Annu. Rev. Astron. Astrophys. 1991. 29: 325-362 Copyright © 1991 by Annual Reviews. All rights reserved

1. INTRODUCTION

The concept of inflation was introduced into cosmology by Guth (48) about a decade ago. It has generated a remarkable degree of response, both positive and negative, from physicists. By hindsight, the idea appears a natural consequence of the concept of the phase transition, which is believed to have occurred in the very early epochs of the big bang universe, when the breakdown of the so-called grand unification symmetry took place. When it was first proposed, the concept was somewhat difficult to understand, however, as it combined ideas from particle physics with those from the general theory of relativity. Even today, controversy remains about important questions, e.g.: Was there really an inflationary phase in the universe? If yes, what was the physical mechanism behind it? Given the mode of inflation, what tangible relics should that era have left for today?

Although excellent reviews are available on this subject (see e.g. 17), they have been written largely by and for the theoreticians working on the frontier between particle physics and cosmology. This review, as its title indicates, is written for astronomers. We present the basic idea in a form that is as free of the jargon of particle physics as possible, and focus attention on the last of the three questions posed above.

To help the reader (and ourselves!), an outline of the notation to be used in this article is presented in Table 1.

Table 1. Notation

FRW: Friedman-Robertson-Walker

MBR: Mirowave background radiation

LSS: the last scattering surface

GUTs: grand unified theories

HDM: hot dark matter

CDM: cold dark matter

a (t) = the expansion factor of the universe at cosmic time t

t0 = the age of the universe, taking a (0) = 0 (thus t = t0 is the present epoch)

H0 = Hubble's constant at the present epoch

h = Hubble's constant today (i.e. at t = t0) in units of 100 kms-1 Mpc-1

c = (3 H02 / 8 G) = critical density separating the closed and open FRW models

= mass density of nonrelativistic particles at t = t0 in units of the present critical density

B = baryonic contribution to at the present epoch

grav = energy density of gravitational waves at t = t0 in units of the present critical density

Q0 = any physical quantity Q evaluated at t = t0

T0 = temperature of MBR at t = t0

= T0 / (2.75 K)

teq = epoch when the matter and radiation energy densities were equal

tdec = epoch when matter (baryons and leptons) decoupled from radiation

H = Hubble's constant during the inflationary phase

1 MeV = 106 electron volts (this energy corresponds to a temperature of 1.16 x 1010 K)

tp = Planck epoch; mp = Planck mass.