Three pioneering scientists in the field of inflationary theory shared the Dirac Medal of the ICTP in 2002.

Universal Inflation

What exactly happened at the moment the universe was created some 14 billion years ago? Can we trace the universe's origins to a huge fireball that followed on the heels of the Big Bang?
Andrei Linde, a professor of physics at Stanford University, USA, and recipient of the 2002 Dirac Medal, has been one of the main architects of an alternative theory of the universe's origins--one based on inflation.
"The cosmos," he explains, "became exponentially large in the blink of an eye--an infinitesimal fraction of a second. Only then did the universe begin to evolve according to the standard 'post-bang' principles of the Big Bang--an endless process marked by cooling and slow, continual expansion."
Linde adds that he doesn't think "our universe is unique." In fact, he contends that an infinite number of ever-expanding universes, similar to the one in which we live, must exist and that each of these universes, in turn, is capable of spawning additional universes in a never-ending tale of cosmic proportions.
Linde has emerged as the most vocal proponent of inflationary theory. But he would be the first to admit that he is not alone in this effort. Indeed Alan Guth, Victor Weisskopf professor of physics at the Massachusetts Institute of Technology, and Paul Steinhardt, Albert Einstein professor of physics at Princeton University, USA, have also made critical intellectual contributions to the theory. For this reason, Linde, Guth and Steinhardt shared the Dirac Medal of the ICTP in 2002.

ICTP director Katepalli Sreenivasan with
2002 Dirac Medal winners Alan Guth, Paul Steinhardt and Andrei Linde

Inflationary theory--a construct built on the power of intellectual insight and the elegance of mathematical analyses--has recently received a boost from observational science, thanks to 'cosmic' data collected by astronomical satellites and probes.
Linde, now 55, was born in the former Soviet Union, which was also the birthplace of inflationary theory. Alexei Starobinsky, a scientist at the Landau Institute of Theoretical Physics in Moscow, presented the first theory of inflation in 1979. However, the idea remained largely confined to a small group of Soviet cosmologists who, while well known in science circles within their own country, had only limited contact and visibility among their counterparts in western Europe and the United States.
Meanwhile, Linde received his doctorate degree in physics from the University of Moscow's Lebedev Physical Institute in 1974, focussing his dissertation on cosmological phase transitions. Based on the findings of his dissertation but first fully articulated in 1976, his notion that phase transitions experienced in a supercooled vacuum state may be sufficient to transform a cold universe into a hot one subsequently became a fundamental element of inflationary cosmology.
In 1981, Alan H. Guth, a postdoctoral student in physics at Cornell University, USA, devised his own inflationary model, independent of any knowledge of the work being done in the Soviet Union.
Indeed it was Guth who chose the word 'inflation' to describe the universe's exponential expansion in an infinitesimal moment (estimated to be 10 to minus 37 seconds). During this fleeting time, matter and energy, at first compressed into a space the size of a proton, filled the void that became the universe--creating on a cosmic scale the same dynamics that unfolds when an airbag is released inside an automobile.
Despite its elegance, Guth's theory contained several shortcomings. Seeking to refine and improve Guth's model, two other youthful American physicists, Andreas Albrecht and Paul Steinhardt, working together at the University of Pennsylvania, constructed yet another model of inflation and, in 1982, Linde began to offer his contributions to the field. One year after the fall of communism in 1989, Linde left for the United States, bringing the disparate strands of inflationary theory closer together.
What accounts for the growing force of inflationary theory within the larger field of cosmology? Part of its appeal is that the theory answers several difficult cosmological questions that remain unanswered by the Big Bang theory and therefore provides a more coherent framework for understanding how the universe began and evolved.
For example, why does the universe's broad landscape appear to be flat when Einstein's theory of relativity indicates that the universe should be curved? The answer? Inflationary theory holds that the universe is so large that we can see only a tiny portion of it. So, from our limited perspective, the universe seems flat when in reality it is curved.
Similarly, why have we not been able to detect primeval monopoles--particles with a single magnetic pole that theoretical physicists have concluded should be present in our universe? Again proponents of inflationary theory contend that the universe is so large that such particles may be literally lost in the vastness of space and thus beyond the 'detection' capabilities of our current instruments.
Finally, what accounts for the universe's homogeneity and isotropy at a large scale? Again proponents of inflationary theory have an answer. Inhomogeneous elements that were present before inflation were eliminated--or, as cosmologists prefer to say, were 'stretched' or 'ironed'--during the breathtaking fragmentary moment that led to the universe's creation. At the same time, inflation produced its own inhomogeneities resulting from small quantum fluctuations that rushed into the vacuum. Stars and galaxies emerged from these 'matter-and-energy' ripples.
"In the past," says Linde, "cosmologists could often only discuss their ideas as abstractions or philosophical musings. Today, however, we are able to probe the outer reaches of space to prove or disprove our theories. Even the harshest critics now acknowledge that our analytical description of the universe's origins is rooted more in science than science fiction. That's because cosmology's theories can increasingly be tested experimentally."
"Cosmologists," Linde is quick to add, "can no longer contend that a theory is right because it's beautiful and elegant, as sometimes happened in the past. Experimental data now plays a crucial role in our research, requiring us to prove that what we say may be true is, in fact, true."
Linde's personal observations about his work are backed by satellite observations in space. In 1992, for example, the US National Aeronautics and Space Administration's (NASA) Cosmic Background Explorer (COBE) satellite took the first 'pictures' of the energy remnants associated with the early universe--photos in the form of fossilised microwave radiation casting faint, frozen images lodged in an environment that existed when the universe was just 200,000 years old and its temperature decreased to about 3000 degrees Kelvin. Browsing through COBE's picture album you can gaze at the predicted elemental ripples of matter and energy from which new galaxies may be spawned in the future.
In 1999, the Boomerang mission over Antarctica, a scientific balloon experiment jointly managed by US and Italian space agencies and universities, enabled scientists to collect even more detailed data on the universe that seemed to confirm COBE's results. The data, however, was based only on a small patch of the universe and more surveys are currently underway to verify the Boomerang mission's findings.
Then, just a few months ago, NASA's Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2001, provided a triumphant confirmation of inflationary theory through its recording of far-away detailed pictures of the remnants of the early universe that parallel theorists' concepts of what we might expect to find.
While Linde welcomes these emerging proofs of his theories, he has recently taken inflationary principles beyond their initial theoretical boundaries into new intellectual frontiers. He hypothesises that the entire universe may consist of an infinite number of inflationary balls that emerge one from another "much like a 'cosmic tree' that grows exponentially over time generating new seedlings that ultimately take root on their own."
"What I am proposing," he notes, "is the existence of a 'fractal-like,' self-reproducing inflationary universe in which each new universe abides by its own physical laws, creating--or denying--conditions that can support life as we know it within our own universe."
"The inflationary paradigm," Linde proudly notes, "is no longer an obscure segment of the Big Bang theory, which was the case just 20 years ago. On the contrary, the Big Bang theory has now become part of the inflationary model."
Inflationary theory's journey from the backstage to the centrestage of cosmology has made its most 'star' players--Guth, Linde and Steinhardt--worthy recipients of the Dirac Medal of the ICTP.

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