Ghost Modes Research Articles

Orientation-dependent fiber-optic accelerometer based on eccentric fiber Bragg grating.

A highly localized eccentric fiber Bragg grating (EFBG) accelerometer was proposed, and its orientation-dependent measurement results were demonstrated experimentally. An EFBG was inscribed point-by-point (PbP) in a single-mode fiber (SMF) using a femtosecond laser, and the cladding mode was recoupled to excite the ghost mode through an abrupt taper. Owing to the asymmetry caused by the lateral offset of the EFBG, the ghost mode showed a significant directional response to acceleration. Furthermore, monitoring the fundamental core mode resonance can help calibrate accidental power perturbation or cross-sensitivity.

A Broad Bandwidth Metamaterial Pillbox Window for W-Band Traveling-Wave Tubes

A low voltage standing wave ratio (VSWR) broad bandwidth pillbox window with metamaterial is proposed for W-band traveling-wave tubes (TWTs) in this paper. The metamaterial structure is designed on a dielectric surface, which decreases the VSWR and broadens the bandwidth of the pillbox window. The cold-test results of the window with metamaterial are presented, which showed that the metamaterial pillbox window (MPW) achieves a bandwidth of 31.5 GHz with VSWR below 1.2, and a bandwidth of 18 GHz with VSWR below 1.1. The ghost mode and the power capacity of the window were also investigated. The results showed that the proposed metamaterial pillbox window is suitable for medium power TWTs. A Ka-band MPW was fabricated and the test results agreed well with the simulation results.

Borehole Sonic Data Dispersion Analysis With a Modified Differential-Phase Semblance Method

Ruijia Wang, Richard Coates, and Jiajun Zhao The sonic wave fields produced by wireline and logging-while-drilling (LWD) monopole, dipole, and quadrupole tools often consist of multiple borehole modes. Classic frequency-slowness semblance-map methods used to process this data often detect only strongly excited modes and overlook weak ones, and erroneously detect some modes. Conventional dispersion processing methods can be separated into two groups: single-mode and multimode extraction algorithms. Single-mode methods are stable but only return one mode, the most energetic one, at each frequency. Single-mode methods include the differential-phase frequency-semblance (DPFS) method and the weighted spectral-semblance method. Multimode methods can return multiple modes at each frequency but may be unstable in some cases. Due to their assumptions about signal models, multimode methods are often sensitive to unbalanced receiver arrays, poor data quality, and formation heterogeneity. For example, in some extreme cases, such as a formation with strong heterogeneity, multimode methods may yield erroneous ghost modes or discontinuous dispersion curves for each mode. Borehole modes with different slowness have different arrival times. Converting the data to the frequency domain can obscure this critical information or encode these time differences into phase differences between adjacent frequencies. Conventional frequency-semblance approaches, which use only a single frequency independently from adjacent ones, ignore this phase information. In this paper, we propose employing the phase differences between adjacent frequencies to facilitate multimode dispersion analysis. We modify one conventional method to incorporate the arrival time of modes or the phase difference between adjacent frequencies. We validate the proposed approach with synthetic, laboratory, and field data. The results suggest the method can extract a much more comprehensive representation of modes present in the sonic data. Additionally, the method provides reliable estimates, even when the number of receivers is small. Unlike the Prony and matrix-pencil methods based on assumed signal models, the proposed approach, which we denote “Modified Differential-Phase Frequency Semblance” (MDPFS), is a modification of the single-mode differential phase approach. The MDPFS is still a semblance-based approach, and as with other semblance-based processing, it is expected to be less sensitive to unbalanced receiver arrays, poor data quality, and formation heterogeneity than other multimode algorithms.

Non-Perturbative Propagators in Quantum Gravity

We employ non-perturbative renormalisation group methods to compute the full momentum dependence of propagators in quantum gravity in general dimensions. We disentangle all different graviton and Faddeev–Popov ghost modes and find qualitative differences in the momentum dependence of their propagators. This allows us to reconstruct the form factors that are quadratic in curvature from first principles, which enter physical observables like scattering cross sections. The results are qualitatively stable under variations of the gauge fixing choice.

The manufacturing process and spectral features of tilted fiber Bragg gratings

The controllable cross-sensitivities of tilted fiber Bragg gratings (TFBGs) are strongly related to the control of the parameters during the manufacturing process. The TFBG spectrum characterization, which can demonstrate cross-sensitivities, is controlled by the process parameters. Cross-sensitivities analyses in biochemical surrounding refractive index (SRI) (cladding mode) and mechanical sensing (Bragg and ghost mode) are evaluated through processing parameters, including the rotating tilted angles of phase-mask, laser exposure dose time, and reducing fiber diameters. The results of this study show the cross-sensing ability in both biochemical and mechanical sensing that can be caused by a controlled tilted angle was 10° and exposure dose of five minutes while obtaining the cladding and Bragg modes of TFBGs with broad spectral coverage.

Anisotropic instability in a higher order gravity theory

We study a metric cubic gravity theory considering odd-parity modes of linear inhomogeneous perturbations on a spatially homogeneous Bianchi type I manifold close to the isotropic de Sitter spacetime. We show that in the regime of small anisotropy, the theory possesses new degrees of freedom compared to General Relativity, whose kinetic energy vanishes in the limit of exact isotropy. From the mass dispersion relation we show that such theory always possesses at least one ghost mode as well as a very short-time-scale (compared to the Hubble time) classical tachyonic (or ghost-tachyonic) instability. In order to confirm our analytic analysis, we also solve the equations of motion numerically and we find that this instability is developed well before a single e-fold of the scale factor. This shows that this gravity theory, as it is, cannot be used to construct viable cosmological models.

Plasmon resonances and tachyon ghost modes in highly conducting sheets

Plasmon-polariton modes in two-dimensional electron gases have a dual field-matter nature that endows them with unusual properties when electrical conductivity exceeds a certain threshold set by the speed of light. In this regime plasmons display an interesting relation with tachyons, the hypothetical faster-than-light particles. While not directly observable, tachyons directly impact properties of plasmon modes. Namely, in the ``tachyon'' regime, plasmon resonances remain sharp even when the carrier collision rate $\gamma$ exceeds plasmon resonance frequency. Resonances feature a recurrent behavior as $\gamma$ increases, first broadening and then narrowing and acquiring asymmetric non-Lorentzian lineshapes with power-law tails extending into the tachyon continuum $\omega>ck$. This unusual behavior can be linked to the properties of tachyon poles located beneath $\omega>ck$ branch cuts in the complex $\omega$ plane: as $\gamma$ grows, tachyon poles approach the light cone and hybridize with plasmons. Narrow resonances persisting for $\gamma>\omega$, along with the unusual density and temperature dependence of resonance frequencies, provide clear signatures of the tachyon regime.

The ideal hydrodynamic limit and non-Abelian gauge symmetries

We show that the ideal fluid local equilibrium limit, defined as the existence of a flow frame which characterises the direction of both a conserved entropy current and conserved charge currents is incompatible with non-Abelian gauge theory if local color charge density is non-zero. Instead, the equation of state becomes dependent on via modes which are roughly equivalent to ghost modes in the hydrodynamic limit. These modes can be physically imagined as a field of “purcell swimmers” whose “arms and legs” are outstretched in Gauge space. Also, vorticity should couple to the Wilson loop via the chromo-electro-magnetic field tensor, which in local equilibrium is not a “force” but instead represents the polarization tensor of the gluons. We show that because of this coupling vorticity also acquires swirling non-hydrodynamic modes. We then argue that these swirling and swimming non-hydrodynamic modes are the manifestation of gauge redundancy within local equilibrium, and speculate on their role in quark-gluon plasma thermalization.

Gravitational leptogenesis with kination and gravitational reheating

We revisit the gravitational leptogenesis scenario in which the inflaton is coupled to gravity by the Chern-Simons term and the lepton asymmetry is generated through the gravitational anomaly in the lepton number current during inflation. We constrain the possible model parameter space by requiring the absence of ghost modes below the Planck scale, which would suggest the breakdown of the effective theory, and evaluate the net baryon asymmetry for various reheating processes. We find that the mechanism with these requirements is insufficient to explain the observed baryon asymmetry of the Universe for standard reheating scenarios. We show that, however, with the kination scenario realized in e.g. the k- and G-inflation models, a sufficient baryon asymmetry can be generated within a feasible range of the model parameters.

Accelerating universe with decreasing gravitational constant

We study the cosmology of dark energy in an extended theory of gravity in Lyra’s geometry. By analyzing a possible phenomenological interaction between the matter field with a constant equation-of-state parameter ωm (0≤ωm<1) and the geometric displacement field in Lyra’s geometry, we find that the displacement vector field engenders an effective time-dependent gravitational constant Geff constrained in the region 0<Geff(t)≤G. We show that the effective equation-of-state parameter ωeff(t) evolves in the same way as the effective time-dependent gravitational constant which, by decreasing with time, can give rise to the late-time cosmic acceleration with ω=-1 crossing in a flat Robertson-Walker background without adding dynamical ghost mode.

Cosmology in a model with Lagrange multiplier, Gauss–Bonnet and nonminimal kinetic couplings

A scalar-tensor model with Gauss–Bonnet and nonminimal kinetic couplings is considered, in which ghost modes are eliminated via a Lagrange multiplier constraint. A reconstruction procedure is devised for the scalar potential and Lagrange multiplier, valid for any given cosmological scenario. In particular, inflationary and dark energy cosmologies of different types (power-law, Little-Rip, de Sitter, quasi-de Sitter) are reconstructed in such models. It is shown that, for various choices of the kinetic coupling terms, it is possible to obtain a viable inflationary phenomenology compatible with the most accurate values of the observational indices.

The effects of intrinsic dynamical ghost modes in discrete-time Langevin simulations

Using the GJF-2GJ Langevin thermostat, which can produce time-step-independent statistical measures even for large time steps, we analyze and discuss the causes for abrupt deviations in statistical data as the time step is increased for some simulations of nonlinear oscillators. Exemplified by the pendulum, we identify a couple of discrete-time dynamical modes in the purely damped pendulum equation as the cause of the observed discrepancies in statistics. The existence, stability and kinetics of the modes are consistent with the acquired velocity distribution functions from Langevin simulations, and we conclude that the simulation deviations from physical expectations are not due to normal, systematic algorithmic time-step errors, but instead due to the inherent properties of discrete time in nonlinear dynamics.

Cosmological instabilities and the role of matter interactions in dynamical dark energy models

We consider cosmological models with a dynamical dark energy field, and study the presence of three types of commonly found instabilities, namely ghost (when fields have negative kinetic energy), gradient (negative momentum squared) and tachyon (negative mass squared). In particular, we study the linear scalar perturbations of theories with two interacting scalar fields as a proxy for a dark energy and matter fields, and explicitly show how canonical transformations relate these three types of instabilities with each other. We generically show that low-energy ghosts are equivalent to tachyonic instabilities, and that high-energy ghosts are equivalent to gradient instabilities. Via examples we make evident the fact that whenever one of these fields exhibits an instability then the entire physical system becomes unstable, with an unbounded Hamiltonian. Finally, we discuss the role of interactions between the two fields, and show that whereas most of the time interactions will not determine whether an instability is present or not, they may affect the timescale of the instability. We also find exceptional cases in which the two fields are ghosts and hence the physical system is seemingly unstable, but the presence of interactions actually lead to stable solutions. These results are very important for assessing the viability of dark energy models that may exhibit ghost, gradient or tachyonic modes.

The large D membrane paradigm for general four-derivative theory of gravity with a cosmological constant

We find the membrane equations which describe the leading order in 1/D dynamics of black holes in the D → ∞ limit for the most general four-derivative theory of gravity in the presence of a cosmological constant. We work up to linear order in the parameter determining the strength of the four-derivative corrections to the gravity action and hence there are no ghost modes in the theory. We find that the effective membrane equations we obtain are the covariant version of the membrane equations in absence of the cosmological constant. We also find the world-volume stress tensor for the membrane whose conservation gives the membrane equations. We apply the membrane equations to predict the light quasi-normal mode spectrum of black holes and black branes in the theory of gravity under consideration.

Hamiltonian analysis of mimetic scalar gravity revisited

We perform the Hamiltonian analysis of several mimetic gravity models and compare our results with those obtained previously by different authors. We verify that for healthy mimetic scalar-tensor theories the condition for the corresponding part of the Hamiltonian to be bounded from below is the positive value of the mimetic field energy density $\lambda$. We show that for mimetic dark matter possessing a shift symmetry the mimetic energy density remains positive in time, provided appropriate boundary conditions are imposed on its initial value, while in models without shift symmetry the positive energy density can be maintained by simply replacing $\lambda\to e^{\lambda}$. The same result also applies to mimetic $f(R)$ gravity, which is healthy if the usual stability conditions of the standard $f(R)$ gravity are assumed and $\lambda>0$. In contrast, if we add mimetic matter to an unhealthy seed action, the resulting mimetic gravity theory remains, in general, unstable. As an example, we consider a scalar-tensor theory with the higher-derivative term $(\Box \varphi)^2$, which contains an Ostrogradski ghost. We also revisit results regarding stability issues of linear perturbations around the FLRW background of the mimetic dark matter in the presence of ordinary scalar matter. We find that the presence of conventional matter does not revive dynamical ghost modes (at least in the UV limit). The modes, whose Hamiltonian is not positive definite, are non-propagating (have zero sound speed) and are associated with the mimetic matter itself. They are already present in the case in which the ordinary scalar fluid is absent, causing a growth of dust overdensity.

Ghost-free Gauss-Bonnet theories of gravity

In this work we develop a theoretical framework for Gauss-Bonnet modified gravity theories, in which ghost modes can be eliminated at the equations of motion level. Particularly, after we present how the ghosts can occur at the level of equations of motion, we employ the Lagrange multipliers technique, and by means of constraints we are able to eliminate the ghost modes from Gauss-Bonnet theories of the form $f(\mathcal{G})$ and $F(R,\mathcal{G})$ types. Some cosmological realizations in the context of the ghost free $f(\mathcal{G})$ gravity are presented, by using the reconstruction technique we developed. Finally, we explore the modifications to the Newton law of gravity generated by the ghost-free $f(\mathcal{G})$ theory.

Exciting the scalar ghost mode through time evolution

A nonlocal gravity model based on $\frac1{\square} R$ achieves the phenomenological goals of generating cosmic acceleration without dark energy and of suppressing the growth of perturbations compared to the $\Lambda$CDM model. Although the localized version of this model possesses a scalar ghost, the nonlocal version does not suffer from any obvious problem with ghosts. Here we study the possibility that the scalar ghost mode might be excited through time evolution, even though it is initially absent. We present strong evidence that this does not happen.

A Broadband Millimeter-Wave Waveguide Window: A Low-Cost Design for Environmental Protection

In this paper, a full W-band waveguide window for the protection of components from the environment is presented. The window is based on rectangular dielectric block placed inside the rectangular waveguide. Electrical design steps for broadband performance are presented, and a simple cost effective mechanical process flow is employed for the fabrication of windows. Four different dielectric windows are evaluated, and they exhibited less than 0.3-dB insertion loss and better than 20-dB return loss: two windows over full W-band, whereas other two over 3-GHz bandwidth. Temperature testing is carried out for 4 h at −40 °C and +70 °C, and the humidity level is set at 95% at +70 °C. Minimum insertion loss variation of ±0.5 dB over the temperature range is observed. To check the compatibility of the windows with millimeter-wave components, a W-band switch is tested with the windows at its input and output ports. The experimental results show that the windows have low-loss characteristics and are free from ghost modes.

Higgs mechanism in nonlocal field theories

We study spontaneous gauge symmetry breaking and the Higgs mechanism in nonlocal field theories. Motivated by the level truncated action of string field theory, we consider a class of nonlocal field theories with an exponential factor of the d’Alembertian attached to the kinetic and mass terms. Modifications of this kind are known to make mild the UV behavior of loop diagrams and thus have been studied not only in the context of string theory but also as an alternative approach to quantum gravity. In this paper we argue that such a nonlocal theory potentially includes a ghost mode near the nonlocal scale in the particle spectrum of the symmetry broken phase. This is in sharp contrast to local field theories and would be an obstruction to making a simple nonlocal model a UV complete theory. We then discuss a possible way out by studying nonlocal theories with extra symmetries such as gauge symmetries in higher spacetime dimensions.

Ghostbusters in f (R) supergravity

f (R) supergravity is known to contain a ghost mode associated with higher-derivative terms if it contains Rn with n greater than two. We remove the ghost in f (R) supergravity by introducing auxiliary gauge field to absorb the ghost. We dub this method as the ghostbuster mechanism [1]. We show that the mechanism removes the ghost super-multiplet but also terms including Rn with n ≥ 3, after integrating out auxiliary degrees of freedom. For pure supergravity case, there appears an instability in the resultant scalar potential. We then show that the instability of the scalar potential can be cured by introducing matter couplings in such a way that the system has a stable potential.