17 The Inflationary Universe
131
some small fluctuations in density naturally developed, as would be the case in
any fluid. They eventually grew to become galaxies and clusters, etc. at a later
stage of the universe.
There are also other conceptual problems with the standard Big Bang
expansion, i.e. the universe appears to be nearly flat now (not like the surface
of a globe but rather like the surface of a dining table), as opposed to Hubble’s
expansion theory, which would make it look like a globe. For the universe to
appear flat, one has to adjust the curvature very precisely. Inflation solves this
problem by making the universe flat, due to its rapid expansion right from the
beginning.
The inflation ends at a very early moment of the universe—nobody knows
when though. Different theories say it ends at different stages and it is a very
active area of research right now. After inflation ends, familiar and new kinds of
matter start to appear in the universe in a process called reheating in cosmology
jargon, and the universe resumes Hubble expansion. Thus, one beginning of
the universe is a very rapid expansion via inflation, and as inflation ends, there
is another beginning, the beginning of Hubble expansion, which continues
until now. Another new accelerated expansion has been discovered during
the past two decades, which started in recent times due to the existence of a
small cosmological constant. This expansion is not related to the early universe
expansion via inflation. Thus the universe has undergone multiple phases in
its evolution before it reached today’s state. This will continue as the universe
ages further (Fig. 17.1).
Inflation is considered a very successful theory and is now a part of many
physics textbooks. There are however many objections to the way inflation
is supposed to occur. For example, there are many possible ways to get
inflation from a starting theory, but only a small number of those theories are
observationally consistent. Furthermore, it has been shown that inflation once
started does not end everywhere, but only in a small patch of the universe. Also,
it is not clear that there is any way to definitively test inflation, but nonetheless,
it is an interesting and useful paradigm which has cleared up many unpleasant
issues with the Big Bang theory. For a popular exposition of the inflation idea,
see [59].
A key question still remains unanswered about inflation—what is the true
mechanism behind it? Is there another object like the Higgs boson that is
driving inflation, or is it something else? Could it even be that inflation is
not the right theory for the beginning of our universe? We have to wait for
the answer to these questions. Thus, the jury may still be out on inflation as a
theory of the beginning of the universe. At the moment, however, inflation
131
some small fluctuations in density naturally developed, as would be the case in
any fluid. They eventually grew to become galaxies and clusters, etc. at a later
stage of the universe.
There are also other conceptual problems with the standard Big Bang
expansion, i.e. the universe appears to be nearly flat now (not like the surface
of a globe but rather like the surface of a dining table), as opposed to Hubble’s
expansion theory, which would make it look like a globe. For the universe to
appear flat, one has to adjust the curvature very precisely. Inflation solves this
problem by making the universe flat, due to its rapid expansion right from the
beginning.
The inflation ends at a very early moment of the universe—nobody knows
when though. Different theories say it ends at different stages and it is a very
active area of research right now. After inflation ends, familiar and new kinds of
matter start to appear in the universe in a process called reheating in cosmology
jargon, and the universe resumes Hubble expansion. Thus, one beginning of
the universe is a very rapid expansion via inflation, and as inflation ends, there
is another beginning, the beginning of Hubble expansion, which continues
until now. Another new accelerated expansion has been discovered during
the past two decades, which started in recent times due to the existence of a
small cosmological constant. This expansion is not related to the early universe
expansion via inflation. Thus the universe has undergone multiple phases in
its evolution before it reached today’s state. This will continue as the universe
ages further (Fig. 17.1).
Inflation is considered a very successful theory and is now a part of many
physics textbooks. There are however many objections to the way inflation
is supposed to occur. For example, there are many possible ways to get
inflation from a starting theory, but only a small number of those theories are
observationally consistent. Furthermore, it has been shown that inflation once
started does not end everywhere, but only in a small patch of the universe. Also,
it is not clear that there is any way to definitively test inflation, but nonetheless,
it is an interesting and useful paradigm which has cleared up many unpleasant
issues with the Big Bang theory. For a popular exposition of the inflation idea,
see [59].
A key question still remains unanswered about inflation—what is the true
mechanism behind it? Is there another object like the Higgs boson that is
driving inflation, or is it something else? Could it even be that inflation is
not the right theory for the beginning of our universe? We have to wait for
the answer to these questions. Thus, the jury may still be out on inflation as a
theory of the beginning of the universe. At the moment, however, inflation