Astroparticle Physics

12 1 Historical IntroductionBigBangtheorysteady-state universecosmic background radiationof z = 10. Its discovery was made possible through lightamplification by a factor of about 50 resulting from stronggravitational lensing by a very massive galactic cluster in theline of sight to the distant galaxy [1]. As will be discussedin Chap. 8, this implies that the quasar is seen in a statewhen the universe was less than 5% of its present age. Consequently,this quasar resides at a distance of 13 billion lightyears.4 Initially, there was some controversy about whetherthe observed quasar redshifts were of gravitational or cosmologicalorigin. Today, there is no doubt that the observedlarge redshifts are a consequence of the Hubble expansionof the universe.The expansion of the universe implies that it began ina giant explosion, some time in the past. Based on this BigBang hypothesis, one arrives at the conclusion that this musthave occurred about 15 billion years ago. The Big Bangmodel was in competition with the idea of a steady-stateuniverse for quite some time. The steady-state model wasbased on the assumption that the universe as a whole wastime independent with new stars being continuously createdwhile old stars died out. On the other hand, Gamowhad been speculating since the forties that there should bea residual radiation from the Big Bang. According to hisestimate, the temperature of this radiation should be in therange of a few Kelvin. Penzias and Wilson (Nobel Prize1978) detected this echo of the Big Bang by chance in 1965,while they were trying to develop low-noise radio antennae(Fig. 1.14). 5 With this discovery, the Big Bang model finallygained general acceptance. The exact temperature ofthis blackbody radiation was measured by the COBE 6 satellitein 1992 as 2.726 ± 0.005 Kelvin. 74 It has become common practice in the scientific literature thatthe number 10 9 is called a billion, while in other countries thebillion is 10 12 . Throughout this book the notation that a billionis equal to a thousand millions is used.5 The excrements of pigeons presented a severe problem duringan attempt to reduce the noise of their horn antenna. When,after a thorough cleaning of the whole system, a residual noisestill remained, Arno Penzias was reported to have said: “Eitherwe have seen the birth of the universe, or we have seen a pileof pigeon shit.”6 COBE – COsmic ray Background Explorer7 The presently (2004) most accurate value of the blackbodytemperature is 2.725 ± 0.001 K.