Density Fluctuations and Structure
In addition to (potentially, at least) solving the preceding problems of the big
bang, the theory of inflation presents a bonus: detailed considerations indicate
that inflation is capable of producing small density fluctuations that can
provide the seeds to cause matter to begin to clump
together later in the history of the Universe to form the galaxies and other observed
structure.
(See the subsequent discussion of structure growth in the
Universe.) The basis for this formation of seed fluctuations is the stretching of
tiny fluctuations on scales the size of microscopic particles to
(literally!) astronomical scales
by the rapid inflation.
Inflation of Microscopic Fluctuations
The top right diagram illustrates such a fluctuation.
The vacuum is the technical term for a state where there are no particles or
fields present. The theory of quantum mechanics permits excitations of the vacuum like the one shown in
the figure, provided that they occur for very short periods of time. In this example, a
particle-antiparticle pair is created spontaneously from the vacuum and an instant later the particle and
antiparticle destroy each other, leaving us with the vacuum again. This
is called a vacuum fluctuation, and will cause a tiny disturbance
in the region of space where the pair was formed and destroyed. These fluctuations go on all the time
at subatomic levels, but normally they are confined to that size. However, in the period of rapid
inflation a disturbance that is of subatomic size can be stretched to sizes comparable to star clusters
or galaxies if the inflation is rapid enough.
Initiation of Structure Formation
These "stretched" microscopic fluctuations are thought to be the ultimate
seeds for matter concentrations observed
in galaxies and clusters of galaxies. The small density variations
produced by inflation have many of the properties that appear to be favorable for initiating later
structure formation. For example, since fluctuations occurring early in the inflationary period will be
stretched much more than those occurring later, inflation produces density perturbations distributed over
many different sizes. This distribution of density fluctuations is known to be particularly
favorable to the formation of the kind of structure that we see in the present Universe.
However, inflation alone, without the
aid of dark matter, probably has difficulty in accounting for how rapidly structure
began to form in the early Universe. The
strength of the perturbations produced in the visible matter would not by itself
have been large enough to explain why galaxy formation was in vigorous progress less than a
billion years after the big bang. We shall return to the role of dark matter in this rapid onset of
structure formation shortly.
Inflation: Where it Stands
Although inflation has many attractive features, it is not yet a
completely proven theory
because many of the details still do not work out right in realistic calculations
without making assumptions that are poorly justified. Probably most cosmologists
today believe inflation
to be correct at least in its outlines, but further investigation
will be required to establish whether this is indeed so. Certainly the recent
data from anisotropies in the microwave background have given a strong boost
to the idea that inflation is correct, particularly because of the confirmation
of inflation's essential prediction that the Universe must be exactly flat.