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GRAVITY may have varied its strength during the earliest moments of
the Universe, according to an American astrophysicist. Such a variation
could patch up a flaw in so-called ‘inflationary cosmology’. This theory
is so good at explaining many features of today’s Universe that astronomers
would very much like to believe it.

According to inflationary cosmology, the vacuum – empty space – underwent
a ‘phase transition’ when the Universe was less then 10-35 seconds old.
This was akin to the change which occurs when water vapour condenses into
liquid water. As a result of the phase transition, energy poured into the
expansion, causing the Universe to grow at an exponential rate.

When inflation began, the entire observable Universe was contained in
a volume just 10-45 metres across, but in a tiny fraction of a second it
was ‘inflated’ to the size of a grapefruit.

Theorists invoked inflationary cosmology in order to explain why the
initial expansion of the Universe was so smooth and uniform. One piece of
evidence that tells them this was so is the cosmic microwave background
radiation, the remnant radiation of the big bang. This is extremely uniform,
implying that the matter that emitted it early on in the Universe was similarly
uniform (New Å®ÉúСÊÓƵ, Science, 28 April).

In the inflationary picture, the entire Universe grew from a tiny ‘seed’,
which was only a tiny fraction of all there was. The seed was too small
to contain irregularities.

The main problem with the inflationary theory has been what is known
as ‘the graceful exit problem’. At the end of the period of inflation, when
the Universe made the transition to the more leisurely expansion we see
today, irregularities would have been created.

Now, Paul Steinhardt, of the Institute for Advanced Study in Princeton,
thinks he has a solution to the problem. He has found a way to slow the
rate of inflation by allowing the strength of gravity to vary in a way which
is linked to the expansion rate itself. This ‘extended’ inflation still
takes only a fraction of a second, but it ends less abruptly and allows
a smooth transition into the big bang itself (Nature, vol 345, p 47).

Steinhardt’s idea offers a bonus. It explains why the Universe contains
the amount of matter it does.

The original inflationary model requires that the Universe today should
contain almost exactly the amount of matter required to make space ‘flat’.
This is a universe that sits on the borderline between one that expands
forever, or is ‘open’, and one that must one day collapse again under the
pull of its own gravity, or is ‘closed’.

Observations so far have found evidence for only about half of the required
mass in our Universe, and this is impossible according to the original inflationary
models. In Steinhardt’s extended inflation theory, it is indeed possible
that the Universe may end up with half the critical mass, although as he
acknowledges he has not proved that this is likely.

Cosmologists today are convinced that the basic concept of inflation
is such a neat resolution of so many puzzles that it must contain a large
grain of truth. As long as it raises new puzzles while solving old ones,
it is hard to swallow the whole package. Extended inflation, while not a
complete answer, may point the way ahead.