baryon acoustic oscillation (BAO)
naveš-e sedâyik-e bâryoni
Fr.: oscillation acoustique baryonique
In cosmology, one of a series of peaks and troughs that are present in the power spectrum of matter fluctuations after the → recombination era, and on large scales. At the time of the Big Bang, and for about 380,000 years afterwards, Universe was ionized and photons and baryons were tightly coupled. Acoustic oscillations arose from perturbations in the primordial plasma due to the competition between gravitational attraction and gas+photons pressure. After the epoch of recombination, these oscillations froze and imprinted their signatures in both the → CMB and matter distribution. In the case of the photons, the acoustic mode history is manifested as the high-contrast Doppler peaks in the temperature anisotropies. As for baryons, they were in a similar state, and when mixed with the non-oscillating → cold dark matter perturbations, they left a small residual imprint in the clustering of matter on very large scales, ~100 h-1Mpc (h being the → Hubble constant in units of 100 km s-1 Mpc-1). The phenomenon of BAOs, recently discovered using the Sloan Digital Sky Survey data, is a confirmation of the current model of cosmology. Like → Type Ia supernovae, BAOs provide a → standard candle for determining cosmic distances. The measurement of BAOs is therefore a powerful new technique for probing how → dark energy has affected the expansion of the Universe (see, e.g., Eisenstein 2005, New Astronomy Reviews 49, 360; Percival et al. 2010, MNRAS 401, 2148).
Fr.: oscillation épicyclique
Fr.: oscillation forcée
The oscillation of a system or object induced by an external periodic force. See also → free oscillation.
Fr.: oscillation libre
Oscillation of any system in stable equilibrium under the influence of internal forces only, or of a constant force originating outside the system, or of both.
naveš-e laxtinâk, ~ laxtimand
Fr.: oscillation inertielle
1) A periodic motion of a particle that moves, free from external forces, over
the surface of a rotating sphere, such the Earth.
Inertial oscillations result from the → Coriolis force.
For example, a hockey puck launched on a big enough lake in the northern hemisphere
would turn to the right (east) and eventually loop back to nearly the initial
point (actually west of that point). The time it takes for the huckey puck
to return can be computed with the → Coriolis frequency.
Fr.: oscillation des neutrinos
The transition between neutrino types (→ neutrino flavor) which is a probabilistic consequence of → quantum mechanics. A neutrino, when produced, is in a quantum state which has three different masses. Therefore, an electron neutrino emitted during a reaction can be detected as a muon or tau neutrino. In other words, the flavor eigenstates are different from the propagation eigenstates. This phenomenon was discovered in → solar neutrinos as well as in → atmospheric neutrinos. Neutrino oscillation violates the conservation of the → lepton number; it is possible only if neutrinos have a mass. First predicted by Bruno Pontecorvo in 1957, neutrino oscillation has since been observed by several experiments. It resolved the long-standing → solar neutrino problem. The smaller the mass difference between the flavors, the longer the oscillation period, so that oscillations would not occur if all of the flavors were equal in mass or were massless. Moreover, the oscillation period increases with neutrino energy.
The state of any quantity when the value of that quantity is continually changing so that it passes through maximum and minimum values.
Verbal noun of → oscillate.
tarz-e naveš, mod-e ~
Fr.: modes d'oscillation
Same as → pulsation mode.
PLAnetary Transits and Oscillations of stars (PLATO)
A space observatory under development by the → European Space Agency for launch around 2024. Its objective is to detect and characterize → exoplanets by means of their → transit signature in front of a very large sample of → bright stars, and measure the seismic oscillations (→ asteroseismology) of the parent stars orbited by these planets in order to understand the properties of the exoplanetary systems.
Fr.: oscillation de plasma
The oscillatory motion of electrons in a neutral plasma around their equilibrium position while the ions remain at rest. When electrons are displaced by any kind of perturbation with respect to ions, a pair of positive and negative charged regions is formed. The Coulomb force accelerates back the displaced electrons, which get kinetic energy. The electrons overshoot their original positions by the same amount as their first displacement.