were assumed to have been made during the T-Tauri phase of stellar evolution (23), and so, were not taken then to have cosmological significance. It was during the 1970s that BBN fully developed as a tool for probing the universe. This possibility was in part stimulated by Ryter et al. (24), who showed that the T-Tauri mechanism for light element synthesis failed. Furthermore, 2D abundance determinations improved significantly with solar wind measurements (25, 26) and the interstellar work from the Copernicus satellite (27). [Recent Hubble Space Telescope observations reported by Linsky et al. (28) have compressed the local intersteller medium 2D error bars considerably.] Reeves, Audouze, Fowler and Schramm (29) argued for cosmological 2D and were able to place a constraint on the baryon density excluding a universe closed with baryons. Subsequently, the 2D arguments were cemented when Epstein, Lattimer, and Schramm (30) proved that no realistic astrophysical process other than the Big Bang could produce significant 2D. This baryon density was compared with dynamical determinations of density by Gott, Gunn, Schramm, and Tinsley (31). See Fig. 2 for an updated H0-Ω diagram.
In the late 1970s, it appeared that a complimentary argument to 2D could be developed by using 3He. In particular, it was argued (33) that, unlike 2D, 3He was made in stars: thus, its abundance would increase with time. Unfortunately, recent data on 3He in the interstellar medium (34) has shown that 3He has been constant for the last 5 Gyr. Thus, low mass stars are not making a significant addition, contrary to these previous theroetical ideas. Furthermore, Rood, Bania, and Wilson (35) have shown that interstellar 3He is quite variable in the galaxy, contrary to expectations for a low-mass, star-dominated nu
cleus. However, the work on planetary nebulae shows that at least some low-mass stars produce 3He.Nonetheless, the current observational situation clearly shows that arguments based on theoretical ideas about 3He evolution should be avoided (c.f. Hata et al., ref. 36) where their “crisis” is really about 3He problems (and excessively small assumed uncertainties in 4He),not BBN. Because 3He nowseems not to have a well-behaved history, simple 3He or 3He+D inventory arguments are misleading at best. However, one is not free to go to arbitrary low baryon densities and high primordial D and 3He, because processing of D and 3He in massive stars also produces metals that are constrained (37, 38) by the metals in the hot intra-cluster gas, if not the galaxy. In the near future, this problem with 3He evolution will be constrained severely by the extragalactic D/H. In particular, the Tytler D/H=2.6± 0.6×10−5 is almost identical to the presolar D/H=2.4± 0.4×10−5 (39) and less than a factor of 2 above the current interstellar D/H=1.5±0.1×10−5 (40). This tells us that the production of current metal content of the galaxy did not destroy much 2D. Thisimplies either a very different initial mass function to make the metals, or much primordial infall throughout the history of the galaxy.
It was interesting that the abundances of the other light elements led to the requirement that 7Li be near its minimum of 7Li/H~10−10, which was verified by the Population II (Pop II) Li measurements of Spite and Spite and their group (41–43), hence yielding the situation emphasized by Yang et al. (13) that the light element abundances are consistent over nine orders of magnitude with BBN, but only if the cosmological baryon density, Ωb, is constrained to be around 6% of the critical value (for H0 50 km/sec per Mpc). The Li plateau argument was strengthened further with the observation of 6Li in a Pop II star by Smith, Lambert, and Nissen (44). Because 6Li is much more fragile than 7Li, and yet it survived, no