| Briefly describe the sequence of reactions leading to the formation of the light nuclides 2H, 3He, 4He and 7Li in the early universe. | [4] |
| This was discussed in the lectures, but principally by means of diagrams. The 4 marks probably represent one for each nuclide. | |
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Explain why: (i) this process cannot produce nuclides heavier than 7Li (even though fusion processes in stars can and do produce such nuclides); |
[2] |
| This is bookwork. For two marks you need to explain not only why heavy nuclei are not produced in the early universe, but how conditions in stars differ so as to make heavy element production feasible. | |
| (ii) you expect the final yield of 4He to be somewhat less than 30% by mass; | [2] |
| This requires a calculation. Recall that the relevant bit of the Boltzmann distribution is the exponential, exp(-E/kT); in this case the relevant energy is mc2, so the ratio of neutrons to protons is exp(-mnc2/kT)/exp(-mpc2/kT) = exp(-Δmc2/kT). The temperature at which the neutron:proton ratio stabilises is around 1010 K. | |
| (iii) the exact yield depends on the neutron lifetime, the number of neutrino species and H0 as well as on the baryon density. | [2] |
| This is fairly straightforward – the marks indicate about half a mark per point. | |
| [Note that the mass of the proton is 1.6726×10-27 kg, and the neutron mass is 1.6749×10-27 kg.] |
(2006 Q7(a) and (b).)