17 March 1967, Volume 155, Number 3768 S C IE N C E Cosmological Element Production - A record of the fireball phase of the universe may be contained in the abundances of some light nuclei. Robert V. Wagoner A potentially powerful line of at- tack is on the verge of revealing new insights into the structure and evolu- tion of the universe. The method is essentially based on, the fact that a record of the past history of the uni- verse is contained in the abundances of the various elements which we observe today. Thus, for instance, the observa- tion of certain lines in the spectra of the oldest stars in our galaxy may give us information about conditions in the universe when its density was over 1030 times the present value. This approach has become more quantitative mainly because of recent observations of back- ground microwave radiation, improved determinations of abundances, and a better knowledge of rates of nuclear reactions. However, the interpretation of observed abundances in terms of production during a past high-density phase of the universe is strongly af- fected by the fact that most of the elements are believed to have been produced more recently within stars (1) or through other processes. In this method of analyzing the past history of the universe, a given model of the universe is tested by comparing the amount of each element and iso- tope produced according to the model withithe observed abundances in matter whose composition may not have been affected by subsequent processes. Other types of observational evidence allow one to nimit the class of cosmological models which may be considered. The The author is a Research Fellow in Physics at the California Institute of Technology, Pasadena. 17 MARCH 1967 observational results which are of im- portance for this purpose are the fol- lowing. 1) The distribution and red shifts of radio sources and clusters of galaxies appear to be isotropic to within - 30 percent (2). In addition, recent measurements of background micro- wave radiation at 3.2 centimeters (3) indicate its possible anisotropy to be z 3 percent. It is expected that this limit will be further reduced in the near future. 2) The darkness of the night sky (Olber's paradox) (4) and the red shift of the spectral lines from distant galaxies imply that the universe is expanding. The measurements (5) indicate that at present Ho 1 dR) (1010 yr)-1 (1) R dt /o _ 1_( t 1d2R_ < 3 (2) Rk)2~, dt' where Ho is the Hubble expansion con- stant and q0 is the deceleration param- eter. The scale factor R is propor- tional to the distance between clusters of galaxies, which are believed to be the smallest aggregates of visible mat- ter whose motion is due solely to the universal expansion. 3) The present density of visible matter is - 3 X 10-31 - 3 X 10O3 gram per cubic centimeter (6). How- ever, other forms of matter, such as neutrinos or ionized intergalactic gas, may contribute significantly to the total density. 4) The microwave background radia- tion mentioned in observation 1 has also been detected at both 7.3 and 20.7 centimeters (7). All intensities lie very nearly on a 3YK black-body spectrum. In addition, measurements of the rota- tional structure of interstellar absorp- tion bands of CN at 0.26 centimeter (8) are also consistent wiht a photon temperature of 3YK. We shall further restrict our con- sideration of possible models by mak-' ing three rather general assumptions. 1) The universe has emerged from a state of very high temperature and density. This is the hot "big bang," as first envisioned by Lemailtre and Gamow (9). The detection of the 3YK photons provides support for this view, since it appears that the photons were prob- ably thermalized during a higher- density phase of the universe, before the plasma recombined at 104 degrees Kelvin. (Subsequent reheating due to energy liberated in galaxy formation has presumably ionized any present- day intergalactic gas.) In particular, it is assumed that the temperature was once high enough (T : 1012 deg K in most models) to allow free neutrons to be present due to the equilibrium of the weak reactions: ne ? n p + e, e+ + n ; P + 7, and n ; p + e- + T In the case of an oscillating universe, such a temperature would also dissociate nuclei, and thus prevent a continual buildup of heavier elements from cycle to cycle through stellar nucleosynithe- Slis. 2) The universe was fairly homo- geneous during the early high-tempera- ture stage. By homogeneity I mean that a cosmic time t can be chosen such that all locations in the universe are equiv- alent at a given time. As a result, all scalar quantities such as density, pres- sure, and temperature depend only on t. The observations of isotropy, plus the philosophical postulate that we are not occupying a unique location in the universe, lead to the conclusion that, at least within times not too far removed from the present epoch, the 1369