In this chapter, few-body nuclei will be defined as those created through big bang nucleosynthesis, namely lithium, beryllium, and lighter elements. All elements heavier than beryllium and lithium were created much later, by stellar nucleosynthesis in evolving and exploding stars. This is discussed in detail in Chapter 20. In Chapter 11, the groundstate structure and properties of the deuteron have been discussed in detail. Here, the main focus will be on what can be learned from electroproduction from masses A = 2, 3, and 4. In addition, the topics of hypernuclei and fusion will be discussed.
Big bang nucleosynthesis (BBN) began a few minutes after the big bang, when the universe had cooled down sufficiently to allow deuterium nuclei to survive photodisintegration by high-energy photons. At this temperature, nucleosynthesis can take place and protons and neutrons can interact to form deuterium. Most of the deuterium then collided with other protons and neutrons to produce helium and a small amount of tritium. Lithium-7 could also form via the coalescence of one tritium and two deuterium nuclei. The BBN produced the stable nuclei 2H, 3He, 4He, 6Li, and 7Li as well as the radioactive nuclei 3H, 7Be, and 8Be. No elements heavier than beryllium were produced due to the absence of stable nuclei with five or eight nucleons. Note that this bottleneck is overcome in stars by triple collisions of 4He nuclei, producing carbon. However, this process is very slow, taking tens of thousands of years to convert a significant amount of helium to carbon in stars. (See Chapter 20 for further discussion of the Big Bang and astrophysics.)
The theory of BBN predicts that roughly 25% of the mass of the universe consists of helium, with about 0.01% deuterium and smaller quantities of lithium. This prediction depends critically on the density of baryons (neutrons and protons) at the time of nucleosynthesis. The observation that helium is nowhere seen to have an abundance below 23% is strong evidence that the universe went through an early, hot phase. Further support comes from the consistency of the light element abundances for a particular value of baryon density and an independent measurement of this quantity from the anisotropies in the cosmic microwave background (CMB).
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