Astroparticle Physics

222 10 Big Bang Nucleosynthesisfine-tuning of parametersastrochemistry, astrobiologyThe exact value of the decoupling temperature is alsoa complicated mixture of effects, being sensitive, for example,to g ∗ , which depends on the number of relativisticparticle species in thermal equilibrium. If, for example, thedecoupling temperature T f had been not 0.7 MeV but, say,0.1 MeV, then the neutron-to-proton ratio would have beene −1.3/0.1 ≈ 2 × 10 −6 , and essentially no helium would haveformed. On the other hand, if it had been at a temperaturemuch higher than m n − m p , then there would have beenequal amounts of protons and neutrons. Then the entire universewould have been made of helium. Usual hydrogen-burning stars would be impossible, and the universe wouldcertainly be a very different place (see also Chap. 14: Astrobiology).10.6 Detailed BBN“Little by little, time brings out eachseveral thing into view, and reasonraises it up into the shores of light.”LucretiusA detailed modeling of nucleosynthesis uses a system of differentialequations involving all of the abundances and reac-tion rates. The rates of nuclear reactions are parameterizationsof experimental data. Several computer programs thatnumerically solve the system of rate equations are publiclyavailable [18]. An example of predicted mass fractions versustemperature and time is shown in Fig. 10.4.Around t ≈ 1000 s, the temperature has dropped to T ≈0.03 MeV. At this point the kinetic energies of nuclei aretoo low to overcome the Coulomb barriers and the fusionprocesses stop.In order to compare the predictions of Big Bang Nucleosynthesiswith observations, one needs to measure the primordialabundances of the light elements, i.e., as they werejust after the BBN era. This is complicated by the fact thatthe abundances change as a result of stellar nucleosynthesis.For example, helium is produced in stars and deuteriumis broken apart.To obtain the most accurate measurement of the 4 Hemass fraction, for example, one tries to find regions of hotionized gas from ‘metal-poor’ galaxies, i.e., those where rel-mass fractionsstellar nucleosynthesisprimordial 4 He abundance