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Magnetic monopoles as engines for cosmological inflation
STANFORD -- Andrei Linde has given the story of the missing magnetic monopoles a surprising new twist: They may be portals in space-time through which new space flows into our universe.
Writing in the May 19 issue of Physics Letters, Linde, a professor of physics at Stanford University, proposes that magnetic monopoles, subatomic particles with only one magnetic pole, may be unexpected engines for cosmological inflation.
The theory of cosmological inflation was first proposed about 15 years ago, in large part to solve a major problem with the original Big Bang theory.
Modern particle theory predicts that large quantities of superheavy particles that carry magnetic charge should have been created in the conditions of extreme temperature and density that existed 15 billion years ago in the exp losive birth of the universe. Furthermore, such particles should be very stable, and so should still be around. However, scientists have yet to find evidence that these unusual particles exist in the world around us.
Inflationary theory takes care of the monopole problem quite elegantly by proposing that our little patch of the cosmos has gone through a period when it expanded exponentially in size. As a result, what began as a microscopic spot of vacuum in the original universe ballooned up until it became larger than the entire observable universe. Such an inflationary episode puts so much space between individual monopoles that detection becomes highly unlikely.
Despite this and a number of other advantages, the early inflationary theory developed by Alan Guth from the Massachusetts Institute of Technology had some fundamental problems. It was based on a theory developed by Linde and D avid Krizhnits at the Lebedev Institute in Moscow that involved high-temperature phase transitions in the early universe. In certain circumstances, these phase transitions appeared to be capable of creating inflationary expansion. Unfo rtunately, this scenario could not adequately explain the highly homogeneous character of the present-day universe.
Then in 1982 Linde proposed what he calls a "new inflationary scenario," which was free of the main difficulties of the theory suggested by Guth. This relied on a simpler mechanism for generating inflation than those used by pr evious theories. This mechanism was predicated on the existence of a special kind of field, called a scalar field, that particle physicists have introduced to help explain the differences between massless and massive particles.
The closest analog to such a field in everyday life is electrostatic potential, Linde explained. If the entire universe had the same electrostatic potential, say 110 volts, then it would have no effect. Without variations in el ectrostatic potential, however, electric currents and magnetic fields would not exist. Similarly, a cosmic scalar field, although largely invisible, causes particles that interact with it to become heavy while others that do not remain light.
Guth's inflationary scenario also was based on the scalar field theory, but he used a complicated version of the field that produced unacceptably large inhomogeneities in the post-inflationary universe.
New inflation had some problems of its own, chief among them the fact that the inflation process was extremely difficult to initiate. So a year later Linde proposed one more modification, which he called "chaotic inflation." In this theory, it is possible for certain small domains in the universe to begin inflating spontaneously and the probability of inflation occurring is rather large.
Moreover, as Linde showed in 1986, once inflation begins in the chaotic inflation scenario, in some parts of the universe it continues without end, repeatedly creating new domains where our type of life later becomes possible. This was one of the main reasons why the chaotic inflation scenario gradually became more popular than the new inflationary scenario.
"Personally, I prefer chaotic inflation, but we don't know if it is right. So we can't close any windows," Linde said, explaining why he turned his attention back to the new inflationary scenario.
In his latest paper, Linde has concluded that inflation may not only help get rid of the monopoles, but these particles may also help ensure that inflation begins and, once begun, continues without end.
Previous theoretical studies of monopoles have all considered them to be point-like objects that were too small to be affected by inflation. Linde, however, assumes that the monopoles are created while inflation is taking place and so they undergo exponential expansion just like the space around them. Thus, once monopoles are created, they become inflationary centers, he argues. To an outside observer, the monopoles appear as microscopic, magnetically charge d black holes. But, to an inside observer, they appear to expand until they become the size of a universe. As a result, each monopole creates a wormhole into a new inflationary universe.
If the initial inflationary region is visualized as a bubble, then each monopole creates a new bubble extending from the original and, in the process, creates new monopole pairs which begin creating other new bubbles, and so on for eternity.
"This is an amazing new twist. If we begin with at least one monopole, we can create eternal inflation. The possibility that monopoles by themselves solve the monopole problem is so simple that it certainly deserves further in vestigation," Linde said.
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