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R. N. Mohapatra
weak force, somewhere near the time when the age of the universe is a minute,
the Hubble expansion rate overtakes the the rate at which weak forces can
change the neutron to protons by reactions like ν e +n → e
−
+p, etc. After that
time, weak interactions cannot change the protons to neutrons and vice versa
since weak interactions become too slow compared to the expansion rate of the
universe. The number of neutrons and protons at that point remains frozen
and as the universe cools a bit more, these neutrons and protons undergo
nuclear attraction and combine two protons to two neutrons to form the
helium nucleus. Calculation shows that there are more protons than neutrons
at the time when neutron and proton numbers freeze. Since it takes an equal
number of protons and neutrons to make a helium nucleus, some protons are
left over together with the helium nuclei. The leftover protons would form
hydrogen atom later on in the history of the universe and the helium nucleus
would form a helium atom. The formation of atoms happens much later. This
leads to roughly 75% of the mass of the matter as hydrogen and 25% of mass
as helium nuclei and a little bit of other nuclei, which are created from further
nuclear reactions. The latter reactions form smaller amount of lithium and
beryllium, and also some small amount of deuterium whose nucleus has one
neutron and one proton. That is all the nuclei that form at the time of Big
Bang Nucleosynthesis (BBN), as this epoch is called. They continue that way
until later stage in the evolution of the universe. The basic theory of BBN
was proposed by Ralph Alpher, Hans Bethe, and George Gamow (the famous
Alpha Beta gamma paper).
BBN is one of the major triumphs of particle physics and cosmology.
It predicts the abundance of deuterium and helium quite accurately and is
in remarkable agreement with observations. This is especially notable since
the abundance of deuterium is about 10,000 times smaller than that of
helium. The BBN model also predicts the lithium-7 abundance (Fig. 18.1).
This prediction however disagrees with observations by a factor of four. That
is considered one of the puzzles that may be a hint of new physics beyond the
standard model (at least some people think so). Thus the neutrino is the key
particle in the formation of hydrogen and helium, the two starting members
of the periodic table. As noted, at the epoch of BBN, no atoms can form since
the temperature of the universe is about ten thousand million degrees and at
that temperature, the electrons are free and much too energetic to bind with
nuclei to form atoms. They get ionized as soon as they bind to form an atom.
But as the universe cools to about 100,000
◦ C, the electrons are moving slowly
enough not to be able to get away from the attractive force of the proton and
would then form atoms (hydrogen atoms, helium atoms, etc.). The age of the
universe at that time is about 100,000 years, still pretty young compared to
R. N. Mohapatra
weak force, somewhere near the time when the age of the universe is a minute,
the Hubble expansion rate overtakes the the rate at which weak forces can
change the neutron to protons by reactions like ν e +n → e
−
+p, etc. After that
time, weak interactions cannot change the protons to neutrons and vice versa
since weak interactions become too slow compared to the expansion rate of the
universe. The number of neutrons and protons at that point remains frozen
and as the universe cools a bit more, these neutrons and protons undergo
nuclear attraction and combine two protons to two neutrons to form the
helium nucleus. Calculation shows that there are more protons than neutrons
at the time when neutron and proton numbers freeze. Since it takes an equal
number of protons and neutrons to make a helium nucleus, some protons are
left over together with the helium nuclei. The leftover protons would form
hydrogen atom later on in the history of the universe and the helium nucleus
would form a helium atom. The formation of atoms happens much later. This
leads to roughly 75% of the mass of the matter as hydrogen and 25% of mass
as helium nuclei and a little bit of other nuclei, which are created from further
nuclear reactions. The latter reactions form smaller amount of lithium and
beryllium, and also some small amount of deuterium whose nucleus has one
neutron and one proton. That is all the nuclei that form at the time of Big
Bang Nucleosynthesis (BBN), as this epoch is called. They continue that way
until later stage in the evolution of the universe. The basic theory of BBN
was proposed by Ralph Alpher, Hans Bethe, and George Gamow (the famous
Alpha Beta gamma paper).
BBN is one of the major triumphs of particle physics and cosmology.
It predicts the abundance of deuterium and helium quite accurately and is
in remarkable agreement with observations. This is especially notable since
the abundance of deuterium is about 10,000 times smaller than that of
helium. The BBN model also predicts the lithium-7 abundance (Fig. 18.1).
This prediction however disagrees with observations by a factor of four. That
is considered one of the puzzles that may be a hint of new physics beyond the
standard model (at least some people think so). Thus the neutrino is the key
particle in the formation of hydrogen and helium, the two starting members
of the periodic table. As noted, at the epoch of BBN, no atoms can form since
the temperature of the universe is about ten thousand million degrees and at
that temperature, the electrons are free and much too energetic to bind with
nuclei to form atoms. They get ionized as soon as they bind to form an atom.
But as the universe cools to about 100,000
◦ C, the electrons are moving slowly
enough not to be able to get away from the attractive force of the proton and
would then form atoms (hydrogen atoms, helium atoms, etc.). The age of the
universe at that time is about 100,000 years, still pretty young compared to