Chapter 7
Inflationary cosmology
7.1 Introduction
The inflationary universe scenario was devised by Guth [I] to provide a resolution
to two major puzzles in the standard model of the universe, namely the horizon
and flatness problems. As a by-product, inflation also solves problems associated
with excessive abundances of particle relics. There are two basic versions of
cosmological inflation which we shall refer to as 'old' and 'new' (or 'slowroU') inflation. In both versions, the universe undergoes a period of very rapid
expansion driven by a large cosmological constant in the false vacuum. This
period of inflation ends when the universe has evolved to the true vacuum with
zero cosmological constant. In the case of old inflation, the universe supercools
in some high-temperature phase before undergoing a first-order phase transition
to some low-temperature phase. In the case of new inflation, some scalar field
rolls in a very flat region of a potential (where the vacuum energy is large and
positive) and eventually rolls to a minimum of the potential with zero vacuum
energy (cosmological constant).
In the first part of this chapter, we shall discuss old inflation and its successes
and shortcomings. The second part of the chapter contains an exposition of new
inflation (slow roll inflation.) We shall see that, as well as providing a solution to
these cosmological problems, slow-roll inflation is capable of accounting for the
size of the density perturbations in the cosmic microwave background radiation.
7.2 Horizon, flatness and unwanted relics problems
We discuss these three puzzles in the standard model of cosmology in turn.
7.2.1 The horizon problem
The cosmic microwave background radiation (CM BR) is very homogeneous.
However, in the standard model, the present universe consists of many regions
DOl: 10.1201/9780367806637-7
195
Inflationary cosmology
7.1 Introduction
The inflationary universe scenario was devised by Guth [I] to provide a resolution
to two major puzzles in the standard model of the universe, namely the horizon
and flatness problems. As a by-product, inflation also solves problems associated
with excessive abundances of particle relics. There are two basic versions of
cosmological inflation which we shall refer to as 'old' and 'new' (or 'slowroU') inflation. In both versions, the universe undergoes a period of very rapid
expansion driven by a large cosmological constant in the false vacuum. This
period of inflation ends when the universe has evolved to the true vacuum with
zero cosmological constant. In the case of old inflation, the universe supercools
in some high-temperature phase before undergoing a first-order phase transition
to some low-temperature phase. In the case of new inflation, some scalar field
rolls in a very flat region of a potential (where the vacuum energy is large and
positive) and eventually rolls to a minimum of the potential with zero vacuum
energy (cosmological constant).
In the first part of this chapter, we shall discuss old inflation and its successes
and shortcomings. The second part of the chapter contains an exposition of new
inflation (slow roll inflation.) We shall see that, as well as providing a solution to
these cosmological problems, slow-roll inflation is capable of accounting for the
size of the density perturbations in the cosmic microwave background radiation.
7.2 Horizon, flatness and unwanted relics problems
We discuss these three puzzles in the standard model of cosmology in turn.
7.2.1 The horizon problem
The cosmic microwave background radiation (CM BR) is very homogeneous.
However, in the standard model, the present universe consists of many regions
DOl: 10.1201/9780367806637-7
195