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.
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.
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