Recent zbMATH articles in MSC 85https://zbmath.org/atom/cc/852023-11-13T18:48:18.785376ZWerkzeugKernel excess mass test for multimodalityhttps://zbmath.org/1521.620722023-11-13T18:48:18.785376Z"Lee, Seonmi"https://zbmath.org/authors/?q=ai:lee.seonmi"Jang, Woncheol"https://zbmath.org/authors/?q=ai:jang.woncheol"Park, Byeong U."https://zbmath.org/authors/?q=ai:park.byeong-ukSummary: In this paper we propose a new statistical procedure for testing the multimodality of an underlying distribution. \textit{P. Hall} and \textit{M. York} [Stat. Sin. 11, No. 2, 515--536 (2001; Zbl 1026.62047)] developed an innovative idea of calibrating the null distribution for the excess mass test statistic using the empirical distribution function. We find that the qualitative characteristics of a smooth underlying distribution function on the number of modes is barely preserved in the excess mass functional by the non-smooth empirical distribution function. Instead of the empirical distribution function, we propose to use a kernel distribution function estimator. We derive the limiting distribution of the resulting test statistic under strong unimodality, based on which we apply the calibration idea to the proposed test statistic to obtain a cut-off value. Our numerical study suggests that the calibrated kernel excess mass test has greater power than other existing methods. We also illustrate the use of the proposed method in a case study in astronomy which supports an assumption on a physical property of minor planets in the solar system.Symmetric periodic orbits in the Hamiltonian galactic-tidal modelshttps://zbmath.org/1521.700212023-11-13T18:48:18.785376Z"Alberti, Angelo"https://zbmath.org/authors/?q=ai:alberti.angeloSummary: The aim of the present work is to show the existence of symmetric periodic solutions of a class of perturbed generalized Galatic-Tidal models. Combining the discrete symmetries of the Hamiltonian and the Poincaré's continuation method, we give sufficient conditions over the parameters of the problem, for the existence of symmetric periodic solutions. Moreover, we determine the linear stability of these symmetric periodic solutions.Evolution of aligned states within nonlinear dynamoshttps://zbmath.org/1521.769092023-11-13T18:48:18.785376Z"Miller, D."https://zbmath.org/authors/?q=ai:miller.d-claire|miller.david-r|miller.donald-l|miller.david-donald|miller.david-a-b|miller.david-f|miller.donald-g|miller.dawson|miller.david-l|miller.dale-d|miller.david-s|miller.donald-w|miller.damon-a|miller.d-michael|miller.david-marshall|miller.douglas-j|miller.douglas-a|miller.donald-m|miller.daniel-a|miller.darlene|miller.donald-s|miller.david-w|miller.duane-d|miller.donald-j|miller.david-p|miller.daniel-j|miller.douglas-e|miller.douglas-s|miller.david-john|miller.daniel-n|miller.dale-a|miller.daniel-e|miller.david-c|miller.douglas-l|miller.dorothy-jSummary: The Archontis dynamo is a rare example of an MHD dynamo within which forcing drives a dynamo where the flow and magnetic fields are almost perfectly aligned and the energies are approximately equal. In this paper, I expand upon our knowledge of the dynamo by showing that the intermediate steady states of the kinetic and magnetic energies observed by Cameron and Galloway are not a necessary feature of aligned dynamos. Furthermore, I show that the steady state into which the flow and magnetic fields eventually evolve is remarkably robust to the addition of time dependence and asymmetry to the forcing.Effects of boundary conditions on the onset of convection with tilted magnetic field and rotation vectorshttps://zbmath.org/1521.769122023-11-13T18:48:18.785376Z"Proctor, M. R. E."https://zbmath.org/authors/?q=ai:proctor.michael-r-e"Weiss, N. O."https://zbmath.org/authors/?q=ai:weiss.nigel-o"Thompson, S. D."https://zbmath.org/authors/?q=ai:thompson.s-d"Roxburgh, N. T."https://zbmath.org/authors/?q=ai:roxburgh.n-tSummary: The problem of the onset of thermal convection is considered, firstly when a uniform tilted magnetic field is present, and secondly in a frame rotating about an oblique axis. If up-down symmetry is broken we expect to find only bifurcations that lead to travelling waves. Numerical studies show, however, that in a Boussinesq fluid the spectrum of eigenvalues can be symmetrical about the real axis, even when the boundary conditions are asymmetrical. Here we show analytically that this symmetry property indeed holds for a wide range of boundary conditions and hence that both steady solutions and standing waves are allowed.Oscillating scalar dissipating in a mediumhttps://zbmath.org/1521.811952023-11-13T18:48:18.785376Z"Ai, Wen-Yuan"https://zbmath.org/authors/?q=ai:ai.wen-yuan"Drewes, Marco"https://zbmath.org/authors/?q=ai:drewes.marco"Glavan, Dražen"https://zbmath.org/authors/?q=ai:glavan.drazen"Hajer, Jan"https://zbmath.org/authors/?q=ai:hajer.janSummary: We study how oscillations of a scalar field condensate are damped due to dissipative effects in a thermal medium. Our starting point is a non-linear and non-local condensate equation of motion descending from a 2PI-resummed effective action derived in the Schwinger-Keldysh formalism appropriate for non-equilibrium quantum field theory. We solve this non-local equation by means of multiple-scale perturbation theory appropriate for time-dependent systems, obtaining approximate analytic solutions valid for very long times. The non-linear effects lead to power-law damping of oscillations, that at late times transition to exponentially damped ones characteristic for linear systems. These solutions describe the evolution very well, as we demonstrate numerically in a number of examples. We then approximate the non-local equation of motion by a Markovianised one, resolving the ambiguities appearing in the process, and solve it utilizing the same methods to find the very same leading approximate solution. This comparison justifies the use of Markovian equations at leading order. The standard time-dependent perturbation theory in comparison is not capable of describing the non-linear condensate evolution beyond the early time regime of negligible damping. The macroscopic evolution of the condensate is interpreted in terms of microphysical particle processes. Our results have implications for the quantitative description of the decay of cosmological scalar fields in the early Universe, and may also be applied to other physical systems.General freeze-in and freeze-outhttps://zbmath.org/1521.813282023-11-13T18:48:18.785376Z"Redi, Michele"https://zbmath.org/authors/?q=ai:redi.michele"Tesi, Andrea"https://zbmath.org/authors/?q=ai:tesi.andreaSummary: We use the framework of relativistic and non-relativistic conformal field theories (CFT) to derive general results relevant for the production of weakly coupled and strongly coupled dark sectors through thermal interactions. Our result reproduce trivially known formulas for \(2 \rightarrow n\) processes and extend to general \(m\rightarrow n\) processes as well as interacting dark sectors. As concrete examples we consider freeze-in of a relativistic CFT coupled to the SM with contact interactions and derive Sommerfeld enhancement of non-relativistic cross-sections from the theory of fermions at unitarity.Axion string signatures: a cosmological plasma colliderhttps://zbmath.org/1521.813342023-11-13T18:48:18.785376Z"Agrawal, Prateek"https://zbmath.org/authors/?q=ai:agrawal.prateek"Hook, Anson"https://zbmath.org/authors/?q=ai:hook.anson"Huang, Junwu"https://zbmath.org/authors/?q=ai:huang.junwu"Marques-Tavares, Gustavo"https://zbmath.org/authors/?q=ai:marques-tavares.gustavoSummary: We study early and late time signatures of both QCD axion strings and hyperlight axion strings (axiverse strings). We focus on charge deposition onto axion strings from electromagnetic fields and subsequent novel neutralizing mechanisms due to bound state formation. While early universe signatures appear unlikely, there are a plethora of late time signatures. Axion strings passing through galaxies obtain a huge charge density, which is neutralized by a dense plasma of bound state Standard Model particles forming a one dimensional ``atom''. The charged wave packets on the string, as well as the dense plasma outside, travel at nearly the speed of light along the string. These packets of high energy plasma collide with a center of mass energy of up to \(10^9\) GeV. These collisions can have luminosities up to seven orders of magnitude larger than the solar luminosity, and last for thousands of years, making them visible at radio telescopes even when they occur cosmologically far away. The new observables are complementary to the CMB observables for hyperlight axion strings that have been recently proposed, and are sensitive to a similar motivated parameter range.Gauge kinetic mixing and dark topological defectshttps://zbmath.org/1521.813452023-11-13T18:48:18.785376Z"Hiramatsu, Takashi"https://zbmath.org/authors/?q=ai:hiramatsu.takashi"Ibe, Masahiro"https://zbmath.org/authors/?q=ai:ibe.masahiro"Suzuki, Motoo"https://zbmath.org/authors/?q=ai:suzuki.motoo"Yamaguchi, Soma"https://zbmath.org/authors/?q=ai:yamaguchi.somaSummary: We discuss how the topological defects in the dark sector affect the Standard Model sector when the dark photon has a kinetic mixing with the QED photon. In particular, we consider the dark photon appearing in the successive gauge symmetry breaking, \(\mathrm{SU}(2)\rightarrow\mathrm{U}(1)\rightarrow\mathbb{Z}_2\), where the remaining \(\mathbb{Z}_2\) is the center of SU(2). In this model, the monopole is trapped into the cosmic strings and forms the so-called bead solution. As we will discuss, the dark cosmic string induces the QED magnetic flux inside the dark string through the kinetic mixing. The dark monopole, on the other hand, does not induce the QED magnetic flux in the U(1) symmetric phase, even in the presence of the kinetic mixing. Finally, we show that the dark bead solution induces a spherically symmetric QED magnetic flux through the kinetic mixing. The induced flux looks like the QED magnetic monopole viewed from a distance, although QED satisfies the Bianchi identity everywhere, which we call a pseudo magnetic monopole.Kibble mechanism for electroweak magnetic monopoles and magnetic fieldshttps://zbmath.org/1521.814682023-11-13T18:48:18.785376Z"Patel, Teerthal"https://zbmath.org/authors/?q=ai:patel.teerthal"Vachaspati, Tanmay"https://zbmath.org/authors/?q=ai:vachaspati.tanmaySummary: The vacuum manifold of the standard electroweak model is a three-sphere when one considers \textit{homogeneous} Higgs field configurations. For inhomogeneous configurations we argue that the vacuum manifold is the Hopf fibered three sphere and that this viewpoint leads to general criteria to detect electroweak monopoles and Z-strings. We extend the Kibble mechanism to study the formation of electroweak monopoles and strings during electroweak symmetry breaking. The distribution of magnetic monopoles produces magnetic fields that have a spectrum \(B_\lambda\propto\lambda^{-2}\), where \(\lambda\) is a smearing length scale. Even as the magnetic monopoles annihilate due to the confining Z-strings, the magnetic field evolves with the turbulent plasma and may be relevant for cosmological observations.Maxwell's equations and Lorentz transformationshttps://zbmath.org/1521.830022023-11-13T18:48:18.785376Z"Aguirregabiria, J. M."https://zbmath.org/authors/?q=ai:aguirregabiria.juan-maria"Hernández, A."https://zbmath.org/authors/?q=ai:hernandez.amanda|hernandez.alexis-r|hernandez.anderson-melchor|hernandez.alejandro-lopez|hernandez.alan-e-lopez|hernandez.ankai|hernandez.alfonso|hernandez.angelica|hernandez.alejandro-s|hernandez.andres-f|hernandez.a-calvo|hernandez.aracelis|hernandez.arezky-h|hernandez.a-i|melle-hernandez.alejandro|hernandez.adrian|hernandez.adolfo|hernandez.alejo|hernandez.arturo|hernandez.alberto-j|hernandez.aurelio|hernandez.alberto-m|hernandez.alejandro-mario|hernandez.antonio-j|hernandez.alvaro|hernandez.a-e-carcamo|hernandez.araceli|hernandez.anabel"Rivas, M."https://zbmath.org/authors/?q=ai:rivas.miriam|rivas.manuel-a|rivas.mauricio-a|rivas.mariolys|rivas-rodriguez.maria-teresa|rivas.m-teresa|rivas.matthew-l|rivas.martin|rivas.maria-jesus|rivas.migdalia(no abstract)A guiding center implementation for relativistic particle dynamics in the PLUTO codehttps://zbmath.org/1521.830102023-11-13T18:48:18.785376Z"Mignone, A."https://zbmath.org/authors/?q=ai:mignone.andrea"Haudemand, H."https://zbmath.org/authors/?q=ai:haudemand.h"Puzzoni, E."https://zbmath.org/authors/?q=ai:puzzoni.eSummary: We present a numerical implementation of the guiding center approximation to describe the relativistic motion of charged test particles in the PLUTO code for astrophysical plasma dynamics. The guiding center approximation (GCA) removes the time step constraint due to particle gyration around magnetic field lines by following the particle center of motion rather than its full trajectory. The gyration can be detached from the guiding center motion if electromagnetic fields vary sufficiently slow compared to the particle gyration radius and period. Our implementation employs a variable step-size linear multistep method, more efficient when compared to traditional one-step Runge Kutta schemes. A number of numerical benchmarks is presented in order to assess the validity of our implementation.Axion fragmentation on the latticehttps://zbmath.org/1521.830192023-11-13T18:48:18.785376Z"Morgante, Enrico"https://zbmath.org/authors/?q=ai:morgante.enrico"Ratzinger, Wolfram"https://zbmath.org/authors/?q=ai:ratzinger.wolfram"Sato, Ryosuke"https://zbmath.org/authors/?q=ai:sato.ryosuke.1"Stefanek, Ben A."https://zbmath.org/authors/?q=ai:stefanek.ben-aSummary: We analyze the phenomenon of axion fragmentation when an axion field rolls over many oscillations of a periodic potential. This is particularly relevant for the case of relaxion, in which fragmentation provides the necessary energy dissipation to stop the field evolution. We compare the results of a linear analysis with the ones obtained from a classical lattice simulation, finding an agreement in the stopping time of the zero mode between the two within an \(\mathcal{O}(1)\) difference. We finally speculate on the generation of bubbles with different VEVs of the axion field, and discuss their cosmological consequences.An introduction to gravitational waves through electrodynamics: a quadrupole comparisonhttps://zbmath.org/1521.830242023-11-13T18:48:18.785376Z"Dorsch, Glauber Carvalho"https://zbmath.org/authors/?q=ai:dorsch.glauber-carvalho"Antunes Porto, Lucas Emanuel"https://zbmath.org/authors/?q=ai:antunes-porto.lucas-emanuel(no abstract)Analytical effective one-body formalism for extreme-mass-ratio inspirals with eccentric orbitshttps://zbmath.org/1521.830302023-11-13T18:48:18.785376Z"Zhang, Chen"https://zbmath.org/authors/?q=ai:zhang.chen"Han, Wen-Biao"https://zbmath.org/authors/?q=ai:han.wenbiao"Yang, Shu-Cheng"https://zbmath.org/authors/?q=ai:yang.shuchengSummary: Extreme-mass-ratio inspirals (EMRIs) are among the most important sources for future spaceborne gravitational wave detectors. In this kind of system, compact objects usually orbit around central supermassive black holes on complicated trajectories. Usually, these trajectories are approximated as the geodesics of Kerr space-times, and orbital evolution is simulated with the help of the adiabatic approximation. However, this approach omits the influence of the compact object on its background. In this paper, using the effective one-body formalism, we analytically calculate the trajectory of a nonspinning compact object around a massive Kerr black hole in an equatorial eccentric orbit (omitting the orbital inclination) and express the fundamental orbital frequencies in explicit forms. Our formalism includes the first-order corrections for the mass ratio in the conservative orbital motion. Furthermore, we insert the mass-ratio-related terms into the first post-Newtonian energy fluxes. By calculating the gravitational waves using the Teukolsky equations, we quantitatively reveal the influence of the mass of the compact object on the data analysis. We find that the shrinking of geodesic motion by taking small objects as test particles may not be appropriate for the detection of EMRIs.Relativistic gravitational collapse by thermal masshttps://zbmath.org/1521.830572023-11-13T18:48:18.785376Z"Roupas, Zacharias"https://zbmath.org/authors/?q=ai:roupas.zachariasSummary: Gravity and thermal energy are universal phenomena which compete over the stabilization of astrophysical systems. The former induces an inward pressure driving collapse and the latter a stabilizing outward pressure generated by random motion and energy dispersion. Since a contracting self-gravitating system is heated up one may wonder why is gravitational collapse not halted in all cases at a sufficient high temperature establishing either a gravo-thermal equilibrium or explosion. Here, based on the equivalence between mass and energy, we show that there always exists a temperature threshold beyond which the gravitation of thermal energy overcomes its stabilizing pressure and the system collapses under the weight of its own heat.Cancellation mechanism of dark matter direct detection in Higgs-portal and vector-portal modelshttps://zbmath.org/1521.830622023-11-13T18:48:18.785376Z"Cai, Chengfeng"https://zbmath.org/authors/?q=ai:cai.chengfeng"Zeng, Yu-Pan"https://zbmath.org/authors/?q=ai:zeng.yu-pan"Zhang, Hong-Hao"https://zbmath.org/authors/?q=ai:zhang.honghaoSummary: We present two alternative proofs for the cancellation mechanism in the U(1) symmetric pseudo-Nambu-Goldstone-Boson Dark Matter (pNGB DM) model. They help us to have a better understanding of the mechanism from multi-angle, and inspire us to propose some interesting generalizations. In the first proof, we revisit the non-linear representation method and rephrase the argument with the interaction eigenstates. In this picture, the phase mode (DM) can only have a trilinear interaction with a derivative-squared acting on the radial mode when the DM is on-shell. Thus, the DM-quark scattering generated by a mass mixing between the radial mode and the Higgs boson vanishes in the limit of zero-momentum transfer. Using the same method, we can easily generalize the model to an SO(N) model with general soft-breaking structures. In particular, we study the soft-breaking cubic terms and identify those terms which preserve the cancellation mechanism for the DM candidate. In our discussion of the second method, we find that the cancellation relies on the special structure of mass terms and interactions of the mediators. This condition can be straightforwardly generalized to the vector-portal models. We provide two examples of the vector-portal case where the first one is an \(\mathrm{SU}(2)_L\times\mathrm{U}(1)_Y\times\mathrm{U}(1)_X\) model and the second one is an \(\mathrm{SU}(2)_L\times\mathrm{U}(1)_Y \times\mathrm{U}(1)_{B-L}\times\mathrm{U}(1)_X\) model. In the first model the vector mediators are the \(Z_\mu\) boson and a new \(\mathrm{U}(1)_X\) gauge boson \(X_\nu\), while in the second model the mediators are the \(\mathrm{U}(1)_{B-L}\) and \(\mathrm{U}(1)_X\) gauge bosons. The cancellation mechanism works in both models when there are no generic kinetic mixing terms for the gauge bosons. Once the generic kinetic mixing terms are included, the first model requires a fine-tuning of the mixing parameter to avoid the stringent direct detection bound, while the second model can naturally circumvent it.Dark matter from a complex scalar singlet: the role of dark CP and other discrete symmetrieshttps://zbmath.org/1521.830632023-11-13T18:48:18.785376Z"Coito, Leonardo"https://zbmath.org/authors/?q=ai:coito.leonardo"Faubel, Carlos"https://zbmath.org/authors/?q=ai:faubel.carlos"Herrero-García, Juan"https://zbmath.org/authors/?q=ai:herrero-garcia.juan"Santamaria, Arcadi"https://zbmath.org/authors/?q=ai:santamaria.arcadiSummary: We study the case of a pseudo-scalar dark matter candidate which emerges from a complex scalar singlet, charged under a global U(1) symmetry, which is broken both explicitly and spontaneously. The pseudo-scalar is naturally stabilized by the presence of a remnant discrete symmetry: dark CP. We study and compare the phenomenology of several simplified models with only one explicit symmetry breaking term. We find that several regions of the parameter space are able to reproduce the observed dark matter abundance while respecting direct detection and invisible Higgs decay limits: in the resonances of the two scalars, featuring the known as forbidden or secluded dark matter, and through non-resonant Higgs-mediated annihilations. In some cases, combining different measurements would allow one to distinguish the breaking pattern of the symmetry. Moreover, this setup admits a light DM candidate at the sub-GeV scale. We also discuss the situation where more than one symmetry breaking term is present. In that case, the dark CP symmetry may be spontaneously broken, thus spoiling the stability of the dark matter candidate. Requiring that this does not happen imposes a constraint on the allowed parameter space. Finally, we consider an effective field theory approach valid in the pseudo-Nambu-Goldstone boson limit and when the U(1) breaking scale is much larger than the electroweak scale.Dark energy constraint on equation of state parameter in the Weyl type \(f(Q,T)\) gravityhttps://zbmath.org/1521.830682023-11-13T18:48:18.785376Z"Gadbail, Gaurav N."https://zbmath.org/authors/?q=ai:gadbail.gaurav-n"Arora, Simran"https://zbmath.org/authors/?q=ai:arora.simran"Sahoo, P. K."https://zbmath.org/authors/?q=ai:sahoo.pradyumn-kumarSummary: The equation of state parameter is a significant method for characterizing dark energy models. We investigate the evolution of the equation of state parameter with redshift using a Bayesian analysis of recent observational datasets (the Cosmic Chronometer data (CC) and Pantheon samples). The Chevallier-Polarski-Linder parametrization of the effective equation of state parameter, \(\omega_{eff}=\omega_0+\omega_a\left(\frac{z}{1+z}\right)\), where \(\omega_0\) and \(\omega_a\) are free constants, is confined to the Weyl type \(f(Q,T)\) gravity, where \(Q\) represents the non-metricity and \(T\) is the trace of the energy-momentum tensor. We observe the evolution of the deceleration parameter \(q\), the density parameter \(\rho\), the pressure \(p\), and the effective equation of state parameter \(\omega\). The cosmic data limit for \(\omega\) does not exclude the possibility of \(\omega<-1\). It is seen that the parameter \(\omega\) shows a transition from deceleration to acceleration, as well as a shift from \(\omega>-1\) to \(\omega<-1\).The gravito-electromagnetic approximation to the gravimagnetic dipole and its velocity rotation curvehttps://zbmath.org/1521.830702023-11-13T18:48:18.785376Z"Govaerts, Jan"https://zbmath.org/authors/?q=ai:govaerts.janSummary: In view of the observed flat rotation curves of spiral galaxies and motivated by the simple fact that within Newtonian gravity a stationary axisymmetric mass distribution or dark matter vortex of finite extent readily displays a somewhat flattened out velocity rotation curve up to distances comparable to the extent of such a vortex transverse to the galaxy's disk, the possibility that such a flattening out of rotation curves may rather be a manifestation of some stationary axisymmetric space-time curvature of purely gravitational character, without the need of some dark matter particles, is considered in the case of the gravimagnetic dipole carrying opposite
Newman-Unti-Tamburino charges and in the tensionless limit of its Misner string, as an exact vacuum solution to Einstein's equations. Aiming for a first assessment of the potential of such a suggestion easier than a full-fledged study of its geodesics, the situation is analysed within the limits of weak field gravito-electromagnetism and nonrelativistic dynamics. Thereby leading indeed to interesting and encouraging results.Configurational entropy of black hole quantum coreshttps://zbmath.org/1521.830972023-11-13T18:48:18.785376Z"Casadio, R."https://zbmath.org/authors/?q=ai:casadio.roberto|casadio.rita"da Rocha, R."https://zbmath.org/authors/?q=ai:da-rocha.rolado|da-rocha.roldao-jun"Meert, P."https://zbmath.org/authors/?q=ai:meert.p"Tabarroni, L."https://zbmath.org/authors/?q=ai:tabarroni.l"Barreto, W."https://zbmath.org/authors/?q=ai:barreto.wendell-p|barreto.w-oSummary: Two types of information entropy are studied for the quantum states of a model for the matter core inside a black hole geometry. A detailed description is first given of the quantum mechanical picture leading to a spectrum of bound states for a collapsing ball of dust in general relativity with a non-trivial ground state. Information entropies are then computed, shedding new light on the stability of the ground state and the spectrum of higher excited states.Late-time post-merger modeling of a compact binary: effects of relativity, r-process heating, and treatment of transporthttps://zbmath.org/1521.831152023-11-13T18:48:18.785376Z"Haddadi, Milad"https://zbmath.org/authors/?q=ai:haddadi.milad"Duez, Matthew D."https://zbmath.org/authors/?q=ai:duez.matthew-d"Foucart, Francois"https://zbmath.org/authors/?q=ai:foucart.francois"Ramirez, Teresita"https://zbmath.org/authors/?q=ai:ramirez.teresita"Fernández, Rodrigo"https://zbmath.org/authors/?q=ai:fernandez.rodrigo-nobre"Knight, Alexander L."https://zbmath.org/authors/?q=ai:knight.alexander-l"Jesse, Jerred"https://zbmath.org/authors/?q=ai:jesse.jerred"Hébert, Francois"https://zbmath.org/authors/?q=ai:hebert.francois"Kidder, Lawrence E."https://zbmath.org/authors/?q=ai:kidder.lawrence-e"Pfeiffer, Harald P."https://zbmath.org/authors/?q=ai:pfeiffer.harald-p"Scheel, Mark A."https://zbmath.org/authors/?q=ai:scheel.mark-aSummary: Detectable electromagnetic counterparts to gravitational waves from compact binary mergers can be produced by outflows from the black hole-accretion disk remnant during the first 10 s after the merger. Two-dimensional axisymmetric simulations with effective viscosity remain an efficient and informative way to model this late-time post-merger evolution. In addition to the inherent approximations of axisymmetry and modeling turbulent angular momentum transport by a viscosity, previous simulations often make other simplifications related to the treatment of the equation of state and turbulent transport effects. In this paper, we test the effect of these modeling choices. By evolving with the same viscosity the exact post-merger initial configuration previously evolved in Newtonian viscous hydrodynamics, we find that the Newtonian treatment provides a good estimate of the disk ejecta mass but underestimates the outflow velocity. We find that the inclusion of heavy nuclei causes a notable increase in ejecta mass. An approximate inclusion of r-process effects has a comparatively smaller effect, except for its designed effect on the composition.
Diffusion of composition and entropy, modeling turbulent transport effects, has the overall effect of reducing ejecta mass and giving it a speed with lower average and more tightly-peaked distribution. Also, we find significant acceleration of outflow even at distances beyond 10 000 km, so that thermal wind velocities only asymptote beyond this radius and at higher values than often reported.Adaptive polygon rendering for interactive visualization in the Schwarzschild spacetimehttps://zbmath.org/1521.831432023-11-13T18:48:18.785376Z"Müller, Thomas"https://zbmath.org/authors/?q=ai:muller.thomas.5"Schulz, Christoph"https://zbmath.org/authors/?q=ai:schulz.christoph.2"Weiskopf, Daniel"https://zbmath.org/authors/?q=ai:weiskopf.daniel(no abstract)Modified gravity with disappearing cosmological constanthttps://zbmath.org/1521.831702023-11-13T18:48:18.785376Z"Granda, L. N."https://zbmath.org/authors/?q=ai:granda.luis-norbertoSummary: New corrections to General Relativity are considered in the context of modified \(f(R)\) gravity, that satisfy cosmological and local gravity constraints. The proposed models behave asymptotically as \(R - 2\Lambda\) at large curvature and show the vanishing of the cosmological constant at the flat spacetime limit. The chameleon mechanism and thin shell restrictions for local systems were analyzed, and bounds on the models were found. The steepness of the deviation parameter \(m\) at late times leads to measurable signal of scalar-tensor regime in matter perturbations, that allows to detect departures form the \(\Lambda\)CDM model. The theoretical results for the evolution of the weighted growth rate \(f\sigma_8(z)\), from the proposed models, were analyzed.Static neutron stars perspective of quadratic and induced inflationary attractor scalar-tensor theorieshttps://zbmath.org/1521.831742023-11-13T18:48:18.785376Z"Oikonomou, V. K."https://zbmath.org/authors/?q=ai:oikonomou.vasilis-kSummary: This study focuses on the static neutron star perspective for two types of cosmological inflationary attractor theories, namely the induced inflationary attractors and the quadratic inflationary attractors. The two cosmological models can be discriminated cosmologically, since one of the two does not provide a viable inflationary phenomenology, thus in this paper we investigate the predictions of these theories for static neutron stars, mainly focusing on the mass and radii of neutron stars. We aim to demonstrate that although the models have different inflationary phenomenology, the neutron star phenomenology predictions of the two models are quite similar. We solve numerically the Tolman-Oppenheimer-Volkoff equations in the Einstein frame using a powerful double shooting numerical technique, and after deriving the mass-radius graphs for three types of polytropic equations of state, we derive the Jordan frame mass and radii. With regard the equations of state we use polytropic equation of state with the small density part being either the Wiringa-Fiks-Fabrocini, the Akmal-Pandharipande-Ravenhall or the intermediate stiffness equation of state Skyrme-Lyon (SLy). The results of our models will be confronted with quite stringent recently developed constraints on the radius of neutron stars with specific mass.
As we show, the only equation of state which provides results compatible with the constraints is the SLy, for both the quadratic and induced inflation attractors. Thus nowadays, scalar-tensor descriptions of neutron stars are quite scrutinized due to the growing number of constraining observations, which eventually may also constrain theories of inflation.Computation of gravitational particle production using adiabatic invariantshttps://zbmath.org/1521.832042023-11-13T18:48:18.785376Z"Basso, Edward E."https://zbmath.org/authors/?q=ai:basso.edward-e"Chung, Daniel J. H."https://zbmath.org/authors/?q=ai:chung.daniel-j-hSummary: Analytic and numerical techniques are presented for computing gravitational production of scalar particles in the limit that the inflaton mass is much larger than the Hubble expansion rate at the end of inflation. These techniques rely upon adiabatic invariants and time modeling of a typical inflaton field which has slow and fast time variation components. A faster computation time for numerical integration is achieved via subtraction of slowly varying components that are ultimately exponentially suppressed. The fast oscillatory remnant results in production of scalar particles with a mass larger than the inflationary Hubble expansion rate through a mechanism analogous to perturbative particle scattering. An improved effective Boltzmann collision equation description of this particle production mechanism is developed. This model allows computation of the spectrum using only adiabatic invariants, avoiding the need to explicitly solve the inflaton equations of motion.Cosmology with viscous generalized Chaplygin gas in \(f(Q)\) gravityhttps://zbmath.org/1521.832082023-11-13T18:48:18.785376Z"Gadbail, Gaurav N."https://zbmath.org/authors/?q=ai:gadbail.gaurav-n"Arora, Simran"https://zbmath.org/authors/?q=ai:arora.simran"Sahoo, P. K."https://zbmath.org/authors/?q=ai:sahoo.pradyumn-kumarSummary: We use the hybrid model of bulk viscosity and generalized chaplygin gas (GCG), named the viscous generalized chaplygin gas (VGCG) model, which is thought to be an alternate dark fluid of the universe. We explore the dynamics of the VGCG model in the framework of the non-metricity \(f(Q)\) gravity using the functional form \(f(Q)=\beta Q^n\), where \(\beta\) and \(n\) are arbitrary constants. For the purpose of constraining model parameters, we use recent observational datasets such as Observational Hubble data, Baryon Acoustic Oscillations, and Type \(I a\) supernovae data. According to our study, the evolution of the deceleration parameter \(q\) and the equation of state (EoS) parameter \(w\) shows a transition from deceleration to an acceleration phase and its deviation from the \(\Lambda\) CDM model.Warm inflation, neutrinos and dark matter: a minimal extension of the standard modelhttps://zbmath.org/1521.832132023-11-13T18:48:18.785376Z"Levy, Miguel"https://zbmath.org/authors/?q=ai:levy.miguel"Rosa, João G."https://zbmath.org/authors/?q=ai:rosa.joao-g"Ventura, Luís B."https://zbmath.org/authors/?q=ai:ventura.luis-bSummary: We show that warm inflation can be realized within a minimal extension of the Standard Model with three right-handed neutrinos, three complex scalars and a gauged lepton/B-L U(1) symmetry. This simple model can address all the shortcomings of the Standard Model that are not related to fine-tuning, within general relativity, with distinctive experimental signatures that can be probed in the near future. The inflaton field emerges from the collective breaking of the U(1) symmetry, and interacts with two of the right-handed neutrinos, sustaining a high-temperature radiation bath during inflation. The discrete interchange symmetry of the model protects the scalar potential against large thermal corrections and leads to a stable inflaton remnant at late times which can account for dark matter. Consistency of the model and agreement with Cosmic Microwave Background observations naturally yield light neutrino masses below 0.1 eV, while thermal leptogenesis occurs naturally after a smooth exit from inflation into the radiation era.Missing scalars at the cosmological colliderhttps://zbmath.org/1521.832142023-11-13T18:48:18.785376Z"Lu, Qianshu"https://zbmath.org/authors/?q=ai:lu.qianshu"Reece, Matthew"https://zbmath.org/authors/?q=ai:reece.matthew"Xianyu, Zhong-Zhi"https://zbmath.org/authors/?q=ai:xianyu.zhong-zhiSummary: Light scalar fields typically develop spatially varying backgrounds during inflation. Very often they do not directly affect the density perturbations, but interact with other fields that do leave nontrivial signals in primordial perturbations. In this sense they become ``missing scalars'' at the cosmological collider. We study potentially observable signals of these missing scalars, focusing on a special example where a missing scalar distorts the usual oscillatory features in the squeezed bispectrum. The distortion is also a useful signal distinguishing the de Sitter background induced thermal mass from a constant intrinsic mass.Possible discrepancies between cosmological and electroweak observables in Higgs inflationhttps://zbmath.org/1521.832172023-11-13T18:48:18.785376Z"Rodrigues, Jamerson G."https://zbmath.org/authors/?q=ai:rodrigues.jamerson-g"Benetti, Micol"https://zbmath.org/authors/?q=ai:benetti.micol"Alcaniz, Jailson S."https://zbmath.org/authors/?q=ai:alcaniz.jailson-sSummary: In this work, we revisit the non-minimally coupled Higgs Inflation scenario and investigate its observational viability in light of the current Cosmic Microwave Background, Baryon Acoustic Oscillation and type Ia Supernovae data. We explore the effects of the Coleman-Weinberg approximation to the Higgs potential in the primordial universe, connecting the predictions for the Lagrangian parameters at inflationary scales to the electroweak observables through Renormalization Group methods at two-loop order. Initially, we find that electroweak scale measurements may be dissonant to the limits obtained from the cosmological data sets used in the analysis. Specifically, an \(\approx 8 \sigma\)-discrepancy between the inflationary parameters and the value of the Monte Carlo reconstructed top quark mass is found. However, considering the most recent results obtained by the CMS Collaboration from differential cross-section measurements of the top quark production a good agreement is obtained.Improved DHOST genesishttps://zbmath.org/1521.832192023-11-13T18:48:18.785376Z"Zhu, Mian"https://zbmath.org/authors/?q=ai:zhu.mian"Zheng, Yunlong"https://zbmath.org/authors/?q=ai:zheng.yunlongSummary: We improve the DHOST Genesis proposed in [\textit{A. Ilyas} et al., J. High Energy Phys. 2021, No. 1, Paper No. 141, 22 p. (2021; Zbl 1459.85003)], such that the near scale invariant scalar power spectrum can be generated from the model itself, without invoking extra mechanism like a string gas. Besides, the superluminality problem of scalar perturbation plagued in [loc. cit.] can be rescued by choosing proper DHOST action.Strange quark star models from Rastall gravityhttps://zbmath.org/1521.850012023-11-13T18:48:18.785376Z"Banerjee, Ayan"https://zbmath.org/authors/?q=ai:banerjee.ayan"Tangphati, Takol"https://zbmath.org/authors/?q=ai:tangphati.takol"Hansraj, Sudan"https://zbmath.org/authors/?q=ai:hansraj.sudan"Pradhan, Anirudh"https://zbmath.org/authors/?q=ai:pradhan.anirudhSummary: We study the possible existence of strange matter in the interior of compact stars in Rastall gravity. The main feature of the Rastall gravity is the covariant derivative of the energy-momentum tensor does not vanish and depends on the curvature \(R\) and a free parameter \(\eta\). We consider the MIT bag model in the present work and numerically solve the modified Tolman-Oppenheimer-Volkoff (TOV) equations. We investigate the effect of the Rastall parameter \(\eta\) on the mass-radius and the mass-central energy density relation of quark stars. From the quark star masses and radii obtained, we conclude that \(\eta\) has significant consequences on the structure of stellar objects.Spin-orbit gravitational locking -- an effective potential approachhttps://zbmath.org/1521.850022023-11-13T18:48:18.785376Z"Clouse, Christopher"https://zbmath.org/authors/?q=ai:clouse.christopher"Ferroglia, Andrea"https://zbmath.org/authors/?q=ai:ferroglia.andrea"Fiolhais, Miguel C. N."https://zbmath.org/authors/?q=ai:fiolhais.miguel-c-nSummary: The objective of this paper is to study the tidally locked 3:2 spin-orbit resonance of Mercury around the Sun. In order to achieve this goal, the effective potential energy that determines the spinning motion of an ellipsoidal planet around its axis is considered. By studying the rotational potential energy of an ellipsoidal planet orbiting a spherical star on an elliptic orbit with fixed eccentricity and semi-major axis, it is shown that the system presents an infinite number of metastable equilibrium configurations. These states correspond to local minima of the rotational potential energy averaged over an orbit, where the ratio between the rotational period of the planet around its axis and the revolution period around the star is fixed. The configurations in which this ratio is an integer or an half integer are of particular interest. Among these configurations, the deepest minimum in the average potential energy corresponds to a situation where the rotational and orbital motion of the planet are synchronous, and the system is tidally locked. The next-to-the deepest minimum corresponds to the case in which the planet rotates three times around its axis in the time that it needs to complete two orbits around the Sun. The latter is indeed the case that describes Mercury's motion. The method discussed in this work allows one to identify the integer and half-integer ratios that correspond to spin-orbit resonances and to describe the motion of the planet in the resonant orbit.Oscillatory path integrals for radio astronomyhttps://zbmath.org/1521.850032023-11-13T18:48:18.785376Z"Feldbrugge, Job"https://zbmath.org/authors/?q=ai:feldbrugge.job"Pen, Ue-Li"https://zbmath.org/authors/?q=ai:pen.ueli"Turok, Neil"https://zbmath.org/authors/?q=ai:turok.neil-gSummary: We introduce a new method for evaluating the oscillatory integrals which describe natural interference patterns. As an illustrative example of contemporary interest, we consider astrophysical plasma lensing of coherent sources like pulsars and fast radio bursts in radio astronomy. Plasma lenses are known to occur near the source, in the interstellar medium, as well as in the solar wind and the earth's ionosphere. Such lensing is strongest at long wavelengths, hence it is generally important to go beyond geometric optics and into the full wave optics regime.
Our computational method is a spinoff of new techniques two of us, and our collaborators, have developed for defining and performing Lorentzian path integralswith applications in quantum mechanics, condensed matter physics, and quantum gravity. Cauchy's theorem allows one to transform a computationally fragile and expensive, highly oscillatory integral into an exactly equivalent sum of absolutely and rapidly convergent integrals which can be evaluated in polynomial time. We require only that it is possible to analytically continue the lensing phase, expressed in the integrated coordinates, into the complex domain. We give a first-principles derivation of the Fresnel-Kirchhoff integral, starting from Feynman's path integral for a massless particle in a refractive medium. We then demonstrate the effectiveness of our method by computing the detailed diffraction patterns of Thom's caustic catastrophes, both in their ``normal forms'' and within a variety of more realistic, local lens models, for all wavelengths. Our numerical method, implemented in a freely downloadable code, provides a fast, accurate tool for modeling interference patterns in radioastronomy and other fields of physics.Study of decoupled gravastars in energy-momentum squared gravityhttps://zbmath.org/1521.850042023-11-13T18:48:18.785376Z"Sharif, M."https://zbmath.org/authors/?q=ai:sharif.muhammad"Naz, Saba"https://zbmath.org/authors/?q=ai:naz.sabaSummary: In this paper, we generate an exact anisotropic gravastar model using gravitational decoupling technique through minimal geometric deformation in the framework of \(f(\Re,T^2)\) gravity. This novel model explains an ultra-compact stellar configuration whose internal region is smoothly matched to the exterior region. The developed stellar model satisfies some of the essential characteristics of a physically acceptable model such as a positive monotonically decreasing profile of energy density from the center to the boundary and monotonically decreasing behavior of the pressure. The anisotropic factor and Schwarzschild spacetime follows physically acceptable behavior. We find that all the energy bounds are satisfied except strong energy condition inside the ultra-compact stellar structure for the coupling constant of this theory, which is compatible with the regularity condition.Retrograde equatorial surface flows generated by thermal convection confined under a stably stratified layer in a rapidly rotating spherical shellhttps://zbmath.org/1521.850052023-11-13T18:48:18.785376Z"Takehiro, Shin-Ichi"https://zbmath.org/authors/?q=ai:takehiro.shin-ichi"Yamada, Michio"https://zbmath.org/authors/?q=ai:yamada.michio"Hayashi, Yoshi-Yuki"https://zbmath.org/authors/?q=ai:hayashi.yoshi-yukiSummary: Finite-amplitude thermal convection in a rapidly rotating spherical shell associated with a stably stratified layer placed near the outer surface is investigated. Systematic numerical experiments are performed with an Ekman number of \(E = 10^{-3}\), a Prandtl number of \(P = 1\) and an inner/outer radius ratio of \(\eta = 0.4\), and the existence of a strongly stratified upper layer is shown to enhance the generation of equatorial surface retrograde flows when the Rayleigh number is approximately ten times larger than the critical value. The existence of the stable layer causes the bottom of the stable layer to behave as a virtual boundary for the convective motion underneath. Its effective dynamic condition varies from the free-slip condition to the no-slip condition as the Rayleigh number increases. The Reynolds stress of the convective vortices beneath the stable layer is weakened and is dominated by the transport of the planetary angular momentum. As a result, the latitudinal temperature gradient produced at the bottom of the stable layer induces the equatorial retrograde flow through the thermal wind balance. This diffuses through the stable layer by viscosity and produces the equatorial surface retrograde flow.A Reynolds-averaged turbulence modelling approach to the maintenance of the Venus superrotationhttps://zbmath.org/1521.850062023-11-13T18:48:18.785376Z"Yoshizawa, A."https://zbmath.org/authors/?q=ai:yoshizawa.akira"Kobayashi, H."https://zbmath.org/authors/?q=ai:kobayashi.hiroshi|kobayashi.honoka|kobayashi.hirotatsu|kobayashi.hirotomo|kobayashi.harutoshi|kobayashi.hisanori|kobayashi.hirotaka|kobayashi.hiroyuki|kobayashi.hidetsune|kobayashi.hirotada|kobayashi.hiroki|kobayashi.hiromichi|kobayashi.hisato-p|kobayashi.hideaki|kobayashi.hideki|kobayashi.hiroaki|kobayashi.hajime|kobayashi.hisashi|kobayashi.hideo|kobayashi.hiroto|kobayashi.hirokazu"Sugimoto, N."https://zbmath.org/authors/?q=ai:sugimoto.naoyuki|sugimoto.norikazu|sugimoto.noriko|sugimoto.nobumasa|sugimoto.naozo|sugimoto.norihiko"Yokoi, N."https://zbmath.org/authors/?q=ai:yokoi.nobumitsu"Shimomura, Y."https://zbmath.org/authors/?q=ai:shimomura.yoshiki|shimomura.yutaka|shimomura.youichiSummary: A maintenance mechanism of an approximately linear velocity profile of the Venus zonal flow or superrotation is explored, with the aid of a Reynolds-averaged turbulence modelling approach. The basic framework is similar to that of \textit{P. J. Gierasch} [Meridional circulation and maintenance of the Venus atmospheric rotation. J. Atmos. Sci. 32, 1038--1044 (1975)] in the sense that the mechanism is examined under a given meridional circulation. The profile mimicking the observations of the flow is initially assumed, and its maintenance mechanism in the presence of turbulence effects is investigated from a viewpoint of the suppression of energy cascade. In the present work, the turbulent viscosity is regarded as an indicator of the intensity of the cascade. A novelty of this formalism is the use of the isotropic turbulent viscosity based on a non-local time scale linked to a large-scale flow structure. The mechanism is first discussed qualitatively. On the basis of these discussions, the two-dimensional numerical simulation of the proposed model is performed, with an initially assumed superrotation, and the fast zonal flow is shown to be maintained, compared with the turbulent viscosity lacking the non-local time scale. The relationship of the present model with the current general circulation model simulation is discussed in light of a crucial role of the vertical viscosity.Modeling of pitch angle distribution on the dayside of the Earth's magnetospherehttps://zbmath.org/1521.850072023-11-13T18:48:18.785376Z"Smolin, Sergeĭ V."https://zbmath.org/authors/?q=ai:smolin.sergei-vSummary: The mathematical model of pitch-angle diffusion in the magnetosphere which allows to compute theoretically density of phase space or pitch angle distribution of the charged particles is offered. On concrete examples of pitch angle distribution evolution during the 1--7 May 1998 geomagnetic storm within its last 68 hours potential opportunities of offered model of pitch-angle diffusion on the dayside of the Earth's magnetosphere are shown.The dynamics of two interacting compositional plumes in the presence of a magnetic fieldhttps://zbmath.org/1521.850082023-11-13T18:48:18.785376Z"Al-Lawatia, M. A."https://zbmath.org/authors/?q=ai:al-lawatia.m-a"Elbashir, T. B. A."https://zbmath.org/authors/?q=ai:elbashir.t-b-a"Eltayeb, I. A."https://zbmath.org/authors/?q=ai:eltayeb.ibrahim-a"Rahman, M. M."https://zbmath.org/authors/?q=ai:rahman.m-muhammad-mahboob-ur|rahman.m-mahibbur|rahman.md-mizanor|rahman.m-m-hafizur|rahman.md-mostafizur|rahman.md-muktadir|rahman.m-mahbubur|rahman.mohammad-muntasir|rahman.md-muklesur|rahman.mohammad-mansur|rahman.mohammad-mafizur|rahman.md-mazder|rahman.m-matiar|rahman.md-mizanur|rahman.md-monsur|rahman.mohammad-mamunur|rahman.m-majedur|rahman.muhammad-m|rahman.md-mashiar|rahman.mohammad-mahabubur|rahman.md-motiur|rahman.md-mijanur|rahman.md-mahmudur|rahman.m-mazibar"Balakrishnan, E."https://zbmath.org/authors/?q=ai:balakrishnan.easwaranSummary: The dynamics of two compositionally buoyant columns of fluid rising in an infinite less buoyant fluid is studied in the presence of a uniform magnetic field, \(\mathbf{B}_0\). The fluid is thermally stably stratified and has a viscosity, \(\nu\), a thermal diffusivity, \(\kappa\) and magnetic diffusivity, \(\eta\). The stability of the mean state to infinitesimal disturbances is governed by the seven dimensionless parameters: the Reynolds number, \(R\) (\(= UL/\nu\), where \(U\), \(L\) are characteristic velocity and length respectively) which measures the strength of the compositional buoyancy; the dimensionless measures \(x_0\), \(x_1\), \(d\) of the thickness of the two plumes and the distance between them, respectively; the ratio \(\Gamma\) of the strengths of the two plumes (as measured by their basic concentration of light material); the Chandrasekhar number, \(Qc\) (\(= B_0^2L^2/\mu\rho_0\eta\nu\), in which \(\mu\) is the magnetic permeability, \(\rho_0\) the fluid density and \(B_0\) a characteristic unit of magnetic field), is a measure of the magnitude of the magnetic field and the normalized horizontal projection \(\hat{B}_H = \sin\theta\) of the magnetic field, where \(\theta\) measures the inclination of the magnetic field to the vertical. The stability is examined for small values of \(R\). The preferred mode of instability is studied in the parameter space \((x_0, x_1, d, \Gamma, Qc, \hat{B}_H)\). It is shown that the influence of the magnetic field does not change the order of the magnitude of the growth rate from \(\mathrm{O}(R^0)\) of the two non-magnetic interacting plumes and it does not introduce any new modes to the stability problem. However, the presence of the magnetic field introduces novel features to the stability problem. For any fixed set \(x_0\), \(x_1\), \(d\), \(\Gamma\), \(Qc\), the growth rate can either increase with \(\hat{B}_H\) or initially decrease reaching a minimum before it increases again. As \(Qc\) increases, with \(x_0\), \(x_1\), \(d\), \(\Gamma\), \(\hat{B}_H\) fixed, the growth rate can assume one of four different behaviour: (i) it maintains the same value of the non-magnetic case with the disturbance propagating along field lines; (ii) it decreases steadily with \(Qc\); (iii) it maintains the same value as in the absence of the field until a value \(Qcm(x_0, x_1, d, \Gamma, \hat{B}_H)\) is reached when it starts to increase to a maximum before it decreases to zero for large values of \(Qc\) and (iv) it increases from its value for \(Qc = 0\) reaching a maximum before it decreases steadily to zero at some value of \(Qc\) dependent on the other parameters. The helicity and \(\alpha\)-effect have also been studied to find that the unstable motions can produce mean helicity and \(\alpha\)-effect.Convection-driven kinematic dynamos with a self-consistent shear flowhttps://zbmath.org/1521.850092023-11-13T18:48:18.785376Z"Currie, L. K."https://zbmath.org/authors/?q=ai:currie.l-k"Tobias, S. M."https://zbmath.org/authors/?q=ai:tobias.steven-mSummary: It is widely accepted that astrophysical magnetic fields are generated by dynamo action. In many cases, these fields exhibit organisation on a scale larger than that of the underlying turbulent flow (e.g. the 11-year solar cycle). The mechanism for the generation of so-called large-scale fields remains an open problem. In cases where the magnetic Reynolds number (\(Rm\)) is small, dynamo-generated fields are coherent but at (the astrophysically relevant) high \textit{Rm}, the fields are overwhelmed by small-scale fluctuating field. Recently Tobias and Cattaneo have shown that an imposed large-scale shear flow can suppress the small-scale fluctuations and allow the large-scale temporal behaviour to emerge. Shear is also believed to modify the electromotive force by introducing correlations between the flow and the field. However, in previous models at high \(Rm\) the shear is often artificially imposed or driven by an arbitrary body force. Here we consider a simple kinematic model of a convective dynamo in which shear is self-consistently driven by the presence of a horizontal temperature gradient (resulting in a thermal wind) and a rotation vector that is oblique to gravity. By considering a 2.5-dimensional system, we are able to reach high \(Rm\) so that the dynamo approaches the asymptotic regime where the growth rate becomes approximately independent of \textit{Rm}. We find the flows studied here to be excellent small-scale dynamos, but with very little systematic behaviour evident at large \(Rm\). We attribute this to being unable to self-consistently generate flows with both large (net) helicity and strong shear in this setup.Convection in rotating spherical fluid shells with inhomogeneous heat flux at the outer boundaryhttps://zbmath.org/1521.850102023-11-13T18:48:18.785376Z"Gibbons, S. J."https://zbmath.org/authors/?q=ai:gibbons.s-j"Gubbins, D."https://zbmath.org/authors/?q=ai:gubbins.david"Zhang, K."https://zbmath.org/authors/?q=ai:zhang.kekeSummary: The Earth's core is subject to a laterally varying heat flux at the outer boundary, which may account for correlations between the geomagnetic field and lower mantle structure. Studies of nonmagnetic, rotating convection in a spherical shell with fixed temperature boundary conditions have revealed flows resonating with, or locked to, the boundary anomalies when the length scales of the convection are close to those of the boundary condition. Here we study a similar system but for fixed heat flux upper boundary conditions, as in the Earth's core. We first map out the onset of thermal instability in a rotating shell of aspect ratio 0.4 for uniform outer boundary cooling with both rigid and stress-free boundaries. A preference for large scale (azimuthal wavenumber \(m = 1\)) flows, not observed for the uniform temperature case, persists to Ekman numbers down to almost \(10^{-4}\). The preference for large scales is greatest for rigid boundaries and high (\(\geq 1\)) Prandtl numbers. Hemispheric asymmetry appears in the weakly nonlinear regime, with small scale columnar convection coexisting with larger scale flows. We next study the effect of heterogeneous cooling of strength \(\epsilon\) proportional to the \(Y_2^2\) spherical harmonic, which resembles the assumed heat flow at the core-mantle boundary. We illustrate the results with two sets of parameters, (1) when the most unstable mode in the uniform case is \(m = 2\), the same as the heterogeneous boundary condition, and (2) when it is \(m = 1\). We follow the numerical solutions from steady flows dominated by boundary heating, through periodic flows drifting at non-uniform rates, to chaotic flows. In case (1) both thermal convection and boundary-driven flow are dominated by the same azimuthal wavenumber \(m = 2\) as the boundary condition; they give way to periodic flows of the same symmetry (even \(m\)) at low boundary heterogeneity. At higher \(\epsilon\) the symmetry is broken as modes with odd \(m\) are excited. In case (2) at low \(\epsilon\) the \(m = 1\) and \(m = 2\) modes compete, while higher \(\epsilon\) imposes an \(m = 2\) symmetry. Boundary effects depend strongly on the most unstable wavenumber at onset of convection with uniform boundary cooling: these simple linear results are a good guide to the probable behaviour of more complex, nonlinear regimes and have already been used to find suitable parameter ranges in a geodynamo calculation.Dissipative structures in a nonlinear dynamohttps://zbmath.org/1521.850112023-11-13T18:48:18.785376Z"Gilbert, Andrew D."https://zbmath.org/authors/?q=ai:gilbert.andrew-d"Ponty, Yannick"https://zbmath.org/authors/?q=ai:ponty.yannick"Zheligovsky, Vladislav"https://zbmath.org/authors/?q=ai:zheligovsky.vladislav-aSummary: This article considers magnetic field generation by a fluid flow in a system referred to as the Archontis dynamo: a steady nonlinear MHD state is driven by a prescribed body force. The field and flow become almost equal and dissipation is concentrated in cigar-like structures centred on straight-line separatrices. Numerical scaling laws for energy and dissipation are given that extend previous calculations to smaller diffusivities. The symmetries of the dynamo are set out, together with their implications for the structure of field and flow along the separatrices. The scaling of the cigar-like dissipative regions, as the square root of the diffusivities, is explained by approximations near the separatrices. Rigorous results on the existence and smoothness of solutions to the steady, forced MHD equations are given.Effects of a subadiabatic layer on convection and dynamos in spherical wedge simulationshttps://zbmath.org/1521.850122023-11-13T18:48:18.785376Z"Käpylä, P. J."https://zbmath.org/authors/?q=ai:kapyla.petri-j"Viviani, M."https://zbmath.org/authors/?q=ai:viviani.michele|viviani.marco|viviani.milo"Käpylä, M. J."https://zbmath.org/authors/?q=ai:kapyla.maarit-j"Brandenburg, A."https://zbmath.org/authors/?q=ai:brandenburg.axel"Spada, F."https://zbmath.org/authors/?q=ai:spada.fSummary: We consider the effect of a subadiabatic layer at the base of the convection zone on convection itself and the associated large-scale dynamos in spherical wedge geometry. We use a heat conduction prescription based on the Kramers opacity law which allows the depth of the convection zone to dynamically adapt to changes in the physical characteristics such as rotation rate and magnetic fields. We find that the convective heat transport is strongly concentrated towards the equatorial and polar regions in the cases without a substantial radiative layer below the convection zone. The presence of a stable layer below the convection zone significantly reduces the anisotropy of radial enthalpy transport. Furthermore, the dynamo solutions are sensitive to subtle changes in the convection zone structure. We find that the kinetic helicity changes sign in the deeper parts of the convection zone at high latitudes in all runs. This region expands progressively towards the equator in runs with a thicker stably stratified layer.Hydromagnetic waves in a thin rotating spherical shellhttps://zbmath.org/1521.850132023-11-13T18:48:18.785376Z"London, Steven D."https://zbmath.org/authors/?q=ai:london.steven-dSummary: We consider an electrically conducting fluid confined to a thin rotating spherical shell in which the Elsasser and magnetic Reynolds numbers are assumed to be large while the Rossby number is assumed to vanish in an appropriate limit. This may be taken as a simple model for a possible stable layer at the top of the Earth's outer core. It may also be a model for the thin shells which are thought to be a source of the magnetic fields of some planets such as Mercury or Uranus. Linear hydromagnetic waves are studied using a multiple scale asymptotic scheme in which boundary layers and the associated boundary conditions determine the structure of the waves. These waves are assumed to be of the form of an asymptotic series expanded about an ambient magnetic field which vanishes on the equatorial plane and velocity and pressure fields which do not. They take the form of short wave, slowly varying wave trains. The results are compared to the author's previous work on such waves in cylindrical geometry in which the boundary conditions play no role. The approximation obtained is significantly different from that obtained in the previous work in that an essential singularity appears at the equator and nonequatorial wave regions appear.Torus dynamo in the outer rings of galaxieshttps://zbmath.org/1521.850142023-11-13T18:48:18.785376Z"Mikhailov, E. A."https://zbmath.org/authors/?q=ai:mikhailov.evgenii-aleksandrovich"Khokhryakova, A. D."https://zbmath.org/authors/?q=ai:khokhryakova.a-dSummary: The magnetic fields in the inner parts of some spiral galaxies are understood quite well. Their generation is connected with the dynamo mechanism that is based on the joint action of turbulent diffusion and the \(\alpha\)-effect. Usually the galactic dynamo is described with the so-called no-\(z\) approximation which takes into account that the galaxy disc is quite thin, with the implication that some spatial derivatives may be replaced by algebraic expressions. Some galaxies have outer rings that are situated at some distance from the galactic centre. The magnetic field can be described there also using the no-\(z\) model. As the thickness of such objects is comparable with their width, it is necessary to take into account the \(z\)-dependence of the field. We have studied the magnetic field evolution using the no-\(z\) approximation and torus dynamo model for the torus with rectangular cross-section in the axisymmetric case.Chaotic behaviour in low-order models of planetary and stellar dynamoshttps://zbmath.org/1521.850152023-11-13T18:48:18.785376Z"Weiss, N. O."https://zbmath.org/authors/?q=ai:weiss.nigel-oSummary: The behaviour of the geodynamo and the solar cycle can be modelled by low-order systems of coupled nonlinear differential equations. The Earth's magnetic field reverses aperiodically, and similar behaviour is exhibited by disc dynamos that are described by the Lorenz equations. Chaotic behaviour is also a characteristic feature of coupled disc dynamos. In stars like the Sun, magnetic activity varies cyclically, with regular reversals of magnetic fields, but the cyclic activity is modulated on longer timescales. This behaviour can be described by normal form equations that account for symmetry-breaking as well as for variations in amplitude. The Von Kármán Sodium (VKS) experiment has successfully demonstrated magnetic reversals in the laboratory, and these results can be represented by evolution equations also.The small-scale dynamo in a spectral representationhttps://zbmath.org/1521.850162023-11-13T18:48:18.785376Z"Yushkov, E."https://zbmath.org/authors/?q=ai:yushkov.egor-vladislavovich"Lukin, A."https://zbmath.org/authors/?q=ai:lukin.alexander-s"Sokoloff, D."https://zbmath.org/authors/?q=ai:sokolov.denis|sokolov.d-o|sokolov.dmitrii-dmitrievich|sokolov.d-a|sokolev.dmitry|sokolov.danil|sokolov.d-v"Frick, P."https://zbmath.org/authors/?q=ai:frick.p-g|frick.pieter-a|frick.peterSummary: In this paper, we study the correlation functions for magnetic energy and magnetic helicity generated by a small-scale dynamo mechanism in a mirror-asymmetric and isotropic turbulent conductive fluid. Solving the Kazantsev system for small-scale generation by an iterative approach, we restore the correlation functions in spatial and spectral representations. Studying scales of localisation and their dependence on magnetic Reynolds number, we propose approximate functions, which can be applied for energy and helicity in a spectral space. We show the absence of 3/2-spectrum for Kazantsev assumptions at \(k\ll 1\) and helicity sign changing at \(k\gg 1\). Using our results and estimating the relation between helicity and energy at different scales, we speculate about what can be considered as a small scale in a spectral space, and how this scale can be connected with mean-field processes.The continuous wavelet derived by smoothing function and its application in cosmologyhttps://zbmath.org/1521.850172023-11-13T18:48:18.785376Z"Wang, Yun"https://zbmath.org/authors/?q=ai:wang.yun"He, Ping"https://zbmath.org/authors/?q=ai:he.pingSummary: The wavelet analysis technique is a powerful tool and is widely used in broad disciplines of engineering, technology, and sciences. In this work, we present a novel scheme of constructing continuous wavelet functions, in which the wavelet functions are obtained by taking the first derivative of smoothing functions with respect to the scale parameter. Due to this wavelet constructing scheme, the inverse transforms are only one-dimensional integrations with respect to the scale parameter, and hence the continuous wavelet transforms (CWTs) constructed in this way are more ready to use than the usual scheme. We then apply the Gaussian-derived wavelet constructed by our scheme to computations of the density power spectrum for dark matter, the velocity power spectrum and the kinetic energy spectrum for baryonic fluid. These computations exhibit the convenience and strength of the CWTs. The transforms are very easy to perform, and we believe that the simplicity of our wavelet scheme will make CWTs very useful in practice.Mechanisms for magnetic field generation in precessing cubeshttps://zbmath.org/1521.860382023-11-13T18:48:18.785376Z"Goepfert, O."https://zbmath.org/authors/?q=ai:goepfert.o"Tilgner, A."https://zbmath.org/authors/?q=ai:tilgner.andreasSummary: It is shown that flows in precessing cubes develop at certain parameters large axisymmetric components in the velocity field which are large enough to either generate magnetic fields by themselves, or to contribute to the dynamo effect if inertial modes are already excited and acting as a dynamo. This effect disappears at small Ekman numbers. The critical magnetic Reynolds number also increases at low Ekman numbers because of turbulence and small-scale structures.The onset of thermo-compositional convection in rotating spherical shellshttps://zbmath.org/1521.860442023-11-13T18:48:18.785376Z"Silva, Luis"https://zbmath.org/authors/?q=ai:silva.luis-g-silva-e|silva.luis-m-a|silva.luis-nuno|silva.luis-o|silva.luis-f-p|silva.luis-carlos"Mather, James F."https://zbmath.org/authors/?q=ai:mather.james-f"Simitev, Radostin D."https://zbmath.org/authors/?q=ai:simitev.radostin-dSummary: Double-diffusive convection driven by both thermal and compositional buoyancy in a rotating spherical shell can exhibit a rather large number of behaviour often distinct from that of the single diffusive system. In order to understand how the differences in thermal and compositional molecular diffusivities determine the dynamics of thermo-compositional convection we investigate numerically the linear onset of convective instability in a double-diffusive setup. We construct an alternative equivalent formulation of the non-dimensional equations where the linearised double-diffusive problem is described by an effective Rayleigh number, Ra, measuring the amplitude of the combined buoyancy driving, and a second parameter, \(\alpha\), measuring the mixing of the thermal and compositional contributions. This formulation is useful in that it allows for the analysis of several limiting cases and reveals dynamical similarities in the parameters space which are not obvious otherwise. We analyse the structure of the critical curves in this Ra-\(\alpha\) space, explaining asymptotic behaviour in \(\alpha\), transitions between inertial and diffusive regimes, and transitions between large-scale (fast drift) and small-scale (slow drift) convection. We perform this analysis for a variety of diffusivities, rotation rates and shell aspect ratios showing where and when new modes of convection take place.