18.2 Condensation of Stars and Galaxies
277
18.2 Condensation of Stars and Galaxies
Stars and galaxies began to form early in the history of the universe, about 200
million years after the end of the radiation era, and they are still being formed. A star
forms when a large enough ball of gas with a density excess begins to contract due
to gravity. As the ball contracts it heats up, and eventually the temperature reaches
the point at which thermonuclear fusion begins in the gas (Chandrasekhar 1939;
Misner 1973). The dominant overall nuclear fusion reaction is 4 p → He + 2e
+ with
several neutrinos also being emitted. Energy released by fusion stops the gravitational
contraction by increasing the pressure due to heat and by direct radiation pressure.
The star then becomes a stable energy emitting member of the main sequence for
billions of years. Finally, enough of the fusible elements are used up that the star dies,
sometimes in a supernova explosion. We will not go into such stellar astrophysics
more deeply here since it is a major subject in itself and we can proceed with a
sketchy understanding for our study of cosmology (Carroll 2017; Dar 2006).
A galaxy forms when billions of stars combine into a large system. Galactic
evolution is an active field of research with many unknowns. Oddly enough we seem
to know more about quarks and the composition of nucleons than about galaxies
(Dar 2006; Quigg 2006).
We may think of stars forming from gas clouds by instability to gravitational
attraction as a sort of stellar condensation process. We may similarly think of galaxies
as forming from stars and gas as a sort of galactic condensation process.
18.3 Condensation of Atoms
Let us again run the cosmic clock backwards. As we have discussed in Chap. 17 the
universe grows hotter until a temperature of about 3000 K (roughly kT = 0.3 eV)
is reached, which happens at about 10
5 years or 10
12 s after the big bang. Before
this recombination time atoms cannot exist because radiation and collisions ionize
them, so the universe is largely composed of a plasma of hydrogen and helium nuclei
(protons and alpha particles) and electrons along with many photons and neutrinos.
We can view this process as atoms condensing from the hot plasma. The plasma
is of course charged and thus opaque to light and its contents (except perhaps the
neutrinos) are in thermal equilibrium due to electromagnetic interactions between
charged particles (Peebles 1993). Early on when the mixture is very hot it is fairly
well described by an ideal gas equation of state with p = ρ/3.
Note that neutrinos produced in early times interact very weakly and should still
exist now in the form of a neutrino background, analogous to the CMB radiation,
but the neutrino sea has not yet been detected. The role of neutrinos in cosmology is
presently under intense study; it depends on their mass, which is small but nonzero
(Dvorkin 2019).
277
18.2 Condensation of Stars and Galaxies
Stars and galaxies began to form early in the history of the universe, about 200
million years after the end of the radiation era, and they are still being formed. A star
forms when a large enough ball of gas with a density excess begins to contract due
to gravity. As the ball contracts it heats up, and eventually the temperature reaches
the point at which thermonuclear fusion begins in the gas (Chandrasekhar 1939;
Misner 1973). The dominant overall nuclear fusion reaction is 4 p → He + 2e
+ with
several neutrinos also being emitted. Energy released by fusion stops the gravitational
contraction by increasing the pressure due to heat and by direct radiation pressure.
The star then becomes a stable energy emitting member of the main sequence for
billions of years. Finally, enough of the fusible elements are used up that the star dies,
sometimes in a supernova explosion. We will not go into such stellar astrophysics
more deeply here since it is a major subject in itself and we can proceed with a
sketchy understanding for our study of cosmology (Carroll 2017; Dar 2006).
A galaxy forms when billions of stars combine into a large system. Galactic
evolution is an active field of research with many unknowns. Oddly enough we seem
to know more about quarks and the composition of nucleons than about galaxies
(Dar 2006; Quigg 2006).
We may think of stars forming from gas clouds by instability to gravitational
attraction as a sort of stellar condensation process. We may similarly think of galaxies
as forming from stars and gas as a sort of galactic condensation process.
18.3 Condensation of Atoms
Let us again run the cosmic clock backwards. As we have discussed in Chap. 17 the
universe grows hotter until a temperature of about 3000 K (roughly kT = 0.3 eV)
is reached, which happens at about 10
5 years or 10
12 s after the big bang. Before
this recombination time atoms cannot exist because radiation and collisions ionize
them, so the universe is largely composed of a plasma of hydrogen and helium nuclei
(protons and alpha particles) and electrons along with many photons and neutrinos.
We can view this process as atoms condensing from the hot plasma. The plasma
is of course charged and thus opaque to light and its contents (except perhaps the
neutrinos) are in thermal equilibrium due to electromagnetic interactions between
charged particles (Peebles 1993). Early on when the mixture is very hot it is fairly
well described by an ideal gas equation of state with p = ρ/3.
Note that neutrinos produced in early times interact very weakly and should still
exist now in the form of a neutrino background, analogous to the CMB radiation,
but the neutrino sea has not yet been detected. The role of neutrinos in cosmology is
presently under intense study; it depends on their mass, which is small but nonzero
(Dvorkin 2019).