18
From Quarks to Protons and Neutrons and
Then to Helium and Beryllium and the Dance
of Atoms
The period from inflation to the beginning of the Hubble phase of expansion
takes only a minute fraction of the age of the universe—about 10
−45 s to
somewhere around 10
−30 s or so. After the Hubble expansion begins, again
the universe rushes through various events where it starts from one phase
and enters another, depending on the kind of symmetries at play in the early
universe. Each kind of symmetry represents a point where the so-called phase
transition takes place. A simple and familiar example of phase transition is
something we see every morning at breakfast time when we boil water to
make tea. When water boils, it goes from liquid to vapor phase. A similar
thing happened at least twice or could have happened more often during the
Hubble expansion phase of the universe. In one of those phases, the weak
bosons W and Z get their mass since they are massless prior to that time. This
happens when the universe is about 10
−10 s old. This is followed by another
phase transition around 10
−4 s when the quarks lose their separate identity and
bind together to form protons and neutrons, pions, K-mesons, etc. Then when
the universe is about a minute old, something very important happens: what
physicists call Big Bang Nucleosynthesis.
18.1 Big Bang Nucleosynthesis
Before the universe is one minute old, the number of neutrons and protons
remains almost same, due to back and forth reactions involving the weak force
that changes a proton to neutron and vice versa. Due to the weakness of the
© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer
Nature Switzerland AG 2021
R. N. Mohapatra, The Neutrino Story: One Tiny Particle’s Grand Role in the Cosmos,
https://doi.org/10.1007/978-3-030-51846-2_18
135
From Quarks to Protons and Neutrons and
Then to Helium and Beryllium and the Dance
of Atoms
The period from inflation to the beginning of the Hubble phase of expansion
takes only a minute fraction of the age of the universe—about 10
−45 s to
somewhere around 10
−30 s or so. After the Hubble expansion begins, again
the universe rushes through various events where it starts from one phase
and enters another, depending on the kind of symmetries at play in the early
universe. Each kind of symmetry represents a point where the so-called phase
transition takes place. A simple and familiar example of phase transition is
something we see every morning at breakfast time when we boil water to
make tea. When water boils, it goes from liquid to vapor phase. A similar
thing happened at least twice or could have happened more often during the
Hubble expansion phase of the universe. In one of those phases, the weak
bosons W and Z get their mass since they are massless prior to that time. This
happens when the universe is about 10
−10 s old. This is followed by another
phase transition around 10
−4 s when the quarks lose their separate identity and
bind together to form protons and neutrons, pions, K-mesons, etc. Then when
the universe is about a minute old, something very important happens: what
physicists call Big Bang Nucleosynthesis.
18.1 Big Bang Nucleosynthesis
Before the universe is one minute old, the number of neutrons and protons
remains almost same, due to back and forth reactions involving the weak force
that changes a proton to neutron and vice versa. Due to the weakness of the
© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer
Nature Switzerland AG 2021
R. N. Mohapatra, The Neutrino Story: One Tiny Particle’s Grand Role in the Cosmos,
https://doi.org/10.1007/978-3-030-51846-2_18
135