Hawking, Linde spar
over birth of the universe
BY DAVID F. SALISBURY
Fleet Street has dubbed it
a battle over the birth of the universe. The
"battle" is in fact a scientific disagreement
between Andrei Linde, a physics professor at Stanford and
well-known proponent of the inflationary universe theory,
and Stephen Hawking, a prominent Cambridge University
physicist and author of the best-selling book A Brief
History of Time.
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The subject is a new
theory for the origin of the universe that has been
announced by Hawking and his Cambridge colleague Neil
Turok. In their theory they propose that the universe
began as an "instanton," a pea-sized cocktail
of space, time, matter and energy that explodes and
transforms itself into an infinite universe, one that
continues to grow and expand forever.
In the March 14 issue of
the London Telegraph, Turok wrote that they
"have made what we consider to be a very exciting
discovery regarding the possible beginning of our
universe. We think this discovery takes us one step
closer towards a 'theory of everything.'"
Linde is not convinced. He
thinks that they have applied some of their mathematics
incorrectly and that their method would create empty
universes where matter and energy are so scarce that it
would be impossible for life to form. For the last few
weeks, Linde has been exchanging electronic critiques and
counterarguments with Hawking and Turok over the Internet
in the form of manuscripts of articles awaiting
publication in physics journals.
Scientists frequently have
this kind of debate over the validity of new theories. It
is one of the mechanisms that scientists have developed
to uncover errors and come up with the best possible
explanations for a wide variety of phenomena. But two
things make the Linde-Hawkins debate exceptional. One is
the subject matter: nothing less than the origin and fate
of the universe. The other is the media coverage.
Propelled by the celebrity status of Hawking, who
frequently has been compared to Einstein, the London
Telegraph, Manchester Guardian and Science
magazine have given their readers a glimpse of this
esoteric and politely passionate exchange.
Last week the discussion
came to Stanford. At Linde's invitation, Hawking
described his and Turok's proposal in a lecture on April
23. Using a voice synthesizer, Hawking, who cannot talk
and is largely paralyzed by motor-neuron disease,
summarized their basic argument before an overflow crowd
in the new SEQ Learning Center Auditoria.
For the past 40 years,
scientists generally have accepted the idea that the
universe began about 15 billion years ago in a uniform
energy broth that immediately began to fly apart: the
so-called big bang. As the soup expanded, it cooled,
allowing matter to precipitate out and clump up to form
stars and galaxies. This simple idea successfully
explains the astronomical evidence that the universe is
expanding today. It also can explain the observation that
the entire sky is filled with an even microwave glow
the cosmic background radiation as fossil radiation
left over from an early period when the universe was much
hotter. Third, the big-bang theory accurately predicts
the relative abundance of hydrogen, deuterium, helium and
lithium, the lightest stable elements in nature.
Many scientists simply
have assumed that the universe contains exactly the right
amount of matter so that its gravitational attraction is
great enough to slow and eventually halt the universe's
expansion, resulting in a flat universe. This balances
the universe precisely between two different kinds of
fates. Slightly more matter and the universe is closed.
An object traveling in a straight line ultimately returns
to the point where it started. Such a universe expands to
a point and then reverses course and begins to contract.
Slightly less matter, on the other hand, and the universe
is open: It is unbounded and continues to expand forever.
Recent astronomical
evidence strongly suggests that the universe contains
only about one-fifth the amount of matter required to
counteract the universe's expansion. "I thought for
many years that the astronomers had simply overlooked
something, that there was enough matter to close the
universe," Hawking confessed.
In 1983, Hawking
collaborated with James Hartle at the University of
California-Santa Barbara to develop an alternative method
for the origin of the universe. They called it the
"no-boundary proposal" because it pictures the
universe as emerging with a finite size from imaginary
space/time. The approach had been largely ignored,
however, because it was limited to closed universes. So
Hawking was intrigued when Turok suggested that there was
a different way of looking at the no-boundary proposal
that could produce open, rather than closed, universes.
The key to this new
approach is another cosmological idea called inflation.
Proposed simultaneously in Moscow and the United States,
inflationary theory postulates that in the first fraction
of a second in its existence the universe went through a
period of dramatic expansion. Such a period would arise
because of conditions that caused the force of gravity to
act as a force of repulsion, rather than attraction. Such
a reversal would cause an exceedingly rapid expansion
where large amounts of matter and energy were created
freely.
Inflationary theory helps
explain some important characteristics of the universe as
it appears today, including the extreme uniformity of the
universe. If you take a very small piece of even a very
heterogeneous object and blow the piece up until it is
very large, the inflated piece will be very homogeneous.
Similarly, the extreme uniformity of the cosmic
background radiation that has been observed can be
explained if all parts of the visible universe were once
close enough together to have been in communication with
each other. Inflation also can account for the
"ripples" required to explain how the structure
of the present-day universe could have arisen from a
perfectly uniform energy broth. These ripples were simply
microscopic "quantum fluctuations" that are
exploded to the size of galaxies.
Initially, inflation was
tied to a condition called a false vacuum. This is a
condition in which empty space acquires an extra charge
of energy. But Linde freed the inflationary model from
this limitation by showing that it could be produced
simply by the presence of a special kind of field, called
a scalar field, that particle physics has invoked to
explain why particles have mass. Hawking and Turok
applied one of these techniques to the no-boundary
proposal. This allowed them to use the
"instanton" process to produce a tiny
"bubble universe" that would undergo inflation
and expand forever.
"This is a logical
extension of Andrei's idea. I don't know if he agrees
with it yet, but I think he's coming around,"
Hawking said.
A vaccination
But in a lecture Linde
gave to the SLAC theory group on the previous Friday, he
warned that his talk was part advertisement for Hawking's
talk and part vaccination.
Linde pointed out that the
Turok/Hawking approach tends to produce universes that
are nearly empty of matter and energy. The team attempts
to address this problem by invoking what is called the
anthropic principle. This holds that the only universes
worth considering are those that are capable of
supporting life. This allows them to ignore the empty
universes and concentrate on those with the most matter.
"Even after using the
anthropic principle," Linde said, "Hawking and
Turok predict a universe which has a matter density at
least 20 times smaller than the number given by
astronomical observations." Hawking and Turok
maintain that they should be able to fine-tune their
approach to produce universes with more matter.
Linde also questioned the
way the pair used the Hartle-Hawking equations that are
the very basis of the no-boundary proposal. These
equations do not represent the probability that a
universe of a given type will be created. Instead, they
describe that universe's ground-state condition, what it
is like when it is completely empty, he argued.
"Stephen has too much
confidence in this basic mathematical approach, which he
has used a number of times with great effect," said
Linde. "You need to make sure that you are applying
the mathematics correctly. In this case, my intuition
tells me that he has not done so.
Linde champions a
different approach to universe creation called the
tunnelling hypothesis. It is very similar to the
Hartle-Hawking approach, but uses a different method to
calculate the probability of creation of the universe.
This method can be used to produce a whole range of
inflationary, open universes, he maintains.
"My conclusion is
that the old method for creating an open universe is
still the best," he said.
In Hawking's opinion the
tunneling hypothesis is "either not well defined or
it gives the wrong answers." He said that it creates
universes where small fluctuations would be enhanced in
ways that make the universe unstable. "There is no
way that this could lead to a reasonable universe,"
he said.
It may be several years
before this dispute can be resolved. Turok and Hawking's
mechanism predicts a distinctive pattern of fluctuations
in the microwave sky. These may be detectable by two
satellites due to launch in the next few years: NASA's
Microwave Anisotropy Probe, with a launch date of 2000,
and the European Space Agency's Planck spacecraft, due to
fly in 2006.
"I consider Stephen
my friend and I hope we will remain friends after this is
over," Linde said. "He is a very brilliant man.
A number of times he has come up with surprising
conclusions that, at first, seem like they are wrong. But
in several instances he turned out to be right. In other
cases, he was wrong. We will just have to wait to see
which it is this time." SR
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