Primordial Black Holes and Cosmological Phase Transitions Report ...

PBHs and Cosmological Phase Transitions 11
Figure 1: The time evolution of the abundances of the light elements produced
during Primordial Nucleosynthesis. It is also shown the decrease of the neutron
abundance (adapted from Burles et al., 1999).
reaction could then proceed with the formation of 3 1H (tritium), 3 2He, 4 2He, 6 3Li,
7
3Li and, 7 4Be. The reason why Primordial Nucleosynthesis did not progress
to produce elements with higher mass numbers is due to two factors: i) the
temperature of the universe which is by this time lower than required and,
ii) there are no stable nuclides with mass number A = 5 or A = 8. When
the Universe become ∼ 1000 s old the formation of nuclides effectively ceased
leaving a Universe with primary matter content of hydrogen (mainly protons,
i.e., 1 1H) and helium (mainly 4 2He), with trace amounts of beryllium and lithium
(e.g. Jones & Lambourne, 2004, Figure 1).
The radiation and matter densities in the Universe decrease as the expansion
dilutes the number of atoms and photons. Radiation also decreases due to the
cosmological redshift (see equation 25), so its density falls faster than that of
matter. Looking back in time, there was an instant, which corresponds to an
age of the Universe of ∼ 104 years (redshift z ≈ 3200, e.g. Bennett et al., 2003;
Hinshaw et al., 2008), when matter and radiation densities were just equal (cf.
Table 1, Figure 2). Before that time the Universe was radiation–dominated.
At an age of ∼ 105 years the universe had expanded and cooled enough
(T ∼ 4000 K), allowing nuclei and electrons to combine in order to form neutral
atoms. This process, which is known as recombination 6 , can be numerically
defined as the instant in time when the number density of ions is equal to the
number density of neutral atoms (e.g. Ryden, 2003).
6 Some authors suggest that we should use the term combination instead because this is
the very first time in the history of the universe when electrons and ions combined to form
neutral atoms (e.g. Ryden, 2003).