Generic Instabilities Research Articles

Stability studies of first-order spin-hydrodynamic frameworks

We study the stability of first-order dissipative spin-hydrodynamic frameworks. We considered two different first-order dissipative spin-hydrodynamic frameworks. The first one considers the spin-chemical potential (${\ensuremath{\omega}}^{\ensuremath{\alpha}\ensuremath{\beta}}$) to be first order [$\mathcal{O}(\ensuremath{\partial})$] in the hydrodynamic gradient expansion. The hydrodynamic gradient ordering of the spin-chemical potential is a debatable issue within the frameworks of spin hydrodynamics. Therefore, as a second choice, we also consider the spin-hydrodynamic equations with ${\ensuremath{\omega}}^{\ensuremath{\alpha}\ensuremath{\beta}}\ensuremath{\sim}\mathcal{O}(1)$. We find that, for both frameworks, at the level of linear perturbations some spin modes can be unstable. To remove these generic instabilities, we consider the Frenkel condition. We argue that the Frenkel condition helps get rid of the unstable solutions in both cases but with a physical drawback for the case where ${\ensuremath{\omega}}^{\ensuremath{\mu}\ensuremath{\nu}}\ensuremath{\sim}\mathcal{O}(\ensuremath{\partial})$.

Instabilities in a growing system of active particles: scalar and vectorial systems.

The physics of micron-scale biological colonies usually benefits from different out-of-equilibrium sources. In bacterial colonies and cellular tissues, the growth process is among the important active sources that determine the dynamics. In this article, we study the generic dynamical instabilities associated with the growth phenomena that may arise in both scalar and vectorial systems. In vectorial systems, where the rotational degrees of particles play a role, a phenomenological growth-mediated torque can affect the rotational dynamics of individual particles. We show that such a growth-mediated torque can result in active traveling waves in the bulk of a growing system. In addition to the bulk properties, we analyze the instabilities in the shape of growing interfaces in both scalar and vectorial systems.

Linear stability of black holes with static scalar hair in full Horndeski theories: Generic instabilities and surviving models

In full Horndeski theories, we show that the static and spherically symmetric black hole (BH) solutions with a static scalar field~$\phi$ whose kinetic term~$X$ is nonvanishing on the BH horizon are generically prone to ghost/Laplacian instabilities. We then search for asymptotically Minkowski hairy BH solutions with a vanishing $X$ on the horizon free from ghost/Laplacian instabilities. We show that models with regular coupling functions of $\phi$ and $X$ result in no-hair Schwarzschild BHs in general. On the other hand, the presence of a coupling between the scalar field and the Gauss-Bonnet (GB) term $R_{\rm GB}^2$, even with the coexistence of other regular coupling functions, leads to the realization of asymptotically Minkowski hairy BH solutions without ghost/Laplacian instabilities. Finally, we find that hairy BH solutions in power-law $F(R_{\rm GB}^2)$ gravity are plagued by ghost instabilities. These results imply that the GB coupling of the form $\xi(\phi)R_{\rm GB}^2$ plays a prominent role for the existence of asymptotically Minkowski hairy BH solutions free from ghost/Laplacian instabilities.

Zones of Occult Instability: The Birth of the Novel in Africa

Zones of Occult Instability: The Birth of the Novel in Africa

Generic instabilities in the relativistic Chapman–Enskog heat conduction law

We address the well-posedness of the Cauchy problem corresponding to the relativistic fluid equations, when coupled with the heat-flux constitutive relation arising within the relativistic Chapman-Enskog procedure. The resulting system of equations is shown to be non hyperbolic, by considering general perturbations over the whole set of equations written with respect to a generic time direction. The obtained eigenvalues are not purely imaginary and their real part grows without bound as the wave-number increases. Unlike Eckart's theory, this instability is not present when the time direction is aligned with the fluid's direction. However, since in general the fluid velocity is not surface-forming, the instability can only be avoided in the particular case where no rotation is present.

Active matter in a viscoelastic environment

Active matter systems such as eukaryotic cells and bacteria continuously transform chemical energy to motion. Hence living systems exert active stresses on the complex environments in which they reside. One recurring aspect of this complexity is the viscoelasticity of the medium surrounding living systems: bacteria secrete their own viscoelastic extracellular matrix, and cells constantly deform, proliferate, and self-propel within viscoelastic networks of collagen. It is therefore imperative to understand how active matter modifies, and gets modified by, viscoelastic fluids. Here, we present a two-phase model of active nematic matter that dynamically interacts with a passive viscoelastic polymeric phase and perform numerical simulations in two dimensions to illustrate its applicability. Motivated by recent experiments we first study the suppression of cell division by a viscoelastic medium surrounding the cell. We further show that the self-propulsion of a model keratocyte cell is modified by the polymer relaxation of the surrounding viscoelastic fluid in a non-uniform manner and find that increasing polymer viscosity effectively suppresses the cell motility. Lastly, we explore the hampering impact of the viscoelastic medium on the generic hydrodynamic instabilities of active nematics by simulating the dynamics of an active stripe within a polymeric fluid. The model presented here can provide a framework for investigating more complex dynamics such as the interaction of multicellular growing systems with viscoelastic environments.

Across the Ocean: Historicizing SF Studies in Japan and Beyond

518 SCIENCE FICTION STUDIES, VOLUME 47 (2020) discussion of “intergroup dialogue,” defined as a “facilitated, face-to-face encounter that aims to foster innovative levels of understanding between two or more identity groups” (109), uses Butler’s writing alongside other texts from contemporary popular culture in a rigorous, multiphase approach to classroom discussion. Indeed, Parker’s chapter offers the most pragmatic (and, arguably, radical) guide to teaching Butler’s science fiction; the appendix of exercises alone is absolutely true to the productive unsettlement generated by Butler’s work. The volume concludes with a section on “The Aesthetics of Afrofuturism” that serves as a valuable recapitulation of a historical literary moment. Here we return to a more familiar interpretation of Butler, circling back to the more literary-critical reception of her work. From models of Black female leadership (as identified by Stanley) to the generic instabilities of a text such as “Bloodchild,” moving through science fiction and historicism to the “liberation politics coupled with critical race theory” (132) that inform Isaiah Lavender’s definition of Afrofuturism, the concept is rendered urgent, timely, and perhaps necessarily expansive in scope. Yet, despite the importance of students becoming cognizant of Afrofuturist aesthetics as they play out “across music, art, film, and other cultural practices” (147), it would have been desirable to see a little more nuance in positioning Octavia E. Butler as an Afrofuturist author. Marisa Parham, for instance, uses the term “astrofuturism” to define the more literary and text-based praxis of Butler, who, indeed, expressed bemusement at being grouped with other contemporary black artists such as Samuel R. Delany (let alone musicians such as George Clinton and Sun Ra). Given that this final subsection is the shortest, containing only three chapters, it might have been more effective to incorporate them into the preceding sections. Stanley’s “Teaching Afrofuturistic Thought Leadership in Butler’s Fiction” would be a fine fit for the “Social Justice” section, for instance, whereas Lavender’s critical reading of Parable of the Sower would be entirely at home in the first subsection on literary approaches. Furthermore, all subsections would have benefited from drawing a little more upon the abundant archival materials held at the Huntington Library which houses Octavia E. Butler’s papers, as well as more fully using some of the interview materials cited in the excellent first section. Butler, in her papers and interviews, had much to say on issues of pedagogy and politics. Nonetheless, this volume presents a wide-ranging and generous framework for teaching the works of Octavia E. Butler in a variety of contexts and educational levels, and I thoroughly recommend it.—Phoenix Alexander, University of Liverpool Across the Ocean: Historicizing SF Studies in Japan and Beyond. Takayuki Tatsumi, ed. Trans-Pacific Cultural Studies. Vol. III: Science Fiction and Cyber Culture. New Delhi: Sage, 2019. 289 pp. $1100 hc. (4-volume set, not sold individually) 519 BOOKS IN REVIEW [For the sake of consistency with Tatsumi’s series, Chinese and Japanese names appear as they would in English, with given name first and surname second. I would like to thank Baryon Tensor-Posadas at the University of Minnesota for his advice on the Japanese titles listed here.] Takayuki Tatsumi’s introduction to this volume argues that “without the existence of Japan, postwar science fiction could not have developed the transpacific imagination. It is true that Japan played the leading role of trans-pacific science fiction” (vii-viii). Global popular culture since at least the 1960s would be vastly different were it not for the circulation of images, tropes, and topoi between and within the United States and Asia, and particularly without Japan’s successful internationalization of all manner of franchisable characters, from the cool to the cute (volume IV is devoted entirely to the topic of “Cool Asia” and is worth finding in your library). While the four-volume series in toto—which also includes “Trans-Pacific Americanism” (vol. I) and “Trans-Pacific Literary Studies” (vol. II)—features a more truly trans-Pacific focus, the introduction to this particular volume and its contents are focused more squarely on Japan and on the influences of the United States during the Cold-War era rather than...

Active morphogenesis of epithelial monolayers

During typical early-stage embryo development, single-cell-thick tissues of tightly bound epithelial cells autonomously generate profound changes in their shape, forming the basis of organism anatomy. We report on a (covariant) active-hydrodynamic theory of such monolayer morphogenesis that is closed under its shape-changing dynamics-i.e., the degrees of freedom that encode monolayer geometry appear properly as broken-symmetry variables. In our theory, the salient physics of tissue-scale deformations emerges from a balance between the displacement and/or shear of a low-Reynolds-number embedding fluid (the "yolk") and cell-autonomous stresses, themselves a result of combining apical contractile stresses with an elastic-like mechanical response under the constraint of constant cell volume. The leading-order hydrodynamic instabilities include both passive constrained-buckling and active deformation, which can be further categorized by cell shape changes that are either "squamous to columnar" or "regular-prism to truncated-pyramid." The deformations resulting from the latter qualitatively reproduce in vivo observations of the onset of both mesoderm and posterior midgut invaginations, which take place during gastrulation in the model organism Drosophila melanogaster.

Axionic instabilities and new black hole solutions

The coupling between scalar and vector fields has a long and interesting history. Axions are one key possibility to solve the strong CP problem and axion-like particles could be one solution to the dark matter puzzle. Given the nature of the coupling, and the universality of free fall, nontrivial important effects are expected in regions where gravity is strong. Here, we show that i. A background EM field induces an axionic instability in flat space, for large enough electric fields. Conversely, a homogeneous harmonic axion field induces an instability in the Maxwell sector. When carried over to curved spacetime, this phenomena translates into generic instabilities of charged black holes (BHs). ii. In the presence of charge, BH uniqueness results are lost. We find solutions which are small deformations of the Kerr-Newman geometry and hairy stationary solutions without angular momentum, which are `dragged' by the axion. Axion fields must exist around spinning BHs if these are immersed in external magnetic fields. The axion profile can be obtained perturbatively from the electro-vacuum solution derived by Wald. iii. Ultralight axions trigger superradiant instabilities of spinning BHs and form an axionic cloud in the exterior geometry. The superradiant growth can be interrupted or suppressed through axionic or scalar couplings to EM. These couplings lead to periodic bursts of light, which occur throughout the history of energy extraction from the BH. We provide numerical and simple analytical estimates for the rates of these processes. iv. Finally, we discuss how plasma effects can affect the evolution of superradiant instabilities.

Generic Instabilities of Non-Singular Cosmologies

Generic Instabilities of Non-Singular Cosmologies

Cosmological perturbations and stability of nonsingular cosmologies with limiting curvature

We revisit nonsingular cosmologies in which the limiting curvature hypothesis is realized. We study the cosmological perturbations of the theory and determine the general criteria for stability. For the simplest model, we find generic Ostrogradski instabilities unless the action contains the Weyl tensor squared with the appropriate coefficient. When considering two specific nonsingular cosmological scenarios(one inflationary and one genesis model), we find ghost and gradient instabilities throughout most of the cosmic evolution. Furthermore, we show that the theory is equivalent to a theory of gravity where the action is a general function of the Ricci and Gauss-Bonnet scalars, and this type of theory is known to suffer from instabilities in anisotropic backgrounds. This leads us to construct a new type of curvature invariant scalar function. We show that it does not have Ostrogradski instabilities, and it avoids ghost and gradient instabilities for most of the interesting background inflationary and genesis trajectories. We further show that it does not possess additional new degrees of freedom in an anisotropic spacetime. This opens the door for studying stable alternative nonsingular very early universe cosmologies.

Generic instabilities of nonsingular cosmologies in Horndeski theory: A no-go theorem

The null energy condition can be violated stably in generalized Galileon theories, which gives rise to the possibilities of healthy non-singular cosmologies. However, it has been reported that in many cases cosmological solutions are plagued with instabilities or have some pathologies somewhere in the whole history of the universe. Recently, this was shown to be generically true in a certain subclass of the Horndeski theory. In this short paper, we extend this no-go argument to the full Horndeski theory, and show that non-singular models (with flat spatial sections) in general suffer either from gradient instabilities or some kind of pathology in the tensor sector. This implies that one must go beyond the Horndeski theory to implement healthy non-singular cosmologies.

On the Stability Problem in Special Relativistic Thermodynamics: Implications of the Chapman–Enskog Formalism

Extended theories are widely used in the literature to describe relativistic fluids. The motivation for this is mostly due to the causality issues allegedly present in the first order in the gradients theories. However, the decay of fluctuations in the system is also at stake when first order theories that couple heat with acceleration are used. This paper shows that although the introduction of the Maxwell–Cattaneo equation in the description of a simple relativistic fluid formally eliminates the generic instabilities identified by Hiscock and Lindblom (Phys Rev D 31:725–733, 1985), the hypothesis on the order of magnitude of the corresponding relaxation term contradicts the basic ordering in Knudsen’s parameter present in the kinetic approach to hydrodynamics. It is shown that the time derivative, stabilizing term is of second order in such parameter and thus does not belong to the Navier–Stokes regime where the so-called instability arises.

Generic instabilities in a fluid membrane coupled to a thin layer of ordered active polar fluid

We develop an effective two-dimensional coarse-grained description for the coupled system of a planar fluid membrane anchored to a thin layer of polar ordered active fluid below. The macroscopic orientation of the active fluid layer is assumed to be perpendicular to the attached membrane. We demonstrate that activity or nonequilibrium drive of the active fluid makes such a system generically linearly unstable for either signature of a model parameter [Formula: see text] [Formula: see text] that characterises the strength of activity. Depending upon boundary conditions and within a range of the model parameters, underdamped propagating waves may be present in our model. We discuss the phenomenological significance of our results.

Instabilities and diffusion in a hydrodynamic model of a fluid membrane coupled to a thin active fluid layer

We construct a coarse-grained effective two-dimensional (2d hydrodynamic theory as a theoretical model for a coupled system of a fluid membrane and a thin layer of a polar active fluid in its ordered state that is anchored to the membrane. We show that such a system is prone to generic instabilities through the interplay of nonequilibrium drive, polar order and membrane fluctuation. We use our model equations to calculate diffusion coefficients of an inclusion in the membrane and show that their values depend strongly on the system size, in contrast to their equilibrium values. Our work extends the work of S. Sankararaman and S. Ramaswamy (Phys. Rev. Lett., 102, 118107 (2009)) to a coupled system of a fluid membrane and an ordered active fluid layer. Our model is broadly inspired by and should be useful as a starting point for theoretical descriptions of the coupled dynamics of a cell membrane and a cortical actin layer anchored to it.

Linearized flavor-stability analysis of dense neutrino streams

Neutrino-neutrino interactions in dense neutrino streams, like those emitted by a core-collapse supernova, can lead to self-induced neutrino flavor conversions. While this is a nonlinear phenomenon, the onset of these conversions can be examined through a standard stability analysis of the linearized equations of motion. The problem is reduced to a linear eigenvalue equation that involves the neutrino density, energy spectrum, angular distribution, and matter density. In the single-angle case, we reproduce previous results and use them to identify two generic instabilities: The system is stable above a cutoff density ("cutoff mode"), or can approach an asymptotic instability for increasing density ("saturation mode"). We analyze multi-angle effects on these generic types of instabilities and find that even the saturation mode is suppressed at large densities. For both types of modes, a given multi-angle spectrum typically is unstable when the neutrino and electron densities are comparable, but stable when the neutrino density is much smaller or much larger than the electron density. The role of an instability in the SN context depends on the available growth time and on the range of affected modes. At large matter density, most modes are off-resonance even when the system is unstable.

Improving relativistic modified Newtonian dynamics with Galileonk-mouflage

We propose a simple field theory reproducing the MOND phenomenology at galaxy scale, while predicting negligible deviations from general relativity at small scales thanks to an extended Vainshtein ("k-mouflage") mechanism induced by a covariant Galileon-type Lagrangian. The model passes solar-system tests at the post-Newtonian order, including those of local Lorentz invariance, and its anomalous forces in binary-pulsar systems are orders of magnitude smaller than the tightest experimental constraints. The large-distance behavior is obtained as in Bekenstein's tensor-vector-scalar (TeVeS) model, but with several simplifications. In particular, no fine-tuned function is needed to interpolate between the MOND and Newtonian regimes, and no dynamics needs to be defined for the vector field because preferred-frame effects are negligible at small distances. The field equations depend on second (and lower) derivatives, and avoid thus the generic instabilities related to higher derivatives. Their perturbative solution around a Schwarzschild background is remarkably simple to derive. We also underline why the proposed model is particularly efficient within the class of covariant Galileons.

Country Matters: Rural Fiction, Urban Modernity, and the Problem of American Regionalism

Mark Storey, "Country Matters: Rural Fiction, Urban Modernity, and the Problem of American Regionalism" (pp. 192–213) This essay intervenes in the critical debates surrounding nineteenth-century American regionalism, arguing that such debates have tended to ignore the possibility of a shared and trans-regional category of "rural fiction." Developing this notion, I suggest that literary representations of rural life in the late nineteenth century are a crucial and neglected way of understanding the geographically indiscrete transformations of urban-capitalist modernity. Further, by examining these transformations through the prism of rural fiction, we can challenge the urban-centric tendency of postbellum American literary history. Drawing on several writers who have been the focus of much of critics' attentions on regionalism (Edward Eggleston, Hamlin Garland, and Sarah orne Jewett in particular), this essay considers both the generic and thematic instabilities of rural fiction, arguing that these instabilities serve to encode and refract the social and cultural context from which this fiction emerges. Reading rural fiction against the background of the increasing similarities between geographically distinct areas of rural life, and reconsidering many of the works that we currently gather under the regionalist rubric as, instead, rural, a distinct perspective can be gained on the standardizing and flattening processes of modernity itself.

On the nature of the so-called generic instabilities in dissipative relativistic hydrodynamics

It is shown that the so-called generic instabilities that appear in the framework of relativistic linear irreversible thermodynamics, describing the fluctuations of a simple fluid close to equilibrium, arise due to the coupling of heat with hydrodynamic acceleration which appears in Eckart's formalism of relativistic irreversible thermodynamics. Further, we emphasize that such behavior should be interpreted as a contradiction to the postulates of linear irreversible thermodynamics (LIT), namely a violation of Onsager's hypothesis on the regression of fluctuations, and not as fluid instabilities. Such contradictions can be avoided within a relativistic linear framework if a Meixner-like approach to the phenomenological equations is employed.

Instability of an elastic film on a viscous layer

Abstract A flat, compressed elastic film on a viscous layer is unstable. The film can form wrinkles to reduce the elastic energy. In this paper, we are interested in the two-dimensional models for thin films bonded to a viscous layer and in particular we focus on generic instabilities evidenced in this context by Suo and coworkers [Huang, Z., Hong, W., Suo, Z., 2005 . Non linear analyses of wrinkles in a film bonded to a compliant substrate. J. Mech. Phys. Solids 53, 2101–2118; Lo, Y.H., 1991. New approach to grow pseudomorphic structures over the critical thickness. Appl. Phys. Lett. 59, 2311–2320]. We present a rigorous linear perturbation analysis for anisotropic materials, that allows the prediction of both the orientation of the corrugations of the thin film, and the wavelength that maximize the growth velocity. Finally, we compare our theoretical estimates to experimental results for a In 0.65 Ga 0.35 As alloy constraint to InP.