Proc. Natl. Acad. Sci. USA Vol. 90, pp. 4782-4788, June 1993 Colloquium Paper This paper was presented at a colloquium entitled "Physical Cosmology," organized by a committee chaired by David N. Schramm, held March 27 and 28, 1992, at the National Academy of Sciences, Irvine, CA. Cosmological implications of light element abundances: Theory DAVID N. SCHRAMM Astronomy and Astrophysics Centers 140, The University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637; and National Aeronautic and Space Administration/Fermilab Astrophysics Center, Fermi National Accelerator Laboratory, Box 500, Batavia, IL 60510-0500 ABSTRACT Primordial nucleosynthesis provides (with number of neutrino families and its subsequent verification the microwave background radiation) one of the two quanti- by the Large Electron Positron Collider (LEP) and the tative experimental tests of the hot Big Bang cosmological Stanford Linear Collider (SLC). model (versus alternative explanations for the observed Hubble Also discussed is the possibility that a first-order quark- expansion). The standard homogeneous-isotropic calculation hadron phase transition could have produced variations from fits the light element abundances ranging from 'H at 76% and the standard homogeneous model. It will be shown that 4He at 24% by mass through 2H and 3He at parts in 105 down contrary to initial indications, first-order quark-hadron- to 7Li at parts in 1010. It is also noted how the recent Large inspired results are consistent with the homogeneous model Electron Positron Collider (and Stanford Linear Collider) results. results on the number of neutrinos (N,,) are a positive labora- Finally, a discussion of the recent B and Be observations tory test of this standard Big Bang scenario. The possible in Population (Pop) II stars will be made. It will be shown that alternate scenario ofquark-hadron-induced inhomogeneities is all the Be and B observations, to date, are best explained by also discussed. It is shown that when this alternative scenario galactic cosmic ray spallation (2) in Pop II environments and is made to fit the observed abundances accurately, the resulting not by any cosmological process. conclusions on the baryonic density relative to the critical This report will draw on two recent reviews (3 and 4) and density (fQb) remain approximately the same as in the standard in some ways is an update (20 years later) ofthe light element homogeneous case, thus adding to the robustness of the stan- summary of ref. 5. dard model and the conclusion that fQb 0.06. This latter point is the driving force behind the need for nonbaryonic dark History of Big Bang Nucleosynthesis (BBN) matter (assuming total density fitt = 1) and the need for dark baryonic matter, since the density of visible matter 1iksib1e < It should be noted that there is a symbiotic connection fQb. The recent Population II B and Be observations are also between primordial nucleosynthesis (hereafter referred to as discussed and shown to be a consequence of cosmic ray BBN) and the 3 K background dating back to Gamow and his spallation processes rather than primordial nucleosynthesis. associates Alpher and Herman. The initial BBN calculations The light elements and N,, successfully probe the cosmological of Gamow's group (6) assumed pure neutrons as an initial model at times as early as 1 sec and a temperature (1) of -1010 condition and thus were not particularly accurate, but their K (-1 MeV). Thus, they provided the first quantitative inaccuracies had little effect on the group's predictions for a arguments that led to the connections of cosmology to nuclear background radiation. and particle physics. Once Hayashi (7) recognized the role of neutron-proton equilibration, the framework for BBN calculations them- selves has not varied significantly. The work of Alpher et al. Yacov Zeldovich (1) noted in material written just before he (8) and Hoyle and Taylor (9), preceding the discovery of the died in 1987 that "the greatest success ofthe Big Bang theory 3 K background, and Peebles (10) and Wagoner et al. (11), is the fact that the quantitative observation of the light immediately following the discovery, and the more recent element abundances agrees with the prediction of the theory work of our group of collaborators (12-18) all do essentially of nucleosynthesis." Such praise from Zeldovich is indeed the same basic calculation, the results of which are shown in pleasing, for the field and, hopefully, the recent develop- Fig. 1. As far as the calculation itself goes, solving the ments that will be described in this paper would not have reaction network is relatively simple by using the numerical detracted from Zeldovich's views. procedures developed slightly earlier for explosive nucleo- This paper will review the present status of primordial synthesis calculations in supernovae (and nuclear weapons nucleosynthesis. After briefly reviewing the history, this tests), with the calculational changes over the last 25 years paper will make special emphasis of the remarkable agree- being mainly in terms of more recent nuclear reaction rates ment of the observed light element abundances with the as input, not as any great calculational insight [although the calculations upon which Zeldovich based his comments. It current Kawano code (18) is somewhat streamlined relative should be remembered that this agreement is one of the two to the earlier Wagoner code (11)]. With the possible excep- prime tests of the Big Bang itself (the other being the tion of 7Li yields (and possibly Be and B to be discussed microwave background) as the successful framework in later), the reaction rate changes over the past 25 years have which to place the observed Hubble expansion. The agree- not had any major affect. The one key improved input is a ment of abundances and predictions works only if the baryon better neutron lifetime determination (19). density is well below the cosmological critical value. The With the exception of the effects of elementary particle review will also mention the nucleosynthesis prediction ofthe assumptions to which we will return, the real excitement for The publication costs of this article were defrayed in part by page charge Abbreviations: BBN, Big Bang nucleosynthesis; HDM, hot dark payment. This article must therefore be hereby marked "advertisement" matter; CDM, cold dark matter; LEP, Large Electron Positron in accordance with 18 U.S.C. §1734 solely to indicate this fact. Collider; SLC, Stanford Linear Collider; Pop, population. 4782 Downloaded by guest on September 29, 2021 Colloquium Paper: Schramm Proc. Natl. Acad. Sci. USA 90 (1993) 4783 situation emphasized by Yang et al. (15) that the light element of with 0.01 0.1 1.0 abundances are consistent over 9 orders magnitude -v 1.0 BBN, but only if the cosmological baryon density is con- strained to be around 5% of the critical value. o i- j 4He(mass ' 10 | BBN was the 2 fraction) The other development of the 1970s for explicit calculation of Steigman (33) showing that the number of neutrino generations, NV, had to be small to avoid over- production of 4He. [Earlier independent work (9, 34, 35) had F 1. BH+rHe commented about a dependence on the energy density of exotic particles but had not done explicit calculations probing 3He Nv] This will subsequently be referred to as the Steigman, Schramm, and Gunn (SSG) limit. To put this in perspective, Z 0.1 1. I 0 0 one should remember that the mid-1970s also saw the dis- 1-8 CONCO RDANCE-"1 covery of charm, bottom, and tau, so that it almost seemed as if each new detector produced new particle discoveries, _j 160- and yet, cosmology was arguing against this "conventional" wisdom. Over the years, the SSG limit on NV improved with 4He abundance measurements, neutron lifetime measure- ments, and limits on the lower bound to the baryon density, hovering at NV, : 4 for most of the 1980s and dropping to density (,q) for a homogeneous universe. below 4 (but not excluding 3) just before LEP and SLC turned on (16, 17, 36, 37). The recent verification of this cosmolog- BBN over the last 25 years has not really been in redoing the ical prediction by the LEP and SLC results (79), where N,, = basic calculation. Instead, the true action is focused on 2.99 ± 0.05, is the first verification of a cosmological pre- understanding the evolution of the light element abundances diction by a high-energy collider. Thus, in some sense LEP and using that information to make powerful conclusions. In (and SLC) have checked the Big Bang model at temperatures the 1960s, the main focus was on 4He, which is very insen- of -101" K and times of -1 sec. sitive to the baryon density. The agreement between BBN The power of homogeneous BBN comes from the fact that predictions and observations helped support the basic Big essentially all of the physics input is well determined in the Bang model but gave no significant information, at that time, terrestrial laboratory. The appropriate temperature regimes, with regard to density. In fact, in the mid-1960s, the other 0.1-1 MeV, are well explored in nuclear physics laboratories. light isotopes (which are, in principle, capable of giving Thus, what nuclei do under such conditions is not a matter of density information) were generally assumed to have been guesswork but is precisely known. In fact, it is known for made by spallation processes during the T-Tauri phase of these temperatures far better than it is for the centers of stars stellar evolution (20) and so were not then taken to have like our sun. The center of the sun is only a little over 1 keV, cosmological significance. It was during the 1970s that BBN thus, below the energy where nuclear reaction rates yield fully developed as a tool for probing the universe.