Astroparticle Physics

14 1 Historical Introductioncharmed mesonsfourth quarkCygnus X1 andgravitational singularitiesblack hole in Cygnus X1?Hawking radiationevaporation of black holesFig. 1.15Decay of an excited charm particle(ψ ′ → ψ + π + + π − , with thesubsequent decay ψ → µ + + µ − )have to be determined by experimental information. However,three neutrino generations with non-zero mass wouldadd another 7 parameters (three for the masses and four mixingparameters). It is generally believed that the standardmodel will not be the final word of the theoreticians.The discovery of charmed mesons in cosmic rays (Niu etal. 1971) and the confirmation, by accelerator experiments,for the existence of a fourth quark (Richter & Ting 1974,Nobel Prize 1976, Fig. 1.15) extended the standard modelof Gell-Mann and Zweig (up, down, strange, and charm).The theory of general relativity and Schwarzschild’sideas on the formation of gravitational singularities weresupported in 1970 by precise investigations of the strong X-ray source Cygnus X1. Optical observations of Cygnus X1indicated that this compact X-ray source is ten times moremassive than our Sun. The rapid variation in the intensityof X rays from this object leads to the conclusion that thissource only has a diameter of about 10 km. A typical neutronstar has a similar diameter to this, but is only three times asheavy as the Sun. An object that was as massive as CygnusX1 would experience such a large gravitational contraction,which would overcome the Fermi pressure of degenerateneutrons. This leads to the conclusion that a black hole mustreside at the center of Cygnus X1.By 1974, Hawking had already managed to unify someaspects of the theory of general relativity and quantumphysics. He was able to show that black holes could evap-orate by producing fermion pairs from the gravitational energyoutside the event horizon. If one of the fermions escapedfrom the black hole, its total energy and thereby itsmass would be decreased (Hawking radiation). The timeconstants for the evaporation process of massive black holes,however, exceed the age of the universe by many orders ofmagnitude.There were some hopes that gravitational waves, whichwould be measured on Earth, could resolve questions on theformation of black holes and other cosmic catastrophes.These hopes were boosted by gravitational-wave experimentsby Weber in 1969. The positive signals of these earlyexperiments have, so far, not been confirmed. It is generallybelieved that the findings of Weber were due to mundaneexperimental backgrounds.In contrast, Taylor and Hulse succeeded in providing indirectevidence for the emission of gravitational waves in