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18 A Brief Historical Overview of the Universe
18.4 Condensation of Nuclei
Again we run the clock backwards, well before the 10
12 s decoupling time, to a time
when the thermal energy is about 1 meV. At higher temperature nuclei cannot exist
because they are disintegrated by collisions and γ rays in the radiation. Thus we can
think of nuclei as condensing from a plasma gas of nucleons at this time, about 1 s.
The process of nuclear formation is commonly called nucleosynthesis. The plasma
before this time is composed mainly of neutrons and protons and electrons and many
photons and neutrinos (Weinberg 1988).
The theory of primordial nucleosynthesis has been very successful in predicting
the abundance of the primordial light elements. For example, the number ratio of
helium to hydrogen atoms is predicted to be about 1/10, in good agreement with
observation. This calculation assumes that essentially all the neutrons become bound
in helium nuclei, and thus it depends critically on the relative abundance r of neutrons
and protons just before the helium nuclei condense. To obtain this ratio requires
analysis of the beta decay reaction during cooling of the nucleon gas,
p + ¯
ν → n + e
+
.
(18.1)
The result is that the ratio was about r = 0.17 when neutrinos decoupled, and that this
dropped to about r = 0.14 due to neutron decay by the time the helium condensed.
It is easy to see that the abundance of helium follows as
He
H
=
r/2
1 − r
= 0.08
(18.2)
as we anticipated.
The predicted abundance of deuterium (heavy hydrogen) is about 10
−4 , that of
helium 3 (light helium) is about 10
−5 , and that of lithium is about 10
−10 ; all are consistent with observation (Wagoner 1967). These predicted abundances are sensitive to
the composition and temperature of the universe at the time of nuclear condensation,
and this places important constraints upon these properties (Freedman 1967). One
interesting result is that the abundance of ordinary nucleonic matter must be about
an order of magnitude less than the present critical density, and thus that the dark
matter is unlikely to be ordinary nuclear matter (Weinberg 1988).
18.5 Condensation of Nucleons
Yet again we run the clock backwards, well beyond the 1 s time of nuclear condensation, to a time when the temperature energy is about 1 GeV. At higher temperature
even nucleons are not stable, but decompose into quarks and gluons. Thus we can
18 A Brief Historical Overview of the Universe
18.4 Condensation of Nuclei
Again we run the clock backwards, well before the 10
12 s decoupling time, to a time
when the thermal energy is about 1 meV. At higher temperature nuclei cannot exist
because they are disintegrated by collisions and γ rays in the radiation. Thus we can
think of nuclei as condensing from a plasma gas of nucleons at this time, about 1 s.
The process of nuclear formation is commonly called nucleosynthesis. The plasma
before this time is composed mainly of neutrons and protons and electrons and many
photons and neutrinos (Weinberg 1988).
The theory of primordial nucleosynthesis has been very successful in predicting
the abundance of the primordial light elements. For example, the number ratio of
helium to hydrogen atoms is predicted to be about 1/10, in good agreement with
observation. This calculation assumes that essentially all the neutrons become bound
in helium nuclei, and thus it depends critically on the relative abundance r of neutrons
and protons just before the helium nuclei condense. To obtain this ratio requires
analysis of the beta decay reaction during cooling of the nucleon gas,
p + ¯
ν → n + e
+
.
(18.1)
The result is that the ratio was about r = 0.17 when neutrinos decoupled, and that this
dropped to about r = 0.14 due to neutron decay by the time the helium condensed.
It is easy to see that the abundance of helium follows as
He
H
=
r/2
1 − r
= 0.08
(18.2)
as we anticipated.
The predicted abundance of deuterium (heavy hydrogen) is about 10
−4 , that of
helium 3 (light helium) is about 10
−5 , and that of lithium is about 10
−10 ; all are consistent with observation (Wagoner 1967). These predicted abundances are sensitive to
the composition and temperature of the universe at the time of nuclear condensation,
and this places important constraints upon these properties (Freedman 1967). One
interesting result is that the abundance of ordinary nucleonic matter must be about
an order of magnitude less than the present critical density, and thus that the dark
matter is unlikely to be ordinary nuclear matter (Weinberg 1988).
18.5 Condensation of Nucleons
Yet again we run the clock backwards, well beyond the 1 s time of nuclear condensation, to a time when the temperature energy is about 1 GeV. At higher temperature
even nucleons are not stable, but decompose into quarks and gluons. Thus we can