88499

2

Origin of the Elements. Isotopes and Atomie Weights

Ch. 1

was once containcd in a primeval nuclcus of immense density (MO⁹⁶ gem^-3) and temperaturę (~10³² K) which, for sonie rcason. cxploded and dislribuled radiation and matter uniformly throughout space. As the univcrse expandcd it cooled; this allowcd the four main types of force to become progressivcly differentiated, and permitted the formation of various types of particie to occur. Nolhing scicntific can be said about the conditions obtaining at times shorter than the Planck time, /p f(G/i/c⁵)^,/2 = l .33 x 10^-43 s] at which moment the forces of gravity and electromagnetism, and the weak and strong nuclear forces were all undifterentiated and equally powerful. At 10^-43 s after the big bang (T = 10³¹ K) gravity separated as a distinct force, and at 10^_35s (lO^K) the strong nuclear force separated from the still combined electro-weak force. Thesc are. of course, inconceivably shorl times and uniniaginably high temperalurcs: for example, it takes as long as 10^-24s for a photon (travclling at the speed of light) to lravcrse a distancc equal to the diameter of an atomie nuclcus. Whcn a time intcrval of 10“^,0s had elapsed from the big bang the temperaturę is calculatcd to have fallen to 10¹⁵ K and this enabled the electromagnetic and weak nuclear forces to separate. By 6 x lO^s (1.4 x 10^,2K) protons and neutrons had becn formed from quarks. and this was followed by stabilization of electrons. One second after the big bang, after a period of extensivc particle-antiparticle annihilation to form electromagnetic photons, the universe was populatcd by particles which sound familiar to chemists — protons, neutrons and electrons.

Shortly thercaftcr, the strong nuclear force ensured that large numbers of protons and neutrons rapidly combined to fonu deuterium nuclci (p + n), then helium (2p + 2n). The process of element building Iwd be#im. During this smali niche of cosmic history. from about 10-500s after the big bang, the entire universe is lhought to have behaved as a colossal homogeneous fusion reactor converting hydrogen into helium. Prcviously no helium nuclci could exist — the temperaturę was so high that the sca of radiation would have immediatcly dccomposed them back to protons and neutrons. Subscqucntly, the continuing cxpansion of the univcrse was such that the particie density was loo Iow for thesc strong (but short-range) interactions to occur. Thus. within the time slot of about cight minutes, it has becn calculatcd that about one-quarter of the mass of the universe was converted to helium nuclci and about threc-quarters rcmained as hydrogen. Simultaneously, a minutę I0~³% was convcrted to deuterons and about IO^-6% to lithium nuclci. These remarkable predictions of the big bang cosmological theory are borne out by cxpcrimcntal obscrvations. Wherever one looks in the universe — the oldest stars in our own galaxy, or the “morę recent*’ stars in remote galaxies — the universal abundance of helium is about 25%. Evcn morę remarkably, the expccted concentration of deuterium has becn detccted in interstcllar clouds. Yct, as we shall shortly sec, stars can only destroy deuterium as soon as it is formed: they cannot create any appreciable equilibrium concentration of deuterium nuclci because of the high temperaturę of the stcllar cnvironment. The sole sourcc of deuterium in the universc sccms to be the big bang. At present no olher cosmological theory can cxplain this observed ratio of H:He:D.

Two other features of the universe lind ready interpretation in terms of the big bang theory. First, as observed originally by E. Hubble in 1929, the light received on earth from distant galaxics is shifted incrcasingly towards the red end of the spectrum as the distance of the source inereases. This implics that the universe is continually expanding and. on ccrtain assumptions, cxtrapolation baekwards in time indicates that the big bang occurred sonie 15 billion ycars ago. Estimates from scvcral other independent lines of cvidcncc givc reassuringly similar valucs for the age of the univcrsc. Secondly. the theory convincingly explains (indeed predicled) the cxistencc of an all-pcrvading isotropic cosmic black-body radiation. This radiation (which corresponds to a temperaturę of 2.735 ± 0.06 K according to the most recent mcasurcments) was discovercd in