17
The Inflationary Universe
According to the Big Bang theory, the universe is undergoing continuous
expansion from its early moments until today. This is evident from the
observation of the cosmic background radiation by Arno Penzias and Wilson,
as well as observation of distant galaxies. Penzias and Wilson got the Nobel
Prize for this discovery in 1978. The rate of expansion is given by Hubble’s
law. The Hubble rate of expansion increases as the square of the temperature
if all the particles are relativistic, and slightly slower if they are non-relativistic.
As the universe is cooling down, this expansion rate is slowing down with time.
It has been realized for almost 40 years that, if this Hubble rate represented
the expansion rate from the beginning of Big Bang until now, there would be
some difficulty with our understanding of the universe’s history. This led to the
suggestion that there is a different expansion phase of the universe before the
Hubble rate of expansion began. This is a phase of extremely rapid expansion
and is called the “inflationary” expansion phase.
17.1 Inflation: One Beginning Before Another
Beginning
Why is inflationary expansion needed? There are several reasons for this. If
the expansion of the universe was given by the Hubble rate all the way from
the beginning of the universe, we would encounter some problems. The first
is a causal connection problem among different parts of the universe today.
Observations say that the universe is almost isotropic in the temperature of
© 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_17
129
The Inflationary Universe
According to the Big Bang theory, the universe is undergoing continuous
expansion from its early moments until today. This is evident from the
observation of the cosmic background radiation by Arno Penzias and Wilson,
as well as observation of distant galaxies. Penzias and Wilson got the Nobel
Prize for this discovery in 1978. The rate of expansion is given by Hubble’s
law. The Hubble rate of expansion increases as the square of the temperature
if all the particles are relativistic, and slightly slower if they are non-relativistic.
As the universe is cooling down, this expansion rate is slowing down with time.
It has been realized for almost 40 years that, if this Hubble rate represented
the expansion rate from the beginning of Big Bang until now, there would be
some difficulty with our understanding of the universe’s history. This led to the
suggestion that there is a different expansion phase of the universe before the
Hubble rate of expansion began. This is a phase of extremely rapid expansion
and is called the “inflationary” expansion phase.
17.1 Inflation: One Beginning Before Another
Beginning
Why is inflationary expansion needed? There are several reasons for this. If
the expansion of the universe was given by the Hubble rate all the way from
the beginning of the universe, we would encounter some problems. The first
is a causal connection problem among different parts of the universe today.
Observations say that the universe is almost isotropic in the temperature of
© 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_17
129