Macroscopic Perturbations Research Articles

Einstein, tea leaves, meandering rivers, and the origin of supermassive black holes in ekpyrotic universe

Central supermassive black holes are found in most massive galaxies. However, their origin is still poorly understood. Observations of quasars show that many supermassive black holes existed less than 700 million years after the Big Bang. To explain the existence of such black holes with masses comparable to the stellar mass of the host galaxy, just 500 million years after the Big Bang, it is probably necessary to assume that they originated from heavy seeds. In an ekpyrotic universe, a hot Big Bang occurs as a result of the collision of two branes. Quantum fluctuations create ripples on the brane surfaces, leading to spatial variations in the timing of collisions, thereby creating density perturbations that can facilitate the formation of massive black hole seeds. I hypothesize that perhaps Rayleigh–Bénard–Marangoni-type convection in the extra dimension is a more efficient source of macroscopic density perturbations than quantum fluctuations.

Peculiarities of polarised light scattering in nematic liquid crystals with large-scale director perturbations

ABSTRACT We examine light scattering in a birefringent material, namely uniaxial nematic liquid crystal possessing macroscopic orientation perturbations (variations of director in planar alignment). Our theoretical analysis and direct experiments proved that the polarisation state of the scattered speckle-field is uniform and elliptical (in a common case). The polarisation ellipse azimuth of scattered waves is mirror reflected to one of the directly transmitted waves relative to the optical axis.

Fast dynamics of radiofrequency emission in FTU plasmas with runaway electrons

Radiofrequency emission in the 0.4–3 GHz range from FTU tokamak in presence of runaway electrons (REs) has been measured in various plasma regimes. Rapid emission bursts associated with enhanced RE pitch-angle scattering reveal kinetic instabilities affecting evolution of the RE population from the buildup phase. Such measurements also provide a sensitive monitor for instabilities during early RE formation. The leading edge of radio bursts is much shorter than interleaving periods of low emission; spectral broadening during growth indicates nonlinear wave coupling as an explanation for the observed intermittency. Both broadband and coherent spectra have been observed. Radio emission disappears at the beginning of post-disruption RE plateaus, and subsequently reappears in the shape of very intense bursts accompanied by macroscopic magnetic perturbations.

Isochoric conditions enhance stability of metastable supercooled water

Supercooling has recently emerged as a highly promising, multi-scale technique for low-temperature preservation of organs and tissues, preventing damaging ice formation while requiring relatively low doses of added cryoprotectants. However, current supercooling techniques are not thermodynamically stable; mild agitations can cause rapid and destructive ice formation throughout the system, rendering them unsuitable for transportation and sharply limiting applicability outside the controlled laboratory environment. In this experimental study, we report a simple thermodynamic alteration to standard supercooling protocols, the use of constant-volume (isochoric) conditions, which substantially increases the stability of the system in the face of various macroscopic perturbations, including drop-impact, vibration, ultrasonication, and thermal fluctuation. We identify this effect as driven by a possible combination of thermodynamic and kinetic factors, including reduction of microscopic density fluctuations, elimination of the air–water interface, and significant resistance to cavitation.

Nanorheology of Polymer Solutions: A Scaling Theory

The current theory of viscoelasticity of polymer fluids deals with their response to time-dependent macroscopic perturbations on arbitrary time scales. We use scaling methods to calculate the elast...

Blazar Jets Perturbed by Magneto-gravitational Stresses in Supermassive Binaries

Abstract We study particle acceleration and radiative processes in blazar jets under recurring conditions set by gravitational perturbations in supermassive binary systems. We consider the action from a companion orbiting a primary black hole of , and perturbing its relativistic jet. We discuss how such conditions induce repetitive magneto-hydrodynamic stresses along the jet, and affect its inner electron acceleration and radiative processes. Specifically, we study how macroscopic perturbations related to increased jet “magnetization” end up in higher radiative outputs in the optical, X-ray, and gamma-ray bands. We first find an increase in magnetic field strength, as gauged in the optical band from the synchrotron emission of electrons accelerated in kinetic processes stimulated by reconnecting magnetic lines. The energetic electrons then proceed to up-scatter the synchrotron photons to GeV energies after the canonical synchrotron self-Compton radiation process. Our model implies a specific recurring pattern in the optical to gamma-ray emissions, made of high peaks and wide troughs. Progressing accelerations caused by spreading reconnections will produce an additional synchrotron keV component. Such outbursts provide a diagnostics for enhanced acceleration of electrons, which can up-scatter photons into the TeV range. We discuss how our model applies to the BL Lac object PG 1553+113, arguably the best candidate to now for high amplitude, recurring modulations in its gamma-ray emissions. We also consider other BL Lacs showing correlated keV–TeV radiations such as Mrk 421.

An outlook review: Mechanochromic materials and their potential for biological and healthcare applications

Macroscopic mechanical perturbations have been observed to result in optical changes for certain compounds and composite materials. This phenomenon could originate from chemical and physical changes across various length scales, from the rearrangement of chemical bonds to alteration of molecular domains on the order of several hundred nanometers. This review classifies the mechanisms and surveys of how each class of mechanochromic materials has been, and can potentially be applied in biological and healthcare innovations. The study of cellular and molecular responses to mechanical forces in biological systems is an emerging field; there is potential in applying mechanochromic principles and material systems for probing biological systems. On the other hand, application of mechanochromic materials for medical and healthcare consumer products has been described in a wide variety of concepts and inventions. It is hopeful that further understanding of mechanochromism and material innovations would initiate concrete, impactful studies in biological systems soon.

Beam quality and dose perturbation of 6 MV flattening-filter-free linac

The aim of this study is twofold: (a) determination of the spectral differences for flattening-filter-free (FFF) versus standard (STD) linac under various clinical conditions, (b) based on an extensive list of clinically important beam configurations, identification of clinical scenarios that lead to higher macroscopic dose perturbations due to the presence of high-Z material. The focus is on dose enhancement due to contrast agents including high-Z elements such as gold or gadolinium.EGSnrc was used to simulate clinical beams under various irradiation conditions: open/IMRT/spit-IMRT fields, in/out-off-field areas, different depths and field sizes. Spectra were calculated and analyzed for about 80 beams and for a total of 480 regions. Quantitative differential effects in beam quality were characterized using energy-dependent and cumulative dose perturbation metrics.Analysis of the spectral database showed that even though the general trends for both linacs (FFF/STD) were the same, there were crucial differences. In general, the relative changes between different conditions were smaller for FFF spectra. This was because of the higher component of low-energy photons of the FFF linac, which already lead to higher dose enhancement than for the STD linac (photon energies were more “uniformly” distributed for FFF spectra and henceforth their perturbation resulted in lesser relative changes). For out-of-field FFF spectra and split-IMRT fields the strongest enhancement were observed (∼25 and ∼5 respectively). Different spectral scenarios lead to different dose enhancements, however, they scale with the higher effective-Z of the materials and were directly related to the lower range of the spectra (<200 keV).

Compressed Beamforming in Ultrasound Imaging

Emerging sonography techniques often require increasing the number of transducer elements involved in the imaging process. Consequently, larger amounts of data must be acquired and processed. The significant growth in the amounts of data affects both machinery size and power consumption. Within the classical sampling framework, state of the art systems reduce processing rates by exploiting the bandpass bandwidth of the detected signals. It has been recently shown, that a much more significant sample-rate reduction may be obtained, by treating ultrasound signals within the Finite Rate of Innovation framework. These ideas follow the spirit of Xampling, which combines classic methods from sampling theory with recent developments in Compressed Sensing. Applying such low-rate sampling schemes to individual transducer elements, which detect energy reflected from biological tissues, is limited by the noisy nature of the signals. This often results in erroneous parameter extraction, bringing forward the need to enhance the SNR of the low-rate samples. In our work, we achieve SNR enhancement, by beamforming the sub-Nyquist samples obtained from multiple elements. We refer to this process as "compressed beamforming". Applying it to cardiac ultrasound data, we successfully image macroscopic perturbations, while achieving a nearly eight-fold reduction in sample-rate, compared to standard techniques.

Correction of gradient echo images for first and second order macroscopic signal dephasing using phase derivative mapping

Gradient echo techniques are often hampered by signal dephasing due to macroscopic phase perturbations because of system imperfections (shimming) or object induced perturbations of the magnetic field (hemorrhagic lesions, calcified tissue, air-tissue interfaces). Many techniques have been proposed to reduce the effects of macroscopic phase variations. Among these techniques are tuned pulse sequences, fitting techniques and reconstruction algorithms. These methods, however, suffer from one or more of the following drawbacks: they require longer acquisition times, require additional acquisitions, compensate only locally, can only be applied to multi-gradient echo data or may result in inaccurate results. In this work a generally applicable post-processing technique is presented to evaluate and compensate signal alterations invoked by first and second order macroscopic phase incoherences. In this technique, the derivatives of the signal phase are determined by applying the Fourier derivative theorem on the complex data. As a result, the phase derivatives are obtained without phase unwrapping and without compromising the resolution. The method is validated for single and multi-echo acquisitions by experiments on a co-axial cylinder phantom with known macroscopic field disturbances. The potential of the method is demonstrated on a multi-gradient echo acquisition on the head of a human volunteer. In general a first order correction is shown to be sufficient, however higher order correction is found to be beneficial near sharp transitions of the magnetic field.

Models for Sub-Alfvénic Magnetodynamics of Fusion Plasmas

The models describing macroscopic magnetic perturbations that evolve slowly compared to the Alfvén velocity are reviewed. The perturbations of interest include tearing modes, resistive interchange and ballooning modes, internal kink modes, resistive wall modes, and resonant magnetic perturbations. Two important features that distinguish the various models are their descriptions of parallel dynamics and of ion gyration. The evolution of macroscopic modes is generally characterized by resonances that result in the development of small scales. For processes involving magnetic reconnection, for example, all scales from the ion down to the electron Larmor radius are generated nonlinearly. The magnetohydrodynamic model assumes that the gradient lengths are always greater than the ion Larmor radius and thus is unable to properly describe the resonances. The drift models rely on a much more detailed description of the motion that enables them to capture many of the features of the short-scale phenomena, but they remain limited by their local description of the effects of gyration, and by their inability to describe the effects of wave-particle interactions in the parallel dynamics. These limitations are remedied by the gyrokinetic model, which provides a consistent, first-principles description of all the dynamics below the ion cyclotron frequency, but this model is computationally costly and its range of practical applicability remains to be established. Lastly, the gyrofluid models constitute a family of closures based on the moments of the gyrokinetic equations. These models offer an attractive compromise between fidelity and computational cost but have only recently begun to be applied to macroscopic evolution.

Phase gradient mapping as an aid in the analysis of object-induced and system-related phase perturbations in MRI

In this note we wish to demonstrate the utility of phase gradient mapping (PGM) as an aid in the analysis and characterization of object-induced and system-related macroscopic phase perturbations in MRI. To achieve this goal, phase gradient maps and, if applicable, field gradient maps were derived from standard phase images via a forward difference operator taking into account phase wraps. By way of phantom experiments, PGM was shown to provide reliable phase and field gradient information, even in regions with multiple phase wraps. Phase gradient mapping was further shown to allow positive identification of local phase and field perturbations and global discrimination between positive and negative local susceptibility deviations. The suitability of PGM for in vivo studies was demonstrated by a 3D brain examination of a healthy volunteer.

Three-dimensional xenon dendrites: Characterization of sidebranch growth

In our experiments, we investigate in situ the growth of three-dimensional xenon crystals into an undercooled pure melt. Experimental studies have been extended from undisturbed growth conditions to more realistic growth conditions. Methods to characterize the transient growth of sidebranches of dendrites have been developed. Two types of sidebranches have been identified: Sidebranches initiated by selective amplification of thermal noise (type N) and sidebranches induced by macroscopic perturbations (type P). Type N sidebranches start to grow 3-7 tip radii behind the tip and are not correlated at the four fins. It has been verified that the sidebranch amplitude grows exponentially to z(2/5) as predicted by Brener and Temkin [E. Brener and D. Temkin, Phys. Rev. E 51, 351 (1995)]. Type P sidebranches are initiated by macroscopic perturbations. They start to grow at the tip and their amplitudes are significantly higher than the ones of type N sidebranches. The growth of type P sidebranches is symmetric at the four fins. The tip positions of type P sidebranches separate from the tip shape of a dendrite at a distance of about 5R from the tip, while for type N sidebranches this distance is about 10R.

Robust and irreversible development in cell society as a general consequence of intra–inter dynamics

A dynamical systems scenario for developmental cell biology is proposed, based on numerical studies of a system with interacting units with internal dynamics and reproduction. Diversification, formation of discrete and recursive types, and rules for differentiation are found as a natural consequence of such a system. “Stem cells” that either proliferate or differentiate to different types stochastically are found to appear when intra-cellular dynamics are chaotic. Robustness of the developmental process against microscopic and macroscopic perturbations is shown to be a natural consequence of such intra–inter dynamics, while irreversibility in developmental process is discussed in terms of the gain of stability, loss of diversity and chaotic instability.

Effects of Shocks on Emission from the Central Engines of Active Galactic Nuclei. I.

In this paper we show that perturbations of the accretion flow within the central engines of some active galactic nuclei (AGNS) are likely to form shock waves in the accreting plasma. Such shocks, which may be either collisional or collisionless, can contribute to the observed high-energy temporal and spectral variability. Our rationale is the following: Observations show that the continuum emission probably originates in an optically thin, hot plasma in the AGN central engine. The flux and spectrum from this hot plasma varies significantly over light crossing timescales. Several authors have suggested that macroscopic perturbations contained within this plasma are the sources of this variability. In order to produce the observed emission the perturbations must be radiatively coupled with the optically thin hot matter and must also move with high velocities. We suggest that shocks, which can be very effective in randomizing the bulk motion of the perturbations, are responsible for this coupling. Shocks should form in the central engine, because the temperatures and magnetic fields are probably reduced below their virial values by radiative dissipation. Perturbations moving at Keplerian speeds, or strong non-linear excitations, result in supersonic and super-Alfvenic velocities leading to shock waves within the hot plasma. We show that even a perturbation smaller than the emitting region can form a shock that significantly modifies the continuum emission in an AGN, and that the spectral and temporal variability from such a shock generally resembles those of radio-quiet AGNS. As an example, the shock inducing perturbation in our model is a small main-sequence star, the capturing and eventual accretion of which are known to be a plausible process. We argue that shocks in the central engine may also provide a natural triggering mechanism for the cold component of Guilbert & Rees two-phase medium and an efficient mecha- nism for angular momentum transfer. Current and future missions, such as ASCA, XTE, XMM, AXAF, and ASTRO-E may determine the importance of shock-related emission from the central engines of AGNS.

Inflation and squeezed quantum states

The inflationary cosmology is analyzed from the point of view of squeezed quantum states. As noted by Grishchuk and Sidorov, the amplification of quantum fluctuations into macroscopic perturbations which occurs during cosmic inflation is a process of quantum squeezing. We carefully develop the squeezed state formalism and derive the equations that govern the evolution of a gaussian initial state. We derive the power spectrum of density perturbations for a simple inflationary model and discuss its features. We conclude that the squeezed state formalism provides an interesting framework within which to study the amplification process, but,in disagreement with the claims of Grishchuk and Sidorov, that it does {\em not} provide us with any new physical results.

Nonlinear stability analysis of miscible displacements

A simulator for three-dimensional horizontal miscible displacements in porous media is developed. Using this simulator, we examine the initiation and development of instabilities, viscous fingers and gravity tongues. With the only perturbations to the system being truncation and round-off errors, a density ratio (the ratio of the density of the displacing fluid to that of the displaced fluid) different from one is responsible for the initiation of the instabilities, and an unfavorable mobility ratio (the ratio of the viscosity of the displaced fluid to that of the displacing fluid) is responsible for the growth of the instabilities. When macroscopic perturbations attributable to initially imperfect contact between the displaced fluid and the displacing fluid at the injection boundary are introduced, an unfavorable mobility ratio is again responsible for the growth of instabilities. Macroscopic perturbations play the same role as a density ratio different from one in initiation of the instabilities, although the manner in which instabilities develop is different.

Phase-mixing by the guiding-centre drifts of charged particles in a plasma

Thermal dispersion of the guiding-centre drifts in a plasma leads to phase-mixing and kinetic damping of macroscopic plasma perturbations. A simple illustration is given by the drifts of a low-density gas of charged particles in an inhomogeneous magnetic field. This results in a substantial kinetic damping on a time scale only slightly longer than that of the Larmor period of gyration. The phase-mixing due to the velocity-dependent magnetic gradient drift is also likely to become one of the important kinetic mechanisms in more complicated situations such as those of a dense, non-dissipative, high-beta plasma. In particular, this should apply to the dynamics and stability of macroscopic plasma perturbations in high-beta systems where there are strong magnetic field inhomogeneities.

Large Larmor radius effects on density perturbations in a magnetized plasma

Electrostatic perturbations are considered in a model where a plasma is situated in a homogeneous magnetic field and is subject to an equivalent gravitation force representing the magnetic inhomogeneity effects. The perturbations are strongly affected by large Larmor radius (LLR) effects when their wavelength becomes comparable to the ion Larmor radius. In the case of a dilute plasma, two time-dependent constants of he motion are found from which a solution can be deduced for the perturbations. The LLR effects are show to 'chop up' the time development of the macroscopic density and electric field perturbations into a pulse-shaped periodic patterns. The latter is repeated at every half of a gyro period and becomes strongly damped between a set of arising narrow peaks. The LR effects have been demonstrated in a simple special case, but similar results are likely to be obtained in more complicated situations, such as in denser plasmas where the electric field from the space charges affects particle dynamics. Consequently, the fine structure of a number of plasma perturbation is expected to become strongly distorted by LLR effects, and this becomes important to plasma stability.

Current fluctuations in discrete transport systems far from equilibrium

A recently developed theoretical approach to transport fluctuations around stable steady states in discrete biological transport systems is used in order to investigate general fluctuation properties at nonequilibrium. An expression for the complex frequency dependent admittance at nonequilibrium is derived by calculation of the linear current response of the transport systems to small disturbances in the applied external voltage. It is shown that the Nyquist or fluctuation dissipation theorem, by which at equilibrium the macroscopic admittance or linear response can be expressed in terms of fluctuation properties of the system, breaks down at nonequilibrium. The spectral density of current fluctuations is decomposed into one term containing the macroscopic admittance and a second term which is bilinear in current. This second term is generated by microscopic disturbances, which cannot be excited by external macroscopic perturbations. At special examples it is demonstrated that this second term is decisive for the occurrence of excess noise e.g. the 1/f(2)-Iorentzian noise generated by the opening and closing of nerve channels in biological membranes.