Homogeneous Relaxation at Strong Coupling from Gravity

Homogeneous relaxation is a ubiquitous phenomenon in semiclassical kinetic theories where the quasiparticles are distributed uniformly in space, and the equilibration involves only their velocity distribution. For such solutions, the hydrodynamic variables remain constant. We construct asymptotically AdS solutions of Einstein's gravity dual to such processes at strong coupling, perturbatively in the amplitude expansion, where the expansion parameter is the ratio of the amplitude of the non-hydrodynamic shear-stress tensor to the pressure. At each order, we sum over all time derivatives through exact recursion relations. We argue that the metric has a regular future horizon, order by order in the amplitude expansion, provided the shear-stress tensor follows an equation of motion. At the linear order, this equation of motion implies that the metric perturbations are composed of zero wavelength quasinormal modes. Our method allows us to calculate the non-linear corrections to this equation perturbatively in the amplitude expansion. We thus derive a special case of our previous conjecture on the regularity condition on the boundary stress tensor that endows the bulk metric with a regular future horizon, and also refine it further. We also propose a new outlook for heavy-ion phenomenology at RHIC and ALICE.Comment: 60 pages, a section titled "Outlook for RHIC and ALICE" has been added, accepted for publication in Physical Review

Crystallization of hard spheres under gravity

We present a simple argument to account for crystallization of hard spheres under the action of a gravitational field. The paper attempts to bridge the gap between two communities of scientists, one working on granular materials and the other on inhomogeneous liquid state theory.Comment: Physica A (in press

Bee-friendly plants in an urban environment : a review of the plant selection from the landscape architecture programme

Pollinatörerna är hotade globalt och i det svenska landskapet, och behöver hjälp för att överleva (Naturvårdsverket u.å.b). Den främsta orsaken till den negativa utvecklingen är förlusten och fragmenteringen av pollinatörernas livsmiljöer (Naturvårdsverket u.å.c). I Sverige är de flesta pollinatörerna insekter (Naturvårdsverket 2023), där bin ingår och anses vara de viktigaste pollinatörerna i skogs- och odlingslandskapet (Linkowski et al. 2004). Med tanke på att pollinatörer minskar i antalet på landsbygden, behöver städerna bidra med vad de kan för att gynna pollinatörer och säkra ekosystemtjänster i staden (Persson & Smith 2014). Olika åtgärder som kan vidtas är att bland annat att öka andelen blommande miljöer och uppnå en lång blomningssäsong (ibid.). Eftersom det är vildbin och inte honungsbin som är hotade bör växtvalen anpassas till vildbins preferenser (ibid.). Landskapsarkitekter har stor inverkan på utemiljöers utformning och funktion i samhället (SACO 2021). Därför är det fördelaktigt om de växter som lärs ut under landskapsarkitekt-utbildningen tillgodoser bins behov eftersom växterna utgör den nyutexaminerade landskaps-arkitektens grundrepertoar. Uppsatsens syfte är därmed att klargöra om utbildningens växtutbud möjliggör för gestaltningar som gynnar bin, och i vilken utsträckning. För att få en grundläggande förståelse om bin och pollinering genomfördes en inledande dokumentstudie. Därefter utfördes datainsamling kring växternas värden för olika bin, vilket sedan sammanställdes och bearbetades i kalkylark. Kalkylarken utgjorde grunden för de tabeller som presenteras i resultatet. Resultatet tydliggör bristen på studier kring ämnet. Fler studier kring honungsbin är utförda jämfört med vildbin, vilket återspeglas i resultatet. Utöver detta visar resultatet att det inte i tillräckligt stor utsträckning finns växter från landskapsarkitektprogrammet som gynnar bin. Resultatet synliggör den ojämna fördelningen mellan de olika växtkategorierna geofyter, perenner och lignoser. Det visar också på en ojämn fördelning av när växterna börjar blomma, hur länge de blommar samt vilket värde växterna har för honungsbin, humlor respektive solitärbin. Sammantaget anses att landskapsarkitektstudenter behöver få kunskap under utbildningen om vilka växter som gynnar bin. Kunskap om bivänliga växter möjliggör för fler blomrika livsmiljöer i städer och på så vis kan den urbana miljön agera brygga mellan olika landskap och öka konnektiviteten mellan livsmiljöer. Städer kan dock aldrig ersätta livsmiljöerna på landsbygden eftersom detta skulle innebära förluster av kulturella och rekreationella värden som anses självklara idag. Som landskapsarkitekt behöver man alltid väga olika intressen mot varandra. Därför diskuteras även möjliga sociala konsekvenser av att införa bivänliga växter som man bör ta hänsyn till i jakten på att gynna bin i städer.The pollinators are threatened globally and in the Swedish landscape, and need help to survive (Naturvårdsverket u.å.b). The main reason for the negative development is the loss and fragmentation of pollinator habitats (Naturvårdsverket u.å.c). In Sweden, most pollinators are insects (Naturvårdsverket 2023), where bees are included and considered to be the most important pollinators in the forest and agricultural landscape (Linkowski et al. 2004). Given that pollinators are declining in rural areas, cities need to contribute with what they can to benefit pollinators and secure ecosystem services in the city (Persson & Smith 2014). Various measures that can be taken include increasing the proportion of flowering environments and achieving a long flowering season (ibid.). Since it is wild bees and not honeybees that are threatened, plant choices should be adapted to the preferences of wild bees (ibid.). Landscape architects have a major impact on the design and function of outdoor environments in the society (SACO 2021) Therefore, it is beneficial if the plants taught in the landscape architect programme meet the needs of bees, as the plants form the basic repertoire of the newly graduated landscape architect. The purpose of the essay is therefore to clarify if the education's plant selection enable designs that benefit bees, and to what extent. To gain a basic understanding about bees and pollination, an initial document study was conducted. Data collection was then carried out around the plants' values for different bees, which was then compiled and processed in spreadsheets. The spreadsheet formed the basis for the tables presented in the results. The result points out the lack of studies on the subject. More studies have been conducted on honeybees compared to wild bees, which is reflected in the results. In addition to this, the result shows that there are not enough plants from the landscape architect program that benefit bees. The result shows the uneven distribution between the different plant categories geophytes, perennials and lignoses. It also shows an uneven distribution of when the plants start to bloom, how long they bloom and what value the plants have for honeybees, bumblebees and solitary bees. Overall, it is considered that landscape architecture students need to gain knowledge during their education about which plants that benefit bees. Knowledge of bee-friendly plants enables more flower-rich habitats in cities and therefore the urban environment can act as a bridge between different landscapes and increase connectivity between habitats. However, cities can never replace rural habitats because this would mean losses of cultural and recreational values that are taken for granted today. As a landscape architect, you always need to weigh different interests against each other. Therefore, possible social consequences of introducing bee-friendly plants are also discussed, which should be considered in the search to benefit bees in cities

Condensation of Hard Spheres Under Gravity

Starting from Enskog equation of hard spheres of mass m and diameter D under the gravity g, we first derive the exact equation of motion for the equilibrium density profile at a temperature T and examine its solutions via the gradient expansion. The solutions exist only when \beta\mu \le \mu_o \approx 21.756 in 2 dimensions and \mu_o\approx 15.299 in 3 dimensions, where \mu is the dimensionless initial layer thickness and \beta=mgD/T. When this inequality breaks down, a fraction of particles condense from the bottom up to the Fermi surface.Comment: 9 pages, one figur

Non equilibrium inertial dynamics of colloidal systems

We consider the properties of a one dimensional fluid of brownian inertial hard-core particles, whose microscopic dynamics is partially damped by a heat-bath. Direct interactions among the particles are represented as binary, instantaneous elastic collisions. Collisions with the heath bath are accounted for by a Fokker-Planck collision operator, whereas direct collisions among the particles are treated by a well known method of kinetic theory, the Revised Enskog Theory. By means of a time multiple time-scale method we derive the evolution equation for the average density. Remarkably, for large values of the friction parameter and/or of the mass of the particles we obtain the same equation as the one derived within the dynamic density functional theory (DDF). In addition, at moderate values of the friction constant, the present method allows to study the inertial effects not accounted for by DDF method. Finally, a numerical test of these corrections is provided.Comment: 13 pages+ 3 Postscript figure

Dynamics of Air-Fluidized Granular System Measured by the Modulated Gradient Spin-echo

The power spectrum of displacement fluctuation of beads in the air-fluidized granular system is measured by a novel NMR technique of modulated gradient spin-echo. The results of measurement together with the related spectrum of the velocity fluctuation autocorrelation function fit well to an empiric formula based on to the model of bead caging between nearest neighbours; the cage breaks up after a few collisions \cite{Menon1}. The fit yields the characteristic collision time, the size of bead caging and the diffusion-like constant for different degrees of system fluidization. The resulting mean squared displacement increases proportionally to the second power of time in the short-time ballistic regime and increases linearly with time in the long-time diffusion regime as already confirmed by other experiments and simulations.Comment: 4 figures. Submited to Physical Review Letters, April 200

Transport Equations from Liouville Equations for Fractional Systems

We consider dynamical systems that are described by fractional power of coordinates and momenta. The fractional powers can be considered as a convenient way to describe systems in the fractional dimension space. For the usual space the fractional systems are non-Hamiltonian. Generalized transport equation is derived from Liouville and Bogoliubov equations for fractional systems. Fractional generalization of average values and reduced distribution functions are defined. Hydrodynamic equations for fractional systems are derived from the generalized transport equation.Comment: 11 pages, LaTe

Phase-space approach to dynamical density functional theory

We consider a system of interacting particles subjected to Langevin inertial dynamics and derive the governing time-dependent equation for the one-body density. We show that, after suitable truncations of the Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy, and a multiple time scale analysis, we obtain a self-consistent equation involving only the one-body density. This study extends to arbitrary dimensions previous work on a one-dimensional fluid and highlights the subtelties of kinetic theory in the derivation of dynamical density functional theory

Retarded versus time-nonlocal quantum kinetic equations

The finite duration of the collisions in Fermionic systems as expressed by the retardation time in non-Markovian Levinson-type kinetic equations is discussed in the quasiclassical limit. We separate individual contributions included in the memory effect resulting in (i) off-shell tails of the Wigner distribution, (ii) renormalization of scattering rates and (iii) of the single-particle energy, (iv) collision delay and (v) related non-local corrections to the scattering integral. In this way we transform the Levinson equation into the Landau-Silin equation extended by the non-local corrections known from the theory of dense gases. The derived nonlocal kinetic equation unifies the Landau theory of quasiparticle transport with the classical kinetic theory of dense gases. The space-time symmetry is discussed versus particle-hole symmetry and a solution is proposed which transforms these two exclusive pictures into each other.Comment: slightly revised, 19 page

Condensation of Hard Spheres Under Gravity: Exact Results in One Dimension

We present exact results for the density profile of the one dimensional array of N hard spheres of diameter D and mass m under gravity g. For a strictly one dimensional system, the liquid-solid transition occurs at zero temperature, because the close-pakced density, $\phi_c$, is one. However, if we relax this condition slightly such that $phi_c=1-\delta$, we find a series of critical temperatures T_c^i=mgD(N+1-i)/\mu_o with \mu_o=const, at which the i-th particle undergoes the liquid-solid transition. The functional form of the onset temperature, T_c^1=mgDN/\mu_o, is consistent with the previous result [Physica A 271, 192 (1999)] obtained by the Enskog equation. We also show that the increase in the center of mass is linear in T before the transition, but it becomes quadratic in T after the transition because of the formation of solid near the bottom