angular momentum catastrophe
negunzâr-e jonbâk-e zâviye-yi
Fr.: catastrophe du moment angulaire
A problem encountered by the → cold dark matter model of galaxy formation. The model predicts too small systems lacking → angular momentum, in contrast to real, observed galaxies. → cusp problem; → missing dwarfs.
A great, often sudden calamity; a complete failure; a sudden violent change in the earth's surface. → cataclysm.
From Gk. katastrophe "an overturning, ruin," from katastrephein "to overturn, ruin" from kata "down" + strephein "to turn."
Negunzâr, from negun "overturned, inverted" + -zâr suffix denoting profusion, abundance, as in kârzâr "a field of battle; combat" šurezâr "unfertile, salty ground; nitrous earth," xoškzâr "arid land," and so forth.
Fr.: catastrophe de Compton
In a compact, steady radio-source where the density of relativistic electrons and the density of synchrotron radiation due to these electrons are very large, the radio photons should be transformed into X-ray and gamma-ray photons through inelastic Compton scatterings onto the relativistic electrons. Thus the radio photons should rapidly disappear and only gamma-ray photons should be observed. This phenomenon does not take place if the radio source is in relativistic expansion.
Fr.: catastrophe ultraviolette
A paradox encountered in the classical theory of thermal radiation (→ Rayleigh-Jeans law), whereby a → blackbody should radiate an infinite amount of energy at infinitely short wavelengths, in contradiction with what is observed. The problem was solved by Max Planck in 1900, who suggested that, rather than being continuous, the energy comes in discrete parcels called quanta. The avoidance of the ultraviolet catastrophe was one of the first great achievements of → quantum mechanics.