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R. N. Mohapatra
and the energy of the relativistic particles becomes smaller and smaller. For
example, the energy of a photon at the epoch when the scale factor is a(t)
becomes half as the scale factor becomes 2a(t). This is called redshift of particle
energy with expansion.
All particles in the beginning were hot and moving extremely rapidly, and
were relativistic, i.e. moving at or close to the speed of light, which is 186,000
miles per second. Most of those particles behaved like massless particles
(similar to the photon). As the universe expanded, the density of energy in
it and the density of particles in it started becoming less and less. Since the
expansion rate is governed by the energy density according to Einstein’s theory
of general relativity, the rate of expansion slows down. The expansion obeys the
Hubble law which says that the speed of a particle (star, galaxy) is proportional
to its distance from the observer, i.e. the further an object is from a given point
in the universe, the faster its speed as observed from that point. This law of
expansion holds even today, except that in recent years, a new accelerating
phase of the universe has been discovered which we discuss later.
At a certain point in its expansion, the particles, which in the beginning
were massless, get their mass, and if the temperature of the universe is less
than the mass, they start moving slower. If they are unstable, they decay away
to lighter particles. During this expansion, several major events happened to
the universe that led to what the present universe looks like. One of the major
ones is the inflationary phase, which we discuss in the next chapter.
16.3 Major Events in the Universe’s Past
The major events from the universe’s past can be summarized as follows: (For
a detailed history of the various stages, see [67, 100].)
1. Inflation begins and ends. Inflation refers to sudden rapid expansion of the
universe at the beginning, soon after the Big Bang.
2. Matter–anti-matter asymmetry is created. The exact moment when this
happened is not known.
3. Quarks turned to protons and neutrons when the universe was about 10
−4 s
old.
4. Protons and neutrons turned to helium, lithium, and beryllium when the
universe was about a minute old. This is known as Big Bang nucleosynthesis.
5. Atoms formed when the universe was about 400,000 years old.
R. N. Mohapatra
and the energy of the relativistic particles becomes smaller and smaller. For
example, the energy of a photon at the epoch when the scale factor is a(t)
becomes half as the scale factor becomes 2a(t). This is called redshift of particle
energy with expansion.
All particles in the beginning were hot and moving extremely rapidly, and
were relativistic, i.e. moving at or close to the speed of light, which is 186,000
miles per second. Most of those particles behaved like massless particles
(similar to the photon). As the universe expanded, the density of energy in
it and the density of particles in it started becoming less and less. Since the
expansion rate is governed by the energy density according to Einstein’s theory
of general relativity, the rate of expansion slows down. The expansion obeys the
Hubble law which says that the speed of a particle (star, galaxy) is proportional
to its distance from the observer, i.e. the further an object is from a given point
in the universe, the faster its speed as observed from that point. This law of
expansion holds even today, except that in recent years, a new accelerating
phase of the universe has been discovered which we discuss later.
At a certain point in its expansion, the particles, which in the beginning
were massless, get their mass, and if the temperature of the universe is less
than the mass, they start moving slower. If they are unstable, they decay away
to lighter particles. During this expansion, several major events happened to
the universe that led to what the present universe looks like. One of the major
ones is the inflationary phase, which we discuss in the next chapter.
16.3 Major Events in the Universe’s Past
The major events from the universe’s past can be summarized as follows: (For
a detailed history of the various stages, see [67, 100].)
1. Inflation begins and ends. Inflation refers to sudden rapid expansion of the
universe at the beginning, soon after the Big Bang.
2. Matter–anti-matter asymmetry is created. The exact moment when this
happened is not known.
3. Quarks turned to protons and neutrons when the universe was about 10
−4 s
old.
4. Protons and neutrons turned to helium, lithium, and beryllium when the
universe was about a minute old. This is known as Big Bang nucleosynthesis.
5. Atoms formed when the universe was about 400,000 years old.