angular momentum problem parâse-ye jonbâk-e zâviye-yi Fr.: problème de moment angulaire 1) The fact that the Sun, which contains 99.9% of the mass of the
→ solar system, accounts for about 2% of the total
→ angular momentum of the solar system. The problem of outward
→ angular momentum transfer has been a main topic of interest for
models attempting to explain the origin of the solar system. |
cosmological constant problem parâse-ye pâyâ-ye keyhânšenâxti Fr.: problème de la constante cosmologique The impressive discrepancy of about 120 orders of magnitude between the theoretical value of the → cosmological constant and its observed value. → Quantum field theory interprets the cosmological constant as the density of the → vacuum energy. This density can be derived from the maximum energy at which the theory is valid, i.e. the → Planck energy scale (10^{18} GeV). The theoretical vacuum → energy density is (10^{18} GeV)^{4} = (10^{27} eV)^{4} = 10^{112} erg cm^{-3}. On the other hand, the observed vacuum energy density is estimated to be about (10^{-3} eV)^{4} = 10^{-8} erg cm^{-3}. There is, therefore, a discrepancy of about 120 orders of magnitude. → cosmological; → constant; → problem. |
cusp problem parâse-ye tizé Fr.: problème des cuspides A problem encountered by the → cold dark matter (CDM) model of galaxy formation. The numerical simulations with CDM predict a large concentration of dark matter in the center of galaxies, with a peaked density distribution, in contrast to the real, observed galaxies. See also: → angular momentum catastrophe; → missing dwarfs. |
existence problem parâse-ye hustumandi, ~ hasti Fr.: problème d'existence Math: The question of whether a → solution to a given → problem exists. |
Fermi-Pasta-Ulam problem parâse-ye Fermi-Pasta-Ulam Fr.: expérience Fermi-Pasta-Ulam A computer experiment that was aimed to study the → thermalization process of a → solid. In other words, the goal was to see whether there is an approximate → equipartition of energy in the system, which would mean that the motion is → chaotic. Using computer simulation, Fermi-Pasta-Ulam studied the behavior of a chain of 64 mass particles connected by → nonlinear springs. In fact, they were looking for a theoretical physics problem suitable for an investigation with one of the very first computers, the he MANIAC (Mathematical Analyzer, Numerical Integrator and Computer). They decided to study how a → crystal evolves toward → thermal equilibrium by simulating a chain of particles, linked by a quadratic interaction potential, but also by a weak nonlinear interaction. Fermi-Pasta-Ulam assumed that if the interaction in the chain were nonlinear, then an exchange of energy among the normal modes would occur, and this would bring forth the equipartition of energy, i.e. the thermalization. Contrary to expectations, the energy revealed no tendency toward equipartition. The system had a simple quasi-periodic behavior, and no → chaoticity was observed. This result, known as the Fermi-Pasta-Ulam paradox, shows that → nonlinearity is not enough to guarantee the equipartition of energy (see, e.g., Dauxois et al., 2005, Eur. J. Phys., 26, S3). E. Fermi, J. Pasta, S. Ulam, 1955, Los Alamos report LA-1940; → problem. |
flatness problem parâse-ye yaxti Fr.: problème de la platitude The observed fact that the → geometry of the → Universe is very nearly flat, in other words its density is very close to the → critical density. This would be an extreme coincidence because a → flat Universe is a special case. Many attempts have been made to explain the flatness problem, and modern theories now include the idea of → inflation. |
horizon problem parâse-ye ofoq Fr.: problème de l'horizon A problem with the standard cosmological model of the Big Bang related to the observational fact that regions of the Universe that are separated by vast distances nevertheless have nearly identical properties such as temperature. This contradicts the fact that light moves with a finite speed and, as a result, certain events which occur in the Universe are completely independent of each other. Inflationary cosmology offers a possible solution. |
Kepler problem parâse-ye Kepler Fr.: problème de Kepler 1) Given the trajectory of a particle moving in a → central force
field, determine the
law governing the central force. |
luminosity problem parâse-ye tâbandegi Fr.: problème de luminosité Low-mass → protostars are about an order of magnitude less luminous than expected. Two possible solutions are that → low-mass stars form slowly, and/or protostellar → accretion is episodic. The latter accounts for less than half the missing luminosity. The solution to this problem relates directly to the fundamental question of the time required to form a low-mass star (McKee & Offner, 2010, astro-ph/1010.4307). → luminosity; → problem. |
magnetic monopole problem parâse-ye takqotbe-ye meqnâtisi Fr.: problème du monopôle magnétique A problem concerning the compatibility of grand unified theories (→ GUTs) with standard cosmology. If standard cosmology was combined with grand unified theories, far too many → magnetic monopoles would have been produced in the early Universe. The → inflation hypothesis aims at explaining the observed scarcity of monopoles. The inflation has deceased their density by a huge factor. |
many-body problem parâse-ye N jesm Fr.: problème à N corps The mathematical problem of solving the equations of motions of any number of bodies which interact gravitationally. More specifically, to find their positions and velocities at any point in the future or the past, given their present positions, masses, and velocities. Many, from M.E. mani, meni, O.E. monig, manig; → body; → problem. |
missing satellites problem (MSP) parâse-ye bandevârhâ-ye gomšodé Fr.: problème des satellites manquants The observed underabundance, by one or two orders of magnitude, of → dwarf galaxies orbiting → spiral galaxies compared to their number predicted by the standard model. The → cold dark matter (CDM) model predicts that dwarf galaxies are the building blocks of large galaxies like the Milky Way and should largely outnumber them. Dwarf galaxies form first, they merge into bigger and bigger galaxies, and galaxies into groups of galaxies. The dark matter halos, however, are very dense, and dwarf halos are not destroyed in the merging, resulting in their large predicted number, in numerical simulations. Probably first dealt with in an article entitled "Where Are the Missing Galactic Satellites?" (Lypin et al. 1999, ApJ 522, 82); → missing mass; → satellite; → problem. |
n-body problem parâse-ye n-jesm Fr.: problème de n-corps The mathematical problem of studying the behavior (e.g., velocities, positions) of any number of objects moving under their mutual gravitational attraction for any time in the past or future. Same as the → many-body problem. |
problem parâsé Fr.: problème 1) Any question or matter involving doubt, uncertainty, or difficulty. M.E., from O.Fr. problème, from L. problema, from Gk. problema "a problem, a question," literally "thing put forward," from proballein "to propose," from → pro- "forward" + ballein "to throw," → ballistics. Parâsé, from pərəs- present tense stem of Av. fras- "to ask, question, inquire," pərəsaiti "asks," to which is related Mod.Pers. pors-, porsidan "to ask;" Mid.Pers. pursidan; O.Pers. fraθ- "to ask, examine, investigate, punish;" Sogd. anfrāsē "question, enquiry;" cf. Skt. prasś- "to ask, long for;" Tokharian prak-/prek- "to ask;" L. prex "request," precor "to ask, to pray;" Lith. prašyti "to ask, to demand;" PIE base *prek- "to ask." |
restricted three-body problem parâse-ye seh jesm-e forudâridé Fr.: problème restreint à trois corps A special case of the → three-body problem in which the → mass of one of the bodies is negligible compared to that of the two others. If the relative motion of the two massive components is a circle, the situation is referred to as the → circular restricted three-body problem. An example would be a space probe moving in the → gravitational fields of the → Earth and the → Moon, which revolve very nearly in circles about their common → center of mass. |
Riemann problem parâse-ye Riemann Fr.: problème de Riemann The combination of a → partial differential equation and a → piecewise constant → initial condition. The Riemann problem is a basic tool in a number of numerical methods for wave propagation problems. The canonical form of the Riemann problem is: ∂u/∂t + ∂f(u)/∂x = 0, x ∈ R, t > 0, u(x,0) = u_{l} if x < 0, and u(x,0) = u_{r} if x > 0 . → Riemann's geometry; → problem. |
solar neutrino problem parâse-ye notrinohâ-ye xoršid Fr.: problème des neutrinos solaires A major discrepancy between the flux of neutrinos detected at Earth from the solar core and that predicted by current models of solar nuclear fusion and our understanding of neutrinos themselves. The problem, lasting from the mid-1960s to about 2002, was a considerably lesser detected number of neutrons compared with theoretical predictions. The discrepancy has since been resolved by new understanding of neutrino physics, requiring a modification of the → standard model of particle physics, in particular → neutrino oscillation. |
three-body problem parâse-ye sé jesm Fr.: problème à trois corps The mathematical problem of studying the positions and velocities of three mutually attracting bodies (such as the Sun, Earth and Moon) and the stability of their motion. This problem is surprisingly difficult to solve, even in the simple case, called → restricted three-body problem, where one of the masses is taken to be negligibly small so that the problem simplifies to finding the behavior of the mass-less body in the combined gravitational field of the other two. See also → two-body problem, → n-body problem. |
two-body problem parâse-ye do jesm Fr.: problème à deux corps In classical mechanics, the study concerned with the dynamics of an isolated system of two particles subject only to the Newtonian gravitational force between them. The problem can be separated into two single-particle problems with the following solutions. The equation of the → center of mass is governed by the equation of the same form as that for a single particle. Moreover, the motion of either particle, with respect to the other as origin, is the same as the motion with respect to a fixed origin of a single particle of → reduced mass acted on by the same internal force. See also → three-body problem, → n-body problem. |
weak wind problem parâse-ye bâd-e nezâr, ~ ~ kamzur Fr.: problème de faible vent The discrepancy between the observed → mass loss rates and the predicted values for → weak-wind O-type stars. |