Big Bang Nucleosynthesis

Fig. 1: Reaction sequence of the Standard Big Bang nucleosynthesis

Fig. 2: Predicted ⁴He, D, ³He, and ⁷Li abundances by SBBN (solid lines). The observed abundances are indicated by boxes.
(Figure from Ref. [1])

The first elements were produced in the first three minutes after the Big Bang when the temperature was cool enough to form deuterons from protons and neutrons. Standard Big Bang nucleosynthesis (SBBN) describes the formation of the lightest elements by a sequence of reactions (see Fig. 1) and predicts their abundances as a function of baryon density. Therefore, a comparison between the predictions and the observations of D, He, and ⁷Li abundances in old stellar objects allows to constrain the baryon to photon density h =n_B/n _γ [1] (see Fig. 2).

So far, only the abundances of D, ³He, ⁴He, and ⁷Li could be used because of the very small primordial yields of other light isotopes (⁶Li, ⁹Be, and ^10,11B). Recently, however, it became possible to detect ⁶Li in metal-poor halo stars [2]. For the interpretation of these observations the ⁴He(d,γ)⁶Li reaction, which is responsible for the production of primordial ⁶Li has to be known with high accuracy. Therefore, the cross section of the ⁴He(d,γ)⁶Li reaction was measured at GSI via the Coulomb dissociation method.

Standard BBN predicts that only the lightest elements H, He, and Li can be synthesized. However, there are non-standard Big Bang models [3], which can bridge the mass gaps at A=5 and 8 thus leading to the synthesis of heavier nuclei. One possible reaction sequence is ⁷Li(n,γ)⁸Li(α,n)¹¹B(n,γ)¹²B(β^-,n)¹²C [4,5]. Subsequent neutron captures on ¹²C and ¹³C will then produce ¹⁴C, which has a half-life of 5730±40 yr. On the time scale of Big Bang nucleosynthesis ¹⁴C can be considered as stable and further proton, alpha, deuteron, and neutron capture reactions on ¹⁴C will result in the production of heavier nuclei (A>20). At GSI we investigated the ¹⁴C(n,γ)¹⁵C reaction with the Coulomb dissociation method.

References

[1] B.D. Fields and S. Sarkar, astro-ph/0601514, 2006
[2] M. Asplund et al., astro-ph/0510636, 2005
[3] J. Applegate, C. Hogan, Phys. Rev. D 31 (1985) 3037
[4] J. Applegate, C. Hogan, R. Scherrer, Ap. j. 329 (1988) 527
[5] R. Malaney, W. Fowler, Ap. J. 333 (1988) 14