killing tensor fields. Moreover, for a potential written as an exponentiation of a combination of exponential potentials with general index, we find nontrivial conformal Killing vector fields. By introducing the coordinate along the conformal Killing vector field, we can solve the equations of motion completely. We also classify the symmetries of multiple scalar field system.
February 2025
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1 Read
Space gravitational wave telescopes are critical in achieving precise interstellar laser interferometry. The coupling coefficient is a key metric for evaluating the ultimate performance of a telescope. However, alignment errors during the assembly phase can degrade the wavefront quality of the telescope, intensify coupling noise, and impair overall performance. Currently, no alignment scheme specifically targets the coupling-noise coefficient of telescopes. To address this, this study proposes a sensitivity matrix model that relates misalignment to the coupling coefficient and establishes clear sampling criteria for coupling noise at the exit pupil of the telescope. The model incorporates second-order correction terms, enabling a more accurate characterisation of the relationship between misalignment and tilt-to-length coupling-noise coefficients. Based on this model, an alignment scheme was developed using the coupling coefficient as the evaluation metric. Owing to the significant differences in the magnitudes of sensitivity among different misalignments and their mutual coupling effects, the predicted alignment parameters often differ substantially from the actual values. To resolve this issue, a misalignment grouping strategy was proposed to reduce prediction errors. Additionally, an iterative algorithm and a sequential adjustment strategy for components were provided to ensure alignment effectiveness. Finally, the feasibility of the proposed method was verified using a typical space gravitational wave telescope model. Experimental results show that the method successfully aligned 500 randomly misaligned samples, with all samples satisfying the requirement of a coupling noise of less than 25 pm μrad⁻¹ after alignment. This method provides new guidance for the alignment of space gravitational wave telescope systems.
February 2025
Cezar Condeescu
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A Micu
In this paper we give an extensive description of Weyl quadratic gravity as the gauge theory of the Weyl group. The previously discovered (vectorial) torsion/non-metricity equivalence is shown to be built-in as it corresponds to a redefinition of the generators of the Weyl group. We present a generalisation of the torsion/non-metricity duality which includes, aside from the vector, also a traceless 3-tensor with two antisymmetric indices and vanishing skew symmetric part. A discussion of this relation in the case of minimally coupled matter fields is given. We further point out that a Rarita-Schwinger field can couple minimally to all the components of torsion and some components of non-metricity. Alongside we present the same gauge construction for the Poincaré and conformal groups. We show that even though the Weyl group is a subgroup of the conformal group, the gauge theory of the latter is actually only a special case of Weyl quadratic gravity.
February 2025
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5 Reads
The differential seismic motion between the internal seismic isolation platforms on the advanced laser interferometer gravitational wave observatory, affects the sensitivity of the detector at lower frequencies (below 1 Hz), because each platform moves independently. This induces noise inside the cavities of the auxiliary optics placed on the platforms, which translates into a higher control effort to maintain stability and resonance. This paper shows that the differential motion between the platforms can be efficiently measured by the capacitive position sensors installed on each platform. We investigate how we can use these sensors to modify the seismic control configuration and help reduce the differential motion between the platforms, reduce the control efforts and help maintain the cavities in resonance. Reduced differential motion is expected to reduce control noise thereby improving sensitivity and improve detector duty cycle by preventing actuator saturation, resulting in loss of optical cavity resonance.
February 2025
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4 Reads
We analyze cosmography as a tool to constrain modified gravity theories. We take four distinct models and obtain their parameters in terms of the cosmographic parameters favored by observational data of strong gravitational lensing. We contrast with the values obtained by direct comparison between each model and the observational data. In general, we find consistency between the two approaches at 2σ for all models considered in this work. Our study bridges the gap between theoretical predictions of modified gravity and empirical observations of strong gravitational lensing, providing a simple methodology to test the validity of these models.
February 2025
We analyze the motion of the spinning body (in the pole-dipole approximation) in the gravitational and electromagnetic fields described by the Mathisson-Papapetrou-Dixon-Souriau equations. First, we define a novel spin condition for the body with the magnetic dipole moment proportional to spin, which generalizes the one proposed by Ohashi-Kyrian-Semerak for gravity. As a result, we get the whole family of charged spinning particle models in the curved spacetime with remarkably simple dynamics (momentum and velocity are parallel). Applying the reparametrization procedure, for a specific dipole moment, we obtain equations of motion with constant mass and gyromagnetic factor. Next, we show that these equations follow from an effective Hamiltonian formalism, previously interpreted as a classical model of the charged Dirac particle.
February 2025
The Newtonian gravitational constant G is one of the most fundamental constants in nature. In the measurement of G with the angular acceleration feedback (AAF) method, the largest error comes from the distances between the geometric centers (GC) of the source masses. In the on-going experiment, the silicon spheres with a more homogeneous density are used as the source masses. Here a scheme of measuring the GC distances between the silicon spheres with the laser interferometer is proposed. The measurement principle is analyzed in detail, and the error sources, such as the laser, the sphere, the alignment of optical path, and the environment are evaluated in detail. With this method, the horizontal and vertical GC distances can be measured with uncertainties of 11 nm and 9 nm, respectively. The simulation with the ZEMAX software is performed to verify the theoretical model of measuring the GC distance, where the maximum deviation of the distance between the simulation result and the theoretical one is only -2.7 nm. When the sphericities of the four silicon spheres are at the level of 0.1 m, the uncertainty of each GC distance after considering the sphericity is about 0.1 m, corresponds to a combined uncertainty of 0.6 ppm for the G measurement with the AAF method. This provides an effective method to reduce the measurement uncertainty of GC distance, and makes it possible to measure G with a higher precision.
February 2025
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1 Read
We study the statistical fluctuations (such as the variance) of causal set quantities, with particular focus on the causal set action. To facilitate calculating such fluctuations, we develop tools to account for correlations between causal intervals with different cardinalities. We present a convenient decomposition of the fluctuations of the causal set action into contributions that depend on different kinds of correlations. This decomposition can be used in causal sets approximated by any spacetime manifold M. Our work paves the way for investigating a number of interesting discreteness effects, such as certain aspects of the Everpresent Λ cosmological model.
February 2025
Gravitational wave (GW) analysis is attracting widespread attention as an emerging research field. As the presence of substantial noise in GW signals, and the characteristics of inspiral and merger stage are different, coupled with the sidelobe effect caused by window length, traditional time-frequency analysis methods face significant challenges in accurately analyzing the frequency variations of GW signals. This poses a major limitation in the precise analysis stage following GW detection. Therefore, we proposed a novel method of Seasonal-Trend decomposition using Loess with Multilayer Perceptron (STLMLP), for predicting and validating the accuracy and effectiveness of GW frequency variations. Experiment results on three noiseless GW templates demonstrate that STLMLP exhibits the adaptability and highest prediction accuracy for the dynamic frequency variations of GW signals compared to five state-of-the-art machine learning and deep learning methods. Furthermore, experiments conducted on three noisy actual GW data compared with the state-of-the-art method of Fourier-based synchrosqueezing transform (FSST) in the signal processing domain confirm that STLMLP maintains lower error in predicting frequency change over the whole duration of the actual noisy GW signals.
February 2025
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2 Reads
Any given prescription of quantum time travel necessarily endows a Hilbert space to the chronology-violating (CV) system on the closed timelike curve (CTC). However, under the two foremost models, Deutsch’s prescription (D-CTCs) and postselected teleportation (P-CTCs), the CV system is treated very differently: D-CTCs assign a definite form to the state on this system, while P-CTCs do not. To further explore this distinction, we present a methodology by which an operational notion of state may be assigned to their respective CV systems. This is accomplished via a conjunction of state tomography and weak measurements, with the latter being essential in leaving any notions of self-consistency intact. With this technique, we are able to verify the predictions of D-CTCs and, perhaps more significantly, operationally assign a state to the system on the P-CTC. We show that, for any given combination of chronology-respecting input and unitary interaction, it is always possible to recover the unique state on the P-CTC, and we provide a few specific examples in the context of select archetypal temporal paradoxes. We also demonstrate how this state may be derived from analysis of the P-CTC prescription itself, and we explore how it compares to its counterpart in the CV state predicted by D-CTCs.
February 2025
We propose a simple modification of space-based gravitational wave (GW) detector optical benches which would enable the measurement of vacuum birefringence of light induced by axion dark matter through its coupling to electromagnetism. Specifically, we propose to change a half-wave plate by a circular polarizer. While marginally affecting the sensitivity to GW by a factor , we show that such an adjustment would make future detectors such as LISA, TianQin, Taiji and Big-Bang Observer the most sensitive experiments at low axion masses.
February 2025
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4 Reads
In general relativity, the motion of an extended test body is influenced by its proper rotation, or spin. We present a covariant and physically self-consistent Hamiltonian framework to study this motion, up to quadratic order in the body’s spin, including a spin-induced quadrupole, and in an arbitrary background spacetime. The choice of spin supplementary condition and degeneracies associated with local Lorentz invariance are treated rigorously with adapted tools from Hamiltonian mechanics. Applying the formalism to a background space- time described by the Kerr metric, we prove that the motion of any test compact object around a rotating black hole defines an integrable Hamiltonian system to linear order in the body’s spin. Moreover, this integrability still holds at quadratic order in spin when the compact object has the deformability expected for an isolated black hole. By exploiting the unique symmetries at play in black hole binaries, our analytical results clarify longstanding numerical conjectures regarding spin-induced chaos in the motion of asymmetric compact binaries, and may provide a powerful framework to improve current gravitational waveform modelling.
January 2025
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2 Reads
General relativistic Gauss equations for osculating elements for bound orbits under the influence of a perturbing force in an underlying Schwarzschild space-time have been derived in terms of Weierstrass elliptic functions. Thereby, the perturbation forces are restricted to act within the orbital plane only. These equations are analytically solved in linear approximation for several different perturbations such as cosmological constant perturbation, quantum correction to the Schwarzschild metric, and hybrid Schwarzschild/post-Newtonian 2.5 order self-force for binary systems in an effective one-body framework.
January 2025
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2 Reads
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1 Citation
We review the description of classical gravitational scatterings of two compact objects by means of the eikonal framework. This encodes via scattering amplitudes both the motion of the bodies and the gravitational-wave signals that such systems produce. As an application, we combine the next-to-leading post-Minkowskian waveform derived in the post-Newtonian PN limit with the 4PM static loss due to the linear memory effect to reproduce known results for the total angular momentum loss in the center-of-mass frame up to O(G4) and 2.5PN order. We also provide similar expressions for the change in the system’s mass dipole, discussing the subtleties related to its sensitivity to the Coulombic components of the field and to the nonlinear memory effect.
January 2025
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5 Reads
Gravitational interactions were studied by measuring the influence of small external field masses on a microwave resonator. It consisted of two spherical mirrors, which acted as independent pendulums individually suspended by strings. Two identical field masses were moved along the axis of the resonator symmetrically and periodically between a near and a far position. Their gravitational interaction altered the distance between the mirrors, changing the resonance frequency, which was measured and found consistent with Newton’s law of gravity. The acceleration of a single mirror caused by the two field masses at the closest position varied from 5.4⋅10−12 ms−2 to 259⋅10−12 ms−2.
January 2025
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4 Reads
It is well-known that the results by Bekenstein, Gibbons and Hawking on the thermodynamics of black holes can be reproduced quite simply in the Euclidean path integral approach to quantum gravity. The corresponding partition function is obtained semiclassically, ultimately requiring only the on-shell Einstein–Hilbert action with opportune asymptotic subtractions. We elaborate on the fact that the same expressions for the thermodynamical quantities can be obtained within teleparallel equivalent theories, based on either torsion or nonmetricity, by employing quasilocal relations. Notably, the bulk integrals of these theories do not vanish on-shell but rather result in boundary terms themselves. Asymptotic subtractions of the latter are able to cancel out the divergences, ultimately leading to Bekenstein–Gibbons–Hawking’s results. As a non-trivial cross-check, we compute the bulk integrals directly without reference to the boundary terms. While the result agrees with the previous method for the torsion-based teleparallel theory, it differs for the nonmetricity theory. Specifically, upon regularizing the bulk integral using a fiducial reference frame, we find that the semiclassical partition function vanishes. To address this problem, we propose a simple prescription for Schwarzschild black holes, which involves keeping the nonmetric connection arbitrary and imposing thermal equilibrium. Generalizations of the results to more general modified gravity theories with antisymmetric degrees of freedom are also discussed.
January 2025
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11 Reads
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1 Citation
The gravitational interaction, as described by the Einstein–Cartan theory, is shown to emerge as the by-product of the spontaneous symmetry breaking of a gauge symmetry in a pre-geometric four-dimensional spacetime. Starting from a formulation a` la Yang–Mills on an SO(1,4) or SO(3,2) principal bundle and not accounting for a spacetime metric, the Einstein–Hilbert action is recovered after the identification of the effective spacetime metric and spin connection for the residual SO(1,3) gauge symmetry of the spontaneously broken phase—i.e. the stabiliser of the SO(1,4) or SO(3,2) gauge group. Thus, the two fundamental tenets of general relativity, i.e. diffeomorphism invariance and the equivalence principle, can arise from a more fundamental gauge principle. The two mass parameters that characterise Einstein gravity, namely the Planck mass and the cosmological constant, are likewise shown to be emergent. The phase transition from the unbroken to the spontaneously broken phase is expected to happen close to the Planck temperature. This is conjectured to be dynamically driven by a scalar field that implements a Higgs mechanism, hence providing mass to new particles, with consequences for cosmology and high-energy physics. The couplings of gravity to matter are discussed after drawing up a dictionary that interconnects pre-geometric and effective geometric quantities. In the unbroken phase where the fundamental gauge symmetry is restored, the theory is potentially power-counting renormalisable without matter, offering a novel path towards a UV completion of Einstein gravity.
January 2025
Building upon earlier work, we explore the limits of using a configuration of satellites to measure the trace of the gravitational gradient tensor using intersatellite laser ranging and timing observables without relying on high-precision external observables such as deep space radio navigation or astrometry with unrealistic accuracy. A refined model, calculated with extended numerical precision, confirms that exceptional sensitivity is possible, placing within reach observational tests of certain modified gravity theories (e.g. Yukawa terms, Galileons) using heliocentric orbits in the vicinity of the Earth. The sensitivity of the experiment improves at larger heliocentric distances. A constellation placed at 30 astronomical units, still well within the domain of feasibility using available propulsion and deep space communication technologies, may approach sensitivities that are sufficient to detect not just the gravitational contribution of the interplanetary medium but perhaps even cosmological dark matter and dark energy constituents.
January 2025
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4 Reads
In the context of ghost-free infinite derivative gravity, we consider the single graviton exchange between two spinless particles, a spinless particle and a photon, or between a spinless particle and a spin-1/2 particle. To this end, we compute the gravitational potential for the three aforementioned cases and derive the O(G h-bar ² ) correction that arises at the linearised level. In the local theory, it is well-known that such a correction appears in the form of a Dirac delta function. Here, we show that this correction is smeared out for the nonlocal theory and, in contrast to the local theory, takes on non-zero values for a non-zero separation between the two particles. In the case of the single graviton exchange between a spinless particle and a spin-1/2 particle, we also compute the O(G h-bar) correction that arises in the non-static case within the non-relativistic approximation and show that it is finite in the nonlocal theory.
January 2025
We generalize the targeted B-statistic for continuous gravitational waves by modeling the h0 -prior as a half-Gaussian distribution with scale parameter H. This approach retains analytic tractability for two of the four amplitude marginalization integrals and recovers the standard B-statistic in the strong-signal limit (H → ∞). By Taylor-expanding the weak-signal regime (H → 0), the new prior enables fully analytic amplitude marginalization, resulting in a simple, explicit statistic that is as computationally efficient as the maximum-likelihood F-statistic, but significantly more robust. Numerical tests show that for day-long coherent searches, the weak-signal Bayes factor achieves sensitivities comparable to the F-statistic, though marginally lower than the standard B-statistic (and the Bero-Whelan approximation). In semi-coherent searches over short (compared to a day) segments, this approximation matches or outperforms the weighted dominant-response F ABw -statistic and returns to the sensitivity of the (weighted) F w -statistic for longer segments. Overall the new Bayes-factor approximation demonstrates state-of-the-art or improved sensitivity across a wide range of segment lengths we tested (from 900 s to 10 days).
January 2025
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2 Reads
Stationary, axisymmetric, dust sourced solutions of Einstein’s equations have been proposed as fully general relativistic models for disc galaxies. These models introduce a novel physical element, i.e., a non-negligible dragging vortex emerging from a full consideration of the essential self-interaction of matter and geometry in general relativity, which might demand a profound recalibration of the inferred amount of dark matter in disc galaxies. Within this framework, we identify the correct observables for redshift-inferred rotation curves of distant galaxies, correcting previously overlooked mistakes in the literature. We find that the presence of the dragging vortex introduces non-negligible corrective terms for the matter density required to maintain a stable physical system. We present the first estimate of the dragging speed which is required to explain a non-negligible fraction of dark matter in disc galaxies. In particular, we show that a sub-relativisitc dragging velocity of tens of kilometers per second in the neighbourhood of the Sun is sufficient to reduce the need of dark matter by 50\% in the Milky Way. Finally, we find that the presence of such a dragging vortex also returns a net contribution to the strong gravitational lensing generated by the galaxy. Thus, we show that the considered class of general relativistic galaxy models, is not only physcially viable, but suggests the need for recalibration of the estimated dark matter content in disc galaxies, with far reaching consequences for astrophysics and cosmology.
January 2025
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9 Reads
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4 Citations
Third generation ground-based gravitational wave (GW) detectors, such as Einstein Telescope and Cosmic Explorer, will operate in the (few−104) Hz frequency band, with a boost in sensitivity providing an unprecedented reach into primordial cosmology. Working concurrently with pulsar timing arrays in the nHz band, and LISA in the mHz band, these 3G detectors will be powerful probes of beyond the standard model particle physics on scales T≳107GeV. Here we focus on their ability to probe phase transitions (PTs) in the early Universe. We first overview the landscape of detectors across frequencies, discuss the relevance of astrophysical foregrounds, and provide convenient and up-to-date power-law integrated sensitivity curves for these detectors. We then present the constraints expected from GW observations on first order PTs and on topological defects (strings and domain walls), which may be formed when a symmetry is broken irrespective of the order of the phase transition. These constraints can then be applied to specific models leading to first order PTs and/or topological defects. In particular we discuss the implications for axion models, which solve the strong CP problem by introducing a spontaneously broken Peccei-Quinn (PQ) symmetry. For post-inflationary breaking, the PQ scale must lie in the 108−1011 GeV range, and so the signal from a first order PQ PT falls within reach of ground based 3G detectors. A scan in parameter space of signal-to-noise ratio in a representative model reveals their large potential to probe the nature of the PQ transition. Additionally, in heavy axion type models domain walls form, which can lead to a detectable GW background. We discuss their spectrum and summarise the expected constraints on these models from 3G detectors, together with SKA and LISA⁷7Invited review for a CQG Focus Issue on the science case for next generation ground based GW detectors. .
January 2025
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30 Reads
Geodesics in general relativity describe the behaviour of test particles in a gravitational field. In 5D Kaluza–Klein, geodesics reproduce the Lorentz force motion of particles in an electromagnetic field. This paper studies geodesic motion on a higher-dimensional M4×K with background metrics encoding general 4D gauge fields and Higgs-like scalars. It shows that the classical mass and charge of a test particle become variable quantities when the geodesic traverses regions of spacetime with massive gauge fields, such as the weak force field, or with non-constant Higgs scalars. This agrees with the physical fact that interactions mediated by massive bosons can change the mass and charge of particles. The variation rates of mass and charge along a geodesic are given by natural geometric formulae. In regions where mass is preserved, there are additional constants of motion, one for every abelian or simple summand in the Killing algebra of K. The last part of the paper discusses traditional difficulties of Kaluza–Klein models, such as the low q/m ratios in the 5D model. It suggests possible ways to circumvent them. It also remarks the naturalness of a model in which elementary particles always travel at the speed of light in higher dimensions.
January 2025
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3 Reads
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1 Citation
Despite the observation of nearly 100 compact binary coalescence (CBC) events up to the end of the Advanced gravitational-wave (GW) detectors’ third observing run (O3), there remain fundamental open questions regarding their astrophysical formation mechanisms and environments. Population analysis should yield insights into these questions, but requires careful control of uncertainties and biases. GW observations have a strong selection bias: this is due first to the dependence of the signal amplitude on the source’s (intrinsic and extrinsic) parameters, and second to the complicated nature of detector noise and of current detection methods. In this work, we introduce a new physically-motivated model of the sensitivity of GW searches for CBC events, aimed at enhancing the accuracy and efficiency of population reconstructions. In contrast to current methods which rely on re-weighting simulated signals (injections) via importance sampling, we model the probability of detection of binary black hole (BBH) mergers as a smooth, analytic function of source masses, orbit-aligned spins, and distance, fitted to accurately match injection results. The estimate can thus be used for population models whose signal distribution over parameter space differs significantly from the injection distribution. Our method has already been used in population studies such as reconstructing the BBH merger rate dependence on redshift.
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Executive Editor
The Australian National University, Australia
Executive Editor
Louisiana State University, USA
Executive Editor
Gran Sasso Science Institute, Italy