Study Of Cosmological Lithium Problem

Big Bang nucleosynthesis(BBN) theory predicts the abundances of the light elements D, 3He, 4He and 7Li produced in the early universe. The primordial abundances of D, 3He and 4He inferred from observational data are in good agreement with predictions, however, the BBN theory overestimates the primordial 7Li abundance by about a factor of three. This is the cosmological lithium problem. Great efforts have been devoted in solving this pending problem, however, solutions involving conventional nuclear physics also seem to be exhausted. The dire potential impact of this longstanding issue on our understanding of the early universe has prompted the introduction of various exotic scenarios, involving, for example, the introduction of new particles and interactions beyond the standard model.The first work in this thesis is to study the impact of non-extensive statistics applied in BBN network reactions on the production of light elements. It is well-known that the classical Maxwell-Boltzmann(MB) velocity distribution has been usually assumed for nuclei in the Big-Bang plasma. However,it is worthwhile to investigate whether the MB distribution still hold for the extremely high-temperature complicated astrophysical environment. In this work, we have investigated the impact on BBN predictions by adopting a generalized distribution to describe the velocities of nucleus in the framework of Tsallis non-extensive statistics. This generalized distribution is characterized by a parameter q, and reduces to the usual MB distribution for q=1. We find excellent agreement between predicted and observed primordial abundances of D, 4He and 7Li for 1.063<q<1.082. For the first time we have found a new solution to the cosmological lithium problem without introducing any mysterious theories and making any correction to SBBN model.The second work in this thesis is to study the thermonuclear 7Be(n,α)4He reaction rate. This reaction is regarded as the secondary important reaction in destructing the 7Be nucleus in BBN. However, the thermonuclear rate of 7Be(n,α)4He has not been well studied so far. This reaction rate was firstly estimated by Wagoner in 1969, which has been generally adopted in the current BBN simulations and the reaction rate library. This simple estimation involved only a direct-capture reaction mechanism, but the resonant contribution should be also considered according to the later experimental results. In this work, we have revised this rate based on the indirect cross-section data available for the 4He(α,n)7Be and 4He(α,p)7Li reactions, with the charge symmetry and detailed-balance principle. Our new result shows that the previous rate is overestimated by about a factor of ten. The BBN simulation shows that the present rate leads to a 1.2% increase in the final 7Li abundance compared to the result using the Wagoner rate, and hence the present rate even worsens the 7Li problem. By the present estimation, the role of 7Be(n,α)4He in destroying 7Be is weakened from the secondary importance to the third, and the 7Be(d,p)24He reaction becomes of secondary importance in destructing 7Be. In addition, the accurate 7Be neutron capture rate is also important for the lithium problem in the non-standard models, in which this revised 7Be(n,α)4He rate might play a very important role.